Patent Application: US-22839802-A

Abstract:
degradable cross - linkers which are used to form polymer networks which degrade under aqueous conditions are described . these cross - linkers comprise a central polyacid , monomeric or oligomeric degradable regions and an optional water soluble regions . these monomers are preferably polymerized using free radical or condensation polymerization . degradation occurs at the ester linkages after cross - linking polymer filaments , and results in soluble polymer filaments which may be cleared from the body . preferred applications of these materials include , for example , controlled release of drugs and cosmetics , tissue engineering , wound healing , hazardous waste remediation , metal chelation , swellable devices for absorbing liquids and the prevention of surgical adhesions .

Description:
this invention discloses a representative synthesis and application of symmetrical biodegradable cross - linking agents for use in cross - linked polymer matrices formed into particles or slabs that may be used e . g ., in drug delivery . the cross - linking agents will be monomers or oligomers of biocompatible units in the preferred biological applications . in the preferred practice of this invention the cross - linker is composed of a central diacid ( such as succinic ); to this diacid is attached one or more biodegradable regions , which are then terminated by reactive moieties which are used for incorporation into the polymer network . this invention requires there be at least two reactive moieties ( two representative cross - linkers are depicted in fig1 ). the cross - linkers may be incorporated into matrices of various sizes ranging from hundreds of cm &# 39 ; s to 10 nm so as to control the diffusion of substance such as drugs e . g ., from the matrix by biodegradation of the cross - linkers under physiological conditions . ultimately the cross - linkers described above may be included in all variety of hydrophilic and hydrophobic polymer networks to which the desirable property of degradation is required . of importance in hydrogel engineering is the control the structural properties of a random polymeric network . in standard stepwise growth of polymers there is heterogeneity in copolymer composition and dispersity in the molecular weight of the polymer filaments thus making it difficult to precisely control bulk material properties of the polymer network such as crystallinity and mesh size . by engineering homogeneous structures into the polymer structure , usefully tuned macromolecular properties such as biodegradability can be obtained . hydrogel networks in the form of colloidal particles which are being explored for use in drug delivery ( kiser et al .) are not biodegradable owing to their carbon - carbon bond containing backbone and their methylene - bis - acrylamide cross - links . this fact initiated the design of a new class of centro - symmetric cross - linking monomers . one of the preferred characteristics of the new material was that it must be easily synthesized . a second preferred characteristic is that the cross - linkers be composed of biocompatible components . the third characteristic which separates this work from all other work in this area is that the biodegradable cross - linker be synthesized to be a single pure molecule and not a mixture . this characteristic should lead to defined biodegradation rates versus the use of a cross - linker mixture as in previous work ( pathak et al .). therefore by utilizing classical organic synthesis methodology to synthesize monodispersed degradable sequences into the monomer structure before polymer formation presents an opportunity to carefully control the overall degradation as well as possibly the release rate of entrapped substances . one of the particularly preferred embodiments of these cross - linkers is that they are composed of a symmetrical diacid each acid attached to a biodegradable regions consisting of acids , such as the alpha - hydroxy acids glycolic or lactic acid for example . these portions are then preferably terminated by the monomer methacrylate . the monomers are composed of a central polyacid as in fig1 and are attached to the degradable region through oxygen , nitrogen , or phosphorous atoms . structure a shows a monomer having a central diacid region (—), and a degradable region (/\/\/\/\/\/\/) which is then terminated by a reactive polymerizable region (------------ x ). structure b is similar and uses the same symbols except that the central core is a triacid symbolized by a t structure . fig2 displays a more specific embodiments of this invention . in structure c , a symmetrical centerpiece ( succinic acid ) is attached to two degradable regions containing alpha - hydroxy esters . these are then attached to a moiety ( r 2 ) which may or may not impart water solubility through the connecting portion labeled y . finally , the cross - linker is terminated with vinyl groups . structure d is again similar to structure c except in this case the monomer is terminated with two nucleophilic moieties which could be used to cross - link preformed polymer chains . these structures are exemplary only . many more are conceivable by those skilled in the art . in a preferred embodiment the network begins with a cross - linker containing two equal degradable regions attached to a central diacid and each containing a terminal reactive group . in a particularly preferred embodiment , the core is made of succinic acid , each degradable region is composed of either symmetrical units of glycolic or lactic acid where n in fig1 is between 1 and 5 and the terminal reactive group is a acrylate type moiety where r2 in fig1 is ch ( ch3 ) ch2co and y is equal to oxygen . in preferred embodiments the central piece can consist of esters of dicarboxylic acids such as malonic succinic , adipic , sebacic , maleic fumaric acids or even possibly ( alpha , omega -( oligo ( ethylene glycol )) dicarboxylic acid ( alpha , omega -( oligo ( propylene glycol )) dicarboxylic acid . other diacids such as aromatic polycarboxylic acids may also be used . in another embodiment tri - acids such as citric acid or tetra and penta acids such as edta and dtpa ( possibly as protected derivatives ) could also be utilized . also protected versions of tartaric , citric , aspartic or glutamic acid may be used in certain embodiments . the biodegradable region is preferred to be hydrolyzable under environmental or in vivo conditions . in the most preferred embodiment the degradable regions will be composed of glycolic or lactic acid domains containing anywhere from one to six members in each oligomeric region attached to the central piece . other hydroxy esters that may be embodied include : ( 3 - hydroxy butyric acid , 2 - hydroxy propanoic acid , and 5 - hydroxy caproic acid . other useful biodegradable regions include amino acids , ortho - esters , anhydrides , phosphazines , phosphoesters and their oligomers and polymers . this region is necessary for the invention because it is the chemical functionality terminating the two or more ends of the cross - linker which will chemically bind polymer filaments together . the preferred method of achieving this end is through an acrylate moiety , with polymerization through free radical generation . free radical generation can be accomplished via thermal , photochemical or redox catalysis initiation systems ( odian ). the preferred polymerizable regions for free radical generation are acrylates , vinyl ethers , diacrylates , oligoacrylates , methacrylates , dimethacrylates , and oligomethacrylates . alternatively another preferred method of cross - linking preformed chains in solution is to attach two or more nucleophiles to the end of the chains which would be reactive with an electrophile attached to the polymer chain . the preferred chemical reactive moieties for this method are carbonate , carbamate , hydrazone , hydrazino , cyclic ether , acid halide , acyl azide , alkylazide , succinimidyl ester , imidazolide , amino groups , alcohol , carbonyl , carboxylic acid , carboxylic ester , alkyl halide , aziridino , nitrile , isocyanate , isothiocyanate , phosphine , phosphonodihalide , sulfide , sulfonate , sulfonamide , sulfate , silane , or silyloxy groups . several initiation systems for the formation of polymer networks are useful with these compounds , depending on the application and the conditions used . for generation of polymer slabs either irradiation of vinyl groups with high energy light such as in the uv is a suitable method for initiation . other preferred methods include the use of thermally activated initiators such as azobisisobutyronitrile or benzoyl peroxide for initiation in water or mixed water / organic solvents , other water soluble alkyl diazo compounds , ammonium persulphate with or without n , n , n ′, n ′- tetramethyethylene diamine . for generation of particles by emulsion polymerization generation of radicals by thermal initiation is convenient . generally this is accomplished with water soluble initiators such as ammonium persulphate . other initiators include the water soluble alkyl diazo compounds . for generation of polymer networks in vivo the most useful initiation system is photochemical . photochemical initiation of free radical polymerization involves light activation of a light absorbing compound ( a dye ), radical abstraction of a hydrogen to generate the initiation radical ( usually an amine ), and attack of this radical on a vinylic moiety beginning the polymerization . this system preferably requires free radicals to be generated locally and within a short time period , preferably in seconds . initiation in this system begins with irradiation of light at the appropriate wavelength . the wavelength is chosen to be as close to the absorption maximum of the dye as possible . the preferred light absorbing compounds which will begin the radical generation process are eosin dyes , 2 , 2 ′- dimethoxy - 2 - phenyl acetophenone and other acetophenone derivatives . other photo redox active dyes include acridine dyes , xanthene dyes and phenazine dyes , for example , acriblarine , rose bengal and methylene blue , respectively . these dyes when photoactivated assume a triplet excited state which can abstract a proton from an amine and thus generate a radical which begins the polymerization . compounds which act as the initiating radical are amines such as triethanolamine , sulfur containing compounds such as ammonium persulphate , and nitrogen containing - heterocycles such as imidazoles . in the preferred embodiment of this invention , these cross - linkers can be incorporated in biodegradable network polymers that are either hydrophilic or are hydrophobic . hydrophobic networks will contain less than 5 % of the total mass of the polymer network as water . whereas hydrophilic networks can contain as great as 99 % water as the total mass . hydrophilic network polymers are known as hydrogels to those skilled in the art . those skilled in the art will generally recognize the polymer structures which are generally considered to be hydrophilic or hydrophobic . one preferred application of these materials is in the use of controlled delivery of bioactive compounds . in this method the cross - linker is homopolymerized or copolymerized with other monomer or polymers which may be charged or uncharged . the drug is placed in the polymer network by polymerizing the network around the drug ( i . e ., by co - dissolving or dispersing the drug with the monomer solution ) or by incubating the resulting polymer with a solution of the drug whereby it diffuses into the polymer network . in this embodiment the drug may be anywhere from 1 to 90 % by weight of the device . the biologically active compounds can be ( but are not limited to ) proteins , peptides , carbohydrates , polysaccharides , antineoplastic agents , water soluble linear and branched polymeric prodrugs , particles containing drug , antibiotics , antibodies , neurotransmitters , psychoactive substances , local anesthetics , anti - inflammatory agents , spermicidal agents , imaging agents , phototherapeutic agents , dna , oligonucleotides and anti - sense oligonucleotides . an alternative method of producing a biodegradable drug delivery system is through the production of particles . the preferred size range is between 10 nm and 10 μm . these particles can be produced by emulsion polymerization in water containing a surfactant such as sodium dodecyl sulfate , an initiator such as ammonium persulphate , and cross - linking monomer and co - monomer ( s ) such as 2 - hydroxypropyl methacrylamide , 2 - hydroxyethylmethacrylate , acrylic acid , methacrylic acid , methyl methacrylate , methyl acrylate , or other suitable monomers by themselves or in mixtures . alternatively the particles can be synthesized by precipitation polymerization in organic solvent containing organic soluble initiator such as azobisisobutronitrile and co - monomer ( s ) such as acrylamide , as 2 - hydroxypropyl methacrylamide , 2 - hydroxyethyl methacrylate , acrylic acid , methacrylic acid , methyl methacrylate or methyl acrylate by themselves or in mixtures . in this method the preferred route of incorporating drug in the particles is by first synthesizing the particle , followed by purification through washing . the particle is then incubated with drug which is bound to the polymer network by either hydrophilic or ionic forces or by entrapment within the network . another method which is well known to those skilled in the art of producing polymer particles includes dissolving the cross - linking monomer , co - monomer , initiator with or with our drug in water and then dispersing this solution in oil . the resulting oil droplets then act as templates for the formation of the gel network . polymerization is initiated either thermally , chemically or photochemically depending on the monomer system and initiator system . which combination of systems to use will be obvious to those skilled in the art . the resulting particles can then be sedimented and isolated and purified . this technique is particularly useful for producing larger particles in the 5 - to 1000 micron in diameter size range . another preferred method for the creation of a drug delivery device is to create a homopolymer network of the cross - linker in organic solvent in the presence of a organic soluble drug . the network is then dried and contains drug dispersed within it . the highly cross - linked network will begin to erode when hydrated and release drug . in this application an important consideration is to copolymerize the biodegradable cross - linker with charged monomers ( either negative charges or positive charges or mixtures thereof ). very high charge densities in the polymer network can be obtained by copolymerization of charge monomers into networks (& gt ; 5 m ). the presence of charges in the polymer network require counterions for electroneutrality . these counterions bind water to a lesser or greater extent , depending on their size and polarizabilities . since the volume of the hydrated gel is equal to the volume of polymer , the volume of water bound to the polymer and the volume of the hydrated ions bound to the polymer , the presence of a large amount of hydrated ions can create a super - water adsorbent hydrogel . the molar ratio of cross - linker to other monomers should be kept as low as possible so as to not inhibit the swellability of the network , preferably in the range of 5 mol % or less . the preferred copolymers include methacrylic acid , acrylic acid , acrylic and methacrylic monomers containing sulfate , alkyl carboxylate , phosphate , amino , quaternary amino and other charged groups and their salts . in this application large batches of the degradable network will be synthesized either by dispersion polymerization or in bulk . the material could be synthesized in the presence of a suitable counterion such as sodium for negatively charged filaments or chloride for positively charged filaments . alternatively the polymer may be formed in its neutral state and then incubated with a suitable acid or base such as hydrochloride in the case of nitrogen containing co - monomers , and soluble metal hydroxides in the case of acidic co - monomers . the most preferred method is to polymerize the cross - linker with the salt form of the co - monomer . another use of the monomer is in temporarily binding two surfaces together . the biodegradable cross - linking monomer and co - monomer or just the biodegradable cross - linking monomer itself are mixed together with a solvent and an initiator by itself or with a co - catalyst . the mixture is then spread on the surfaces which are to be adhered , then polymerization is initiated by addition of heat or by light . in the case of light initiation at least one of the surfaces to be adhered must be transparent to the light beam in order for the polymer network to form . the initiation systems described above can be used to this end . such biodegradable adhesives should have many uses . there is a need for degradable polymers as cell scaffolds in tissue engineering . in this application the tissue scaffold would be synthesized under sterile conditions in a suitable biocompatible buffer . the cross - linking density should be controlled so as to obtain a pore size large enough to allow cell migration . pore size may be determined by scanning electron microscopy and by using macromolecular probes . a cell suspension containing cells such as , but not limited to , keratinocytes , chrondocytes and osteoblasts , would be injected into the polymer network along with suitable growth factors . the cells would then be allowed to grow within the network . as the cells grow the network around them would degrade . bioadhesive moieties such as rgd peptide sequence ( arg - gly - asp ) could be connected to matrix and thereby provide adhesive domains for the growing cells . the timing of the network degradation should coincide with the cells forming their own network ( artificial tissue ) through inter - cell contacts . the following examples are presented to describe preferred embodiments and utilities of this invention but are not intended to limit the use or scope of the methods , compositions or compounds claimed in this invention unless otherwise stated in the claims . taken together , these examples describe the best currently understood mode of synthesizing and incorporating these materials into polymer networks . the synthesis of the four members of the preferred class of molecules claimed herein are given in fig2 . this invention has several advantages over related inventions in this area , including : ( 1 ) the cross - linking agents are biodegradable to biocompatible substances , ( 2 ) the syntheses are both general and flexible , allowing for a variety of monomeric units to be incorporated , ( 3 ) the end groups ( e . g ., acrylate or hydrazide ) can be readily modified to accommodate either condensation or radical - type polymerizations . preparation of di ( s )- 1 -[ benzyloxycarbonyl ] ethyl butane - 1 , 4 - dioate ( bnlacsuc ) ( 3a ). 3a was prepared by reaction of benzyl ( s )-(−) lactate ( 27 . 0 g , 150 mmol ) with pyridine ( 15 . 2 ml , 188 mmol ), and succinyl chloride ( 8 . 21 ml , 75 . 0 mmol ) in dichloromethane ( 100 ml ) at 0 ° c . with subsequent stirring for 16 hours at 25 ° c . an additional aliquot of succinyl chloride ( 1 . 6 ml , 15 mmol ) was then added to ensure complete consumption of benzyl lactate . the reaction was allowed to stir 4 additional hours . after filtering the suspension through activated carbon , the dark solution was washed with 100 ml water , 2 - 50 ml portions of 1m hcl , 2 - 50 ml portions of 2 - 100 ml sat . nahco3 and 100 ml brine . the organic phase was then dried over na2so4 and concentrated in vacuo to a viscous brown oil . yield of 3a : 32 . 3 g ( 97 %). [ a ] d =- 43 . 2 ( c = 1 . 0 , chcl3 ). elution through a short column ( 8 . 5 cm i . d . by 4 cm ) of silica gel ( 70 - 230 mesh ) using 3 : 7 ethyl acetate / hexane resulted in a yellow oil of high purity by nmr . 1h nmr ( cdcl3 ): 1 . 49 ( d , 6h , j = 7 . 1 hz ), 2 . 65 - 2 . 72 ( m , 4h ), 5 . 08 - 5 . 21 ( m , 6h ), 7 . 29 - 7 . 34 ( m , 10h ). 13c nmr ( cdcl3 ): 16 . 63 , 28 . 47 , 66 . 76 , 68 . 68 , 76 . 49 , 77 . 52 , 127 . 91 , 128 . 21 , 128 . 40 , 135 . 13 , 170 . 30 , 171 . 34 . anal . calcd . for c24h26o8 : c , 65 . 15 ; h , 5 . 92 . found : c , 65 . 06 ; h , 6 . 02 . preparation of ( 2s )- 2 -{ 3 -[(( 1s )- 1 - carboxyethyl ) oxycarbonyl ] propanoyloxy } propanoic acid ( lacsuc ) ( 4a ). lacsuc was prepared by hydrogenolysis of bnlacsuc ( 3a ) ( 10 . 2 g , 23 . 1 mmol ) over pd / c ( 1 . 0 g , 10 % wt . pd , degussa type ) in 2 - propanol ( 100 ml ). the material was placed on a parr hydrogenator at 50 psi . at 25 ° c . when hydrogen uptake had ceased , the sample was removed from the hydrogenator , and the pd - c was then removed by filtration through celite . the solvent was removed in vacuo at 40 ° c . ( 16 hours ). the crude product was purified by crystallization of its dicyclohexylamine salt as follows : crude 5 ( 6 . 4 g , 23 mmol ) was dissolved in 50 ml of a toluene / ethyl acetate / ethanol ( 2 : 2 : 1 ) solvent mixture . dicyclohexylamine ( 9 . 2 ml , 46 mmol ) was added to the diacid solution at 0 ° c . crystallization was induced by cooling to − 10 ° c . and scratching the sides of the flask . the white solid was washed with 30 ml portions of ethyl ether . concentration of the mother liquor allowed isolation of a second crop . the first and second crop were combined to give a total yield of 30 . 2 g [ a ] d =− 26 . 9 , ( c = 1 . 0 , chcl3 ). the dicyclohexylamine salt was dissolved in 5 : 1 water / ethanol ( 10 ml ) and subjected to strong cation exchange chromatography ( biorad ag 50w - x4 , 200 - 400 mesh ) to regenerate the dicarboxylic acid form . the eluate was lyophilized to remove water / ethanol . the light yellow oil which resulted was taken up in 100 ml dichloromethane / ethyl acetate ( 5 : 1 ) and dried over na2so4 , to remove residual water . the organic solvents were removed in vacuo , and heating the viscous residue to 65 ° c . under vacuum ( 0 . 5 mm hg ) was required to induce crystallization of the diacid 4a . yield of 4a : 3 . 75 g ( 63 %): mp 59 - 61 ° c . ; [ a ] d =− 54 . 5 , ( c = 1 . 0 , chcl3 ): 1h nmr ( cdcl3 ): 1 . 54 ( d , 6h , j = 7 . 1 hz ), 2 . 72 - 2 . 77 ( m , 4h ), 5 . 13 ( q , 2h , j = 7 . 1 hz ), 10 . 97 ( br , 2h ). 13c nmr ( cdcl3 ): 16 . 56 , 28 . 51 , 68 . 40 , 171 . 62 , 176 . 28 . anal . calcd . for c10h14o8 : c , 45 . 81 ; h , 5 . 38 . found : c , 46 . 01 ; h , 5 . 55 . preparation of di ( 1s )- 1 -{[ 1 - methyl - 2 -( 2 - methylprop - 2 - enoylamino ) ethyl ] oxycarbonyl } ethyl butane - 1 , 4 - dioate ( hpmalacsuc ) ( 5a ). lacsuc ( 4a ) ( 2 . 20 g , 8 . 3 mmol ) was dissolved in dichloromethane ( 30 ml ) and cooled to 0 ° c . under an argon atmosphere in a three - necked flask equipped with a stir bar and a powder addition funnel . the reaction vessel was then charged with cdi ( 2 . 75 g , 17 . 0 mmol ) via the powder addition funnel . upon addition of the cdi the reaction frothed copiously . the reaction vessel was allowed to warm to 25 ° c ., and then hpma ( 2 . 57 g , 17 . 0 mmol ) was added . the reaction was stirred at 25 ° c . for 2 hours , and then washed with 1m nah2po4 ( 2 - 100 ml ), sat . na2co3 , ( 10 ml ) and brine ( 10 ml ). the dichloromethane phase was then dried over na2so4 and concentrated in vacuo to a light yellow , viscous oil . yield of 5a : 4 . 08 g ( 95 %). although the purity was & gt ; 90 % by tlc and nmr , the purity could be improved by flash chromatography . elution on 300 ml silica gel ( 230400 mesh ) using 3 % methanol / dichloromethane resulted in 3 . 22 g ( 75 %) of 5a : [ a ] d =− 21 . 3 , ( c = 1 . 0 chcl3 ) 1h nmr ( cdcl3 ): 1 . 24 - 1 . 29 ( m , 6h ), 1 . 47 - 1 . 51 ( m , 6h ), 1 . 96 ( s , 6h ), 2 . 70 - 2 . 74 ( m , 4h ), 3 . 20 - 3 . 38 ( m , 2h ), 3 . 57 - 3 . 72 ( m , 2h ), 4 . 87 - 5 . 00 ( m , 2h ), 5 . 03 - 5 . 16 ( m , 2h ), 5 . 33 - 5 . 36 ( m , 2h ), 5 . 71 - 5 . 75 ( m , 2h ), 6 . 25 - 6 . 55 ( m , 2h ). 13c nmr ( 62 . 9 mhz , dmso - d6 several peaks exhibited duality which maybe due to hindered rotation or diastereomers ): 16 . 53 , 17 . 21 , 17 . 37 , 18 . 55 , 28 . 20 , 42 . 99 , 54 . 88 , 68 . 74 , 70 . 17 , 70 . 22 , 119 . 11 , 139 . 83 , 139 . 87 , 167 . 68 , 167 . 83 , 169 . 72 , 169 . 89 , 171 . 27 , 171 . 35 . hrms ( fab +) calc for c24h27n2o10 ( m + h ) 513 . 2448 , found 513 . 2418 . all chemicals were reagent grade and were used without purification unless otherwise noted . 1h nmr and 13c nmr spectra were recorded at 400 and 100 . 4 mhz respectively on a varian inova - 400 spectrometer equipped with a temperature - controlled probe . abbreviations for nmr data are as follows : s = singlet , d = doublet , m - multiplet , dd = doublet of doublets , t = triplet . melting points are uncorrected . coupling constants ( j ) are reported in hertz . chemical shifts are reported in parts per million . 1h shifts are referenced to chcl3 ( 7 . 24 ) or to dmso ( 2 . 54 ) and 13c spectra are referenced to chcl3 ( 77 . 14 ) or to dmso ( 40 . 45 ). solvent mixtures are given in volume to volume ratios unless otherwise stated . flash chromatography was performed on sio2 kieselgel 60 ( 70 - 230 mesh e . merck ). mass spectroscopy was performed at the duke university mass spectrometry laboratory . optical rotations were obtained using the na + 589 nm line at in chcl3 or acetone using a perkin - elmer 241 polarimeter in a 1 dm cell . thf was used freshly distilled from sodium benzophenone ketyl under nitrogen . 2 - propanol was dried by distilling from cao and storing over 4a molecular sieves . dichloromethane was distilled from p205 and stored over molecular sieves . all other solvents were obtained in their anhydrous state or stored over molecular sieves before use . hydrogenations were performed on parr hydrogenator at 30 to 50 psi of hydrogen gas . preparation of di [ benzyloxycarbonyl ] methyl butane - 1 , 4 - dioate ( bnglysuc ) ( 3b ). compound 3b was synthesized by dissolving benzyl glycolate ( 15 . 0 g , 90 . 3 mmol ) and pyridine ( 7 . 9 ml , 97 mmol ) in 150 ml of ch2cl2 at 0 ° c . and adding succinyl chloride ( 4 . 7 ml 43 mmol ), via a syringe while stirring under an argon atmosphere . the reaction was allowed to warm to room temperature and stir for 3 hours . after 3 hours , tlc ( 5 : 95 methanol / chcl3 rf = 0 . 5 ) indicated almost complete reaction , and 0 . 5 ml of succinyl chloride was added . the reaction was allowed to stir for 12 more hours . the reaction was washed with 2 - 50 ml of saturated nahco3 followed by 2 - 50 ml 1m nah2po4 and then 1 - 50 ml of brine . the organic layer was dried over na2so4 . the crude brown solid was concentrated in vacuo . the compound was purified using flash chromatography on a 7 cm i . d . by 40 cm bed of sio2 eluting isocratically with chcl3 . alternatively , the solid could be purified by recrystallization from ( 1 : 1 ethyl acetate / hexane ). the pure fractions were combined and concentrated in vacuo to yield 3b as a white solid . yield of 3b : 14 . 6 g ( 82 %). 1h nmr ( cdcl3 ): 2 . 77 ( s , 4h ), 4 . 65 ( s , 4h ), 5 . 17 ( s , 2h1 ), 7 . 29 - 7 . 34 ( m , 10h ); 13c nmr ( cdcl3 ): 28 . 71 , 61 . 01 67 . 21 , 128 . 65 , 135 . 13 , 167 . 58 , 171 . 51 . anal . calcd . for c22h18o10 : c , 59 . 73 ; h , 4 . 10 found : c , 59 . 64 ; h , 4 . 25 . preparation of 2 -{ 3 -[( carboxymethyl ) oxycarbonyl ] propanoyloxy } acetic acid ( glysuc ) ( 4b ). compound 4b was prepared by dissolving 3b ( 5 . 0g , 11 . 3 mmol ) in 2 : 1 2 - propanol / ch2cl2 ( 150 ml ) in the presence of 500 mg of pd / c ( degussa type ). the reaction mixture was place on a parr hydrogenator at 50 psi for 5 hours , at which time uptake of hydrogen gas had stopped . the reaction was filtered through celite to remove the catalyst and the reaction was concentrated in vacuo resulting in a white solid . the solid was triturated with diethyl ether and dried further . attempts to further purify this material through the dicyclohexylamine salt resulted in low yields due to liability of this material in water . however , the nmr of the titurated product displayed no extraneous nmr resonances . yield of 4b 2 . 54 g ( 96 %): 1h nmr ( d6 - dmso ): 2 . 62 ( s , 4h ), 4 . 44 ( s , 4h ), 5 . 74 ( m , 4h ); 13c nmr ( d6 - dmso ): 28 . 40 , 60 . 61 , 169 . 32 , 171 . 30 . hrms ( fab +) calcd . for c8h10o8 ( m + h ) 233 . 0376 , found 233 . 0290 . preparation of di {[ 1 - methyl - 2 -( 2 - methylprop - 2 - enoylamino ) ethyl ] oxycarbonyl } methyl butane - 1 , 4 - dioate ( hpmaglysuc ) ( 5b ). the cross - linker hpmaglysuc was prepared by adding 4b ( 3 . 40 g , 14 . 5 mmol ) to a 100 ml three necked round bottomed flask under an argon atmosphere at 0 ° c . the reaction vessel was evacuated three times and dry dmf ( 25 ml ) was added to the vessel under pressure . cdi ( 4 . 71 g , 29 . 0 mmol ) was added rapidly via a powder addition funnel with vigorous stirring and was accompanied by copious frothing and the formation of the partially soluble diimidazolide . the slurry was allowed to warm to room temperature and hpma ( 1 ) ( 4 . 16 g 29 . 0 mmol ), dissolved in 10 ml of dmf , was added to the reaction through a syringe . the reaction was allowed to stir for 15 hours during which time the precipitate dissolved . tlc of the reaction mixture indicated complete conversion of the hpma ( 10 : 90 methanol / chcl3 rf 5b = 0 . 55 ). the reaction was diluted with ch2cl2 ( 300 ml ) and was washed with 1m nah2po4 , ( 2 - 75 ml ), nahco3 ( 2 - 75 ml ) and of brine ( 100 ml ). the organic layer was dried over na2so4 . the solvent was removed in vacuo ( t & lt ; 35 ° c .) to yield a light yellow oil . the material was purified by flash chromatography on a sio2 column ( 6 cm i . d . by 20 cm ) eluting with ch2cl2 followed by 2 - propanol / ch2cl2 . fractions containing pure product were combined and the solvent removed in vacuo ( t & lt ; 35 ° c .) to yield a colorless oil . yield of 5b : 5 . 83 g ( 83 %). 1h nmr ( cdcl3 ): 1 . 23 ( d , j = 6 . 4 hz , 6h ), 1 . 92 ( s , 6h ), 2 . 74 ( s , 4h ), 3 . 21 - 3 . 28 ( m , 2h ), 3 . 55 - 3 . 62 ( m , 2h ), 4 . 54 ( dd , 4h j1 = 10 hz j2 = 3 . 2 hz ), 5 . 01 - 5 . 12 ( m , 2h ), 5 . 31 ( d , j = 1 . 0 hz , 2h ), 5 . 67 ( d , 2h , j = 1h2 ), 6 . 15 - 6 . 25 ( m , 2h ); 13c nmr ( cdc1 3 ): 17 . 64 , 18 . 70 , 28 . 51 , 28 . 54 , 43 . 95 , 43 . 01 , 61 . 45 , 71 . 90 , 119 . 92 , 119 . 94 , 139 . 86 , 167 . 42 , 167 . 46 , 168 . 66 , 172 . 026 , 172 . 07 . hrms ( fab +) calcd for ( m + h ) c22h33n2o10 485 . 2057 found , 485 . 2123 . preparation of ( 1s )- 1 -[ benzyloxycarbonyl ] ethyl ( 2s )- 2 - hydroxy propionate ( bnlaclacoh ). bnlaclacoh was prepared by the acid catalyzed ring opening of l - lactide . a 250 ml round bottomed flask was charged with l - lactide ( 15 . 0 g , 104 mmol ) benzyl alcohol ( 12 . 4 g , 114 mmol ) and camphor sulfonic acid ( 139 mg , 624 μmol ) along with dry benzene ( 100 ml ). the reaction was refluxed under argon for 36 hours . tlc indicated that the reaction had consumed most of the i - lactide ( thf / hexanes / etoh 45 : 45 : 10 rf lactide = 0 . 1 ( phosphomolybdic acid stain )). the reaction was washed with of 200 mm nahco3 ( 2 - 50 ml ), dried over na2so4 and the solvent was removed in vacuo . the resultant clear oil was fractionally distilled under high vacuum ( 30 mtorr ) using a vacuum - jacketed vigreux column . the product was collected in a fraction between 108 and 115 ° c . yield of bnlaclacoh : 19 . 8 g ( 69 %) 1h nmr ( cdcl3 ) 1 . 40 ( d , 3h j = 6 . 8 hz ), 1 . 49 ( d , 3h j = 3 . 8 hz ), 3 . 00 ( br , 1h ), 4 . 284 . 38 ( q , 2h j = 6 . 8 hz ), 5 . 10 - 5 . 23 ( m , 2h ), 7 . 30 - 7 . 4 ( m 5h ) 13c nmr ( cdcl3 ): 16 . 79 , 20 . 33 , 66 . 68 , 66 . 82 , 67 . 19 , 69 . 26 , 128 . 20 , 128 . 48 , 128 . 59 , 135 . 05 , 170 . 09 , 175 . 00 . hrms ( fab +) calcd c13h16o5 ( m + h ) 253 . 0998 found 253 . 1066 . preparation of ( 1s )- 1 -({( 1s )- 1 [ benzyloxycarbonyl ] ethyl } oxy carbonyl ) ethyl ( 1s )- 1 -({( 1s )- 1 -[ benzyloxycarbonyl ] ethyl } oxy carbonyl ) ethyl butane - 1 , 4 - dioate . bnlaclacoh ( 4 . 00 g , 15 . 9 mmol ), pyridine ( 1 . 32 ml , 16 . 4 mmol ) was dissolved in dichloromethane ( 50 ml ) and cooled to 0 ° c . under a n2 atmosphere . to this mixture was added succinyl chloride ( 0 . 90 ml , 8 . 2 mmol ) over a period of 20 minutes . the reaction vessel was allowed to warm to 25 ° c ., and was stirred for 3 hours . tlc indicated the reaction had nearly reached completion and an additional aliquot of succinyl chloride was added ( 0 . 5 ml , 4 . 5 mmol ). the reaction was stirred for 1 hour more . the reaction was diluted with of ch2cl2 , ( 50 ml ) and poured into water , and washed with 2n hcl ( 2 - 50 ml ), water ( 2 - 50 ml ), 2 m nahco3 , ( 100 ml ) and brine ( 50 ml ). the ch2cl2 phase was then dried over na2so4 , and concentrated in vacuo to a bronze - colored oil . yield of bnlaclacoh : 35 . 8 g ( 85 %). an analytically pure sample of 9 was prepared by flash chromatography on silica gel ( 230 - 400 mesh ) using 30 : 70 ethyl acetate hexane . [ a ] d =− 71 . 60 , ( c = 1 . 0 , chcl3 ); 1h nmr ( cdcl3 ): 1 . 48 ( d , 6h , j = 7 . 0 hz ), 1 . 50 ( d , 6h , j = 7 . 1 hz ), 2 . 60 - 2 . 70 ( m , 4h ), 5 . 06 - 5 . 24 ( m , 8h ), 7 . 28 - 7 . 36 ( m , 1h ). 13c nmr ( cdcl3 ,): 16 . 38 , 16 . 50 , 28 . 36 , 66 . 86 , 68 . 36 , 68 . 87 , 127 . 98 , 128 . 24 , 128 . 37 , 134 . 92 , 169 . 74 , 169 . 83 , 171 . 29 . preparation of ( 1s )- 1 -({( 1s )- 1 [ benzyloxycarbonyl ] ethyl } oxy carbonyl ) ethyl ( 1s )- 1 -({( 1s )- 1 -[ benzyloxycarbonyl ] ethyl } oxycarbonyl ) ethyl butane - 1 , 4 - dioate ( bnlaclacsuc ). lacsuc ( 4a ) ( 19 . 1 g , 73 . 0 mmol ) was dissolved in ch2cl2 ( 75 ml ) and cooled to 0 ° c . under n2 atmosphere . cdi ( 26 . 0 g , 161 mmol ) was then added to the reaction vessel . much bubbling of co2 gas was observed . the reaction vessel was allowed to warm to 25 ° c ., and then benzyl ( s )-(−)- lactate ( 25 . 7 g , 143 mmol ) was added . the reaction was stirred at 25 ° c . for 1 hour , and then washed with 2n hci ( 2 - 100 ml ), water ( 100 ml ), 10 % nahco3 ( 2 - 100 ml ), and brine ( 100 ml ). the ch2cl2 phase was then dried over mgso4 , and concentrated in vacuo to a bronze - colored oil . yield of bnlaclacsuc : 35 . 8 g ( 85 %). an analytically pure sample of bnlaclacsuc was prepared by flash chromatography on silica gel ( 230 - 400 mesh ) using 30 : 70 ethyl acetate / hexane . [ a ] d =− 71 . 4 , ( c = 1 . 0 chcl3 ) 1h nmr ( 250 mhz , cdcl3 ): 1 . 48 ( d , 6h , j = 7 . 0 hz ), 1 . 50 ( d , 6h , j = 7 . 1 hz ), 2 . 60 - 2 . 70 ( m , 4h ), 5 . 06 - 5 . 24 ( m , 8h ), 7 . 28 - 7 . 36 ( m , 1h ). 13c nmr ( 62 . 9 mhz , cdcl3 ): 16 . 38 , 16 . 50 , 28 . 36 , 66 . 86 , 68 . 36 , 68 . 87 , 127 . 98 , 128 . 24 , 128 . 37 , 134 . 92 , 169 . 74 , 169 . 83 , 171 . 29 . anal . calcd . for c30h36o12 : c , 61 . 43 ; h , 5 . 84 . found : c , 61 . 47 ; h , 6 . 01 . preparation of laclacsuc ( 9a ): laclacsuc was prepared by hydrogenation of bnlaclacsuc ( 9a ) ( 35 . 8 g , 60 . 8 mmol ) over 12 . 9g pd - c ( 10 % wt . pd , degussa type ; 6 . 08 mmol pd ) in 100 ml 2 - propanol / ethyl acetate ( 2 : 1 ). positive hydrogen pressure was maintained using a gas dispersion tube for 4 hours at 25 ° c . and then under a balloon of hydrogen for 2 days . the pd - c was then removed by filtration , and the solvent was removed in vacuo . the crude product was purified by crystallization of its dicyclohexylamine salt as follows : dicyclohexylamine ( 24 . 2 ml , 122 mmol ) was added to the crude diacid dissolved in 200 ml 50 % ethyl acetate / hexane at 25 ° c . crystallization was induced by cooling to − 78 ° c . for 16 hours . the white solid was washed with 30 ml portions of 50 % ethyl acetate / hexane . concentration of the mother liquor allowed isolation of a second crop . the first and second crop were combined to give a total yield of 21 . 4 g ([ a ] d = 42 . 5 , c = 1 . 0 , chcl3 ). the dicyclohexylamine salt was dissolved in 25 ml water / ethanol ( 4 : 1 ) and subjected to strong cation exchange chromatography ( dowex 5ox4 - 400 ) to regenerate the dicarboxylic acid form . the fractions containing the pure diacid were saturated with nacl and extracted with 3 - 100 ml portions of ethyl acetate . the combined organic phases were dried over mgso4 , and concentrated in vacuo . the light yellow , viscous oil was then heated to 65 ° c . under vacuum ( 0 . 5 mm hg ) to remove residual solvent . yield of 6a laclacsuc : 9 . 31 g ( 38 %). ([ a ] d =− 86 . 2 , c = 1 . 0 , chcl3 ) 1h nmr ( 250 mhz , cdcl3 ): 1 . 55 ( d , 6h , j = 7 . 1 hz ); 1 . 56 ( d , 6h , j = 7 . 1 hz ); 2 . 70 - 2 . 80 ( m , 4h ); 5 . 09 - 5 . 22 ( m , 4h ); 11 . 07 ( b , 2h ). 13c nmr ( 62 . 9 mhz , cdcl3 ): 16 . 44 , 28 . 42 , 68 . 50 , 170 . 00 , 171 . 61 , 175 . 75 . preparation of hpmalaclacsuc ( 7a ). laclacsuc ( 2 . 01 g , 4 . 92 mmol ) was dissolved in 10 ml dichloromethane and cooled to 0 ° c . under n2 atmosphere . the reaction vessel was then charged with carbonyldiimidazole ( 1 . 78 g , 11 . 0 mmol ). much bubbling of c02 gas was observed . the reaction vessel was allowed to warm to 25 ° c ., and then hpma ( 1 . 43 g , 10 . 0 mmol ) was added . the reaction was stirred at 25 ° c . for 2 hours , and then washed with 3 - 10 ml portions of 5 % citric acid solution , 10 ml water , 10 ml 10 % nahco3 , and 10 ml brine . the dichloromethane phase was then dried over mgso4 , and concentrated in vacuo to a light yellow , viscous oil . yield of 7a : 2 . 55 g ( 79 %). although the purity was & gt ; 90 % by tlc and nmr , the purity could be improved by flash chromatography . elution on 400 ml silica gel ( 230 - 400 mesh ) using 3 % methanol / dichloromethane resulted in 2 . 20g ( 68 %) of hpmalaclacsuc ( 10a ). ([ a ] d =− 24 . 9 , c = 1 . 0 , chcl3 ) 1h nmr ( 250 mhz , cdcl3 ,): 1 . 23 ( d , 6h , j = 6 . 4 hz ); 1 . 39 - 1 . 54 ( m , 12h ); 1 . 92 - 1 . 93 ( m , 6h ); 2 . 63 - 2 . 78 ( m , 4h ); 3 . 18 - 3 . 37 ( m , 2h ); 3 . 55 - 3 . 67 ( m , 2h ); 4 . 91 - 5 . 11 ( m , 6h ); 5 . 29 - 5 . 32 ( m , 2h ); 5 . 68 ( d , 2h , j = 9 . 5 hz ); 6 . 28 - 6 . 33 ( m , 2h ); 13c nmr ( 62 . 9 mhz , cdcl3 ; several peaks exhibited duality which is due to diastereomers ): 16 . 23 , 16 . 44 , 16 . 50 , 17 . 11 , 17 . 19 , 18 . 31 , 28 . 30 , 43 . 41 , 43 . 63 , 68 . 34 , 68 - 51 , 69 . 58 , 71 . 24 , 71 . 36 , 119 . 47 , 119 . 60 , 139 . 40 , 139 . 49 , 168 . 20 , 168 . 38 , 169 . 46 , 169 . 89 , 170 . 16 , 170 . 45 , 171 . 26 . ; hrms ( fab +) calcd mh + c30h44n2o14 657 . 2839 , found 657 . 2849 . preparation of di ({[ benzyloxycarbonyl ] methyl } oxycarbonyl ) methyl butane - 1 , 4 - dioate ( bnglyglysuc ). glysuc ( 4b ) ( 3 . 50g 14 . 95 mmol ) was dissolved in ch2cl2 ( 30 ml ) and anhydrous dmf ( 60 ml ) and cooled to 0 ° c . under argon atmosphere in a three necked flask equipped with a stir bar and a powder addition funnel . the reaction vessel was then charged with cdi ( 4 . 85 g , 30 . 0 mmol ) via a powder addition funnel . upon the addition of the cdi the reaction frothed copiously . the insoluble diimidazolide formed a thick precipitate . the reaction vessel was allowed to warm to 25 ° c ., and then benzyl glycolate ( 3 . 82 ml , 30 . 0 mmol ) was added via a syringe in anhydrous dmf ( 10 ml ). the reaction was allowed to run overnight at 25 ° c . as the reaction proceeded , the reaction mixture slowly became less viscous . the reaction was diluted with ch2cl2 ( 500 ml ) and was washed with 1m nah2po4 ( 2 - 100 ml ), nahc3o3 ( 2 - 100 ml ), and brine ( 100 ml ). the organic layer was dried over na2so4 and the solvent was removed in vacuo yield of 4b : 7 . 67 g 98 % ( a light yellow crystalline solid ). the compound was purified by flash chromatography on a 4 . 5 cm i . d . by 12 cm column over silica gel . the sample was loaded in 2 : 1 ch2cl2 / hexanes ( 100 ml ) eluted with of the same ( 200 ml ), of ch2cl2 ( 200 ml ), of 1 : 99 thf / ch2cl2 ( 200 ml ), and finally with thf / ch2cl2 ( 3 : 97 ). the fractions containing pure product were combined and the solvent removed in vacuo to yield 26 as a pure crystalline solid . yield of 4b : 6 . 57 g ( 83 %). 1h nmr ( mhz , cdcl3 ): 2 . 78 ( s , 4h ), 4 . 71 ( s , 4h ), 4 . 73 ( s , 4h ), 5 . 18 ( s , 4h ), 7 . 33 - 7 . 39 ( m , 10h ); 13c nmr cdcl3 ) 28 . 65 , 60 . 66 , 61 . 21 , 67 . 42 , 128 . 55 , 128 . 74 , 128 . 78 , 135 . 00 , 167 . 03 , 167 . 22 , 171 . 40 ; anal . calcd . for c26h22o14 c , 55 . 92 ; h , 3 . 97 ; found : c , 55 . 64 ; h , 4 . 01 . preparation of 2 -{ 2 -[ 3 -({[ carboxymethyl ) oxycarbonyl ] methyl } oxy carbonyl ) propanoyloxy ] acetyloxy } acetic acid ( 6b ). compound 6b was prepared by suspending 4b ( 4 . 58 g , 8 . 61 mmol ) in 1 : 1 2 - propanol / ch2cl2 ( 250 ml ) in the presence of pd / c ( 2 . 0 g , degussa type ). the reaction mixture was placed on a parr hydrogenator at 50 psi for 18 hours at which time uptake of hydrogen gas had stopped . the reaction was filtered through celite to remove the catalyst , and the solution was concentrated in vacuo , resulting in a white solid . the solid was titurated with diethyl ether , and dried further yielding a white solid . attempts at purification by recrystallization of the dicyclohexylamine salt resulted in complex mixtures upon trying to remove the amine by semi - aqueous ion exchange . this was likely due to the instability of this compound . however the nmr of the titurated product was adequate with a purity & gt ; 95 %. yield of 6b : 2 . 87 g ( 93 %): 1h nmr ( d6 - dmso ): 2 . 80 ( s , 4h ), 4 . 66 ( s , 4h ), 4 . 77 ( s , 4h ), 13c nmr ( d6 - dmso ): 29 . 31 , 61 . 34 , 61 . 91 , 167 . 92 , 169 . 57 , 171 . 97 . hrms ( fab ) calcd for ( m − h ) c12h13o12 , 349 . 0485 found 349 . 0403 . preparation of di [({[ 1 - methyl - 2 -( 2 - methylprop - 2 - enoylamino ) ethyl ] oxycarbon - yl } methyl ) oxycarbonyl ] methylbutane - 1 , 4 - dioate ( hpmaglyglysuc ) ( 7b ). the cross - linker hpmaglyglysuc was prepared by adding 6b ( 1 . 50 g , 4 . 28 mmol ) to 100 ml three - necked round bottomed flask under an argon atmosphere at 0 ° c . the reaction vessel was evacuated three times and dry 1 : 1 dmf / ch2cl2 ( 35 ml ) was added to the vessel under pressure . the cdi ( 1 . 39 g , 8 . 57 mmol ) was added rapidly with vigorous stirring via a powder addition funnel and was accompanied by frothing and the formation of the insoluble diimidazolide . the slurry was allowed to warm to room temperature and hpma ( 1 . 23g , 8 . 57 mmol ) dissolved in dmf ( 10 ml ) was added to the reaction through a syringe . the reaction was allowed to stir for 10 hours during which time the precipitate dissolved . tlc of the reaction mixture indicated complete conversion of the hpma ( 10 : 90 methanol / chcl3 rf 5b = 0 . 73 ). the reaction was diluted with ch2cl2 ( 200 ml ) and was washed with 1m nah2po4 ( 2 - 50 ml ), nahco3 ( 2 - 50 ml ) and brine ( 100 ml ). the organic layer was dried over na2so4 . the solvent was removed in vacuo ( t & lt ; 35 ° c .) to yield a clear oil . the material was purified by flash chromatography on a 6 cm i . d . by 20 cm silica gel column eluting with ch2cl2 followed by 3 : 97 2 - propanol / ch2cl2 . pure fractions were combined and the solvent removed in vacuo ( t & lt ; 35 ° c .) to yield a colorless oil . yield of 7 : 2 . 08g ( 81 %): 1h nmr cdcl3 ): 1 . 23 ( d , 6h j = 6 . 3 hz ), 1 . 89 ( s , 6h ), 2 . 73 ( s , 4h ), 3 . 21 - 3 . 30 ( m , 2h ), 3 . 53 - 3 . 62 ( m , 2h ), 4 . 524 . 8 ( m , 8h ), 5 . 01 - 5 . 17 ( m , 2h ), 5 . 28 ( s , 4h ), 5 . 64 ( s , 4h ), 6 . 23 - 6 . 35 ( b , 2h ), 13c nmr ° cdcl3 ): 17 . 45 , 18 . 62 , 43 . 80 , 60 . 64 , 61 . 54 , 72 . 12 , 119 . 74 , 139 . 84 , 166 . 75 , 167 . 49 , 168 . 67 , 171 . 50 . hrms ( fab +) calcd for c26h36n2o14 ( m + h ) 601 . 2167 found 601 . 2219 . biodegradable hydrogels are synthesized by free radical polymerization of the biodegradable cross - linkers and other monomers described herein using the aps / tmed couple . the vinyl groups on the terminus of the cross - linking structure can be used to form a gel network structure . gels were synthesized using the ammonium persulphate ( aps ) n , n , n ′, n ′- tetramethylethylenediamine ( tmed ) couple as the free - radical initiator system . this system proved very useful in the synthesis of clear isotropic gels , without having to degas the polymerization reactions . the gels in this section were made at a mole feed ratio of 1 . 5 mole % cross - linker , as a copolymer with 98 . 5 mole % hpma . before the gels were polymerized , three 1 . 0 ml plastic syringes to be used as a slab gel template were silylanized by briefly incubating them in a heptane solution containing sigmacote and oven drying at 90 ° c . also , three 8 cm lengths of 25 gauge tungsten wire were silylanized for use in the gel making process and each was threaded through 7 mm suba seal rubber septa . the procedure to form gels was as follows : a 7 ml test tube was charged with hpma ( 2 . 115 g , 14 . 8 mmol [ hpma ] final ˜ 5 m ), the oily compound 5a ( hpmaglysuc ) was adsorbed to the end of a tarred spatula ( 109 . 0 mg , 0 . 225 mmol , [ xl ] final = 0 . 075 m ). the end of the spatula was placed in the test tube and 1 . 5 ml of di water was added to the mixture . the cross - linker was dissolved in the mixture by rapid rotation of the spatula and gentle bath sonication . the dissolution of the hpma has a negative heat of solution but the mixture should not be warmed above room temperature . to this solution was added a solution of aps in water ( 99 mg , 0 . 438 mmol , 166 μl of a 2 . 63 m solution , [ aps ] final = 0 . 143 m ). this was again agitated until homogeneous . to this mixture was added tmed to initiate the polymerization ( 49 mg , 0 . 429 mmol , 204 μl of a 2 . 10 m solution of tmed adjusted to ph 7 with hcl ). in this preferred embodiment the concentration of tmed must be approximately 0 . 15 m or greater . it was important to control the ph of the tmed because tmed solutions in water are basic enough to cause significant degradation of the hydrolytically reactive cross - linker . immediately after the tmed was added the mixture of monomers and aps was vigorously mixed on a vortexer for 15 seconds and then drawn into the 1 . 0 ml plastic syringes by plunger aspiration . the syringe acts as a mold for gel formation . the syringes were inverted and the tungsten wires were inserted into the gel through the opening so that it runs through the center of the forming 1 . 0 ml gel cylinder . the wire was held in place by a septa which was placed over the tip of the syringe , as the solution polymerized . this formed a hole in the center of the cylinder , which was later used as a place to insert a wire hanger for the initially brittle and finally fragile gel , in order to measure its swelling and degradation kinetics as a change of mass with time . gelation occurred within one to five minutes and the syringe was allowed to sit for 4 hours at room temperature . at this point the wire was removed from the center of the solid and the end of the plastic syringe was removed with a razor blade . the plunger was then used to extrude the gel from the syringe in 100 μl increments which were cut into small cylinders as they hung out from the end of the syringe . the gels were then placed on tarred wire holders and the initial mass of the assembly was determined . the resulting clear isotropic gels had the composition of poly ( hpma - co - hpmaglysuc ) 98 . 5 : 1 . 5 . the gels were then incubated in ph 5 , 100 mm sodium acetate buffer for 24 hours . they were then charged into vials of differing ph to study the degradation kinetics . the gels of the four different compositions contained the following amounts of cross - linkers : compound mw mole % xl moles mass ( mg ) hpmasuc 366 . 38 0 . 015 2 . 25e - 04 82 . 4 hpmalacsuc 512 . 5 0 . 015 2 . 25e - 04 115 . 3 hpmaglysuc 484 . 51 0 . 015 2 . 25e - 04 109 . 0 hpmaglyglysuc 600 . 58 0 . 015 2 . 25e - 04 135 . 1 to a 10 ml round bottomed flask was charged hpmalaclacsuc ( 60 . 75 mg , 125 mol ) and azobisisobutryonitrile ( free radical initiator ) ( 4 . 0 mg ). to this was added 1 . 0 ml of a 1 : 1 methanol : water mixture . the contents were dissolved and degassed under n2 for 0 . 5 hours followed by 5 minutes in a bath sonicator under a stream of n2 . the mixture was charged in 300 ml aliquots into 36 × 50 mm glass tubes which have been evacuated and capped with rubber septa . the tubes were placed in a 60 ° c . bath overnight . the next morning the polymer gel had formed . it was removed from the glass tube under vacuum and cut into 2 × 5 mm disks . the disks were incubated in water for two days . the water was changed at 8 hour intervals to remove any water soluble monomer or reaction byproducts . the gels were placed in 15 ml vials containing 10 ml of buffers . the masses of wire holders were determined before the gels were placed on them . the original mass of the gel in its relaxed state was also known by subtraction from the total mass of the assembly . the original dry mass of the gel was determined by drying three gels in their relaxed state from each composition and determining the dry mass of the gel . these values were then used to calculate the inverse of the volume fraction of polymer in the gel ( qv ) respectively , using the densities of the polymer and water . the incubation solutions were changed each time the gel was weighed . the gels were incubated in a gyratory water - bath shaker ( new brunswick scientific , new brunswick , n . j .). the temperature was regulated to be 37 ± 2 ° c . and the shaker was set to 30 rotations per minute . hydrogels are cross - linked structures composed of elastic networks of water - soluble polymers . the maximum degree of swelling is limited by the network elasticity . so as the gel &# 39 ; s network structure degrades the cross - link density decreases and the network becomes more elastic . this allows the network to swell further as it imbibes more water . this swelling results in a increase in the volume fraction of water and a corresponding decrease in the volume fraction of polymer . the property of the change of volume of the polymer network can be measured by weighing the gel at different time points . since the swelling is related to network cross - link density by weighing the macroscopic gels at various times throughout their swelling one can obtain information about the change in cross - link density and thus the rate of degradation . the cross - linkers in the gel degrade hydrolytically by the action of the two hydrolytically active components of water : the hydronium and hydroxide ions . therefore , the rate of degradation is strongly dependent on ph . hpmalacsuc is electronically similar to hpmaglysuc 5b yet the lactic ester shows slower degradation than the glycolic . this is because the lactic ester has a methyl group to the carbonyl where the first step of ester hydrolysis takes place , and is sterically hindered in 5a than 5b . hpmaglyglysuc 7b shows the fastest hydrolysis and swelling kinetics with complete degradation after about 5 days ( see fig4 through 6 ). moreover , since we are not measuring the rate of hydrolysis of individual bonds but measuring the swelling which comes about as a result of cleaving the connection between two polymer chains , the concentration of cleavable sites comes into play . therefore , when comparing gels composed of 5b and 7b at the same cross - link density , compound 5b has four potential sites of cleavage and 7b has six . this increased concentration of cleavable sites may result in a difference in swelling rate depending on the relative microscopic rate constants for hydrolysis of the different bonds making up the cross - linker . preparation of the control non - degradable cross - linker bis - 1 - methyl - 2 -( 2 - methylprop - 2 - enoylamino ) ethyl - 1 , 4 - butanedioate ( hpmasuc ) ( 2 ). to a solution of hpma ( 4 . 00 g , 27 . 9 mmol ), dmap ( 340 mg , 2 . 8 mmol ) and na2co3 ( 3 . 26 g , 30 . 7 mmol ) in ch2cl2 ( 100 ml ) at 0 ° c . was added succinyl chloride ( 1 . 54 ml , 13 . 97 mmol ) dropwise . the reaction was allowed to warm to 25 ° c . and stir for 8 hours at which time another aliquot of succinyl chloride was added ( 0 . 61g , 4 mmol ). the reaction was allowed to stir for another 4 hours . the reaction mixture was poured into 50 ml of water and filtered through activated carbon . the mixture was then washed with 1m nah2po4 ( 50 ml ), sat . nahco3 ( 50 ml ) and brine ( 100 ml ). the organic phase was then dried over na2so4 , and concentrated in vacuo to a tan residue . this was purified by flash chromatography in 15 : 85 2 - propanol / chcl3 on a 3 i . d . by 20 cm column . yield of 2 3 . 51g ( 68 %): mp 103 - 105 ° c ., 1h nmr ° cdcl3 ): 1 . 24 ( d , 6h , j = 6 . 3 hz ); 1 . 93 ( d , 6h , j = 0 . 6 hz ), 2 . 54 - 2 . 65 ( m , 4h ), 3 . 31 - 3 . 38 ( m , 2h ), 3 . 52 - 3 . 60 ( m , 2h ), 4 . 98 - 5 . 05 ( m , 2h ), 5 . 31 ( d , 2h , j = 0 . 4 hz ), 5 . 66 ( s , 2h ), 6 . 23 ( b , 2h ). 13c nmr ° cdcl3 ); ( several peaks exhibited duality which is most likely due to diastereomers ) — 17 . 48 , 18 . 51 , 29 . 38 , 43 . 81 , 43 . 87 , 70 . 48 , 70 . 62 , 119 . 50 , 119 . 56 , 139 . 78 , 168 . 48 , 172 . 24 . anal . calcd . for c18h28n2o6 : c , 58 . 68 ; h , 7 . 65 ; n , 7 . 60 . found : c , 58 . 71 ; h , 7 . 72 ; n , 7 . 48 . release of a soluble macromolecule from a degrading network and degradation of a polymer network labeled with a chromophoric agent . gels were formed by the same method as above , but in this case other compounds were included during the preparation of the gels to study the release of small molecules from the network . in one case , the network itself was labeled with a polymerizable derivative of tetramethyl rhodamine ( tmrahmam ) in order to show the release ( degradation ) of the network itself ( see fig6 ). in the other case fluorescent rhodamine labeled albumin ( molecular probes , eugene oreg .) was included in the uncharged network to show diffusive release of a macromolecule from the network ( see fig7 ). to a 3 ml test tube was charged tmrahmam ( 3 . 0 mg , 5 . 0 μmol ; 30 μl of a 100 mg / ml solution in chcl3 ) which was then placed under a 7 mtorr vacuum for 3 hours . to another 3 ml test tube , 4 . 5 mg of 5 +( 6 ) carboxytetramethylrhodamine labeled albumin ( molecular probes ) was added . to a third 7 . 0 ml test tube hpma was added ( 1 . 692 g , 11 . 8 mmol , [ hpma ] final ˜ 5 m ), the oily compound 7b ( hpmaglyglysuc ) was adsorbed to the end of a tarred spatula ( 108 . 0 mg , 180 μmol , [ xl ] final = 0 . 075 m ). the end of the spatula was placed in the test tube and 1 . 2 ml of di water was added to the mixture . the cross - linker was dissolved in the mixture by rapid rotation of the spatula and gentle bath sonication . hpma has a negative heat of solution but the mixture should not be warmed above room temperature . to this solution was added a solution of aps in water ( 80 mg , 0 . 350 mmol , 79 μl of a 2 . 63 m , [ aps ] final = 0 . 143 m ). this was again agitated until homogeneous . this viscous mixture was separated into 3 - 890 μl aliquots . one was mixed with the polymerizable dye ( tmrahmam ) and the other with fluorescent albumin . all resulting monomer mixtures were thoroughly homogenized . to each of these three 890 μl mixtures was added an aliquot of tmed to initiate the polymerization ( 13 . 2 mg , 114 μmol , 54 μl of a 2 . 10 m solution of tmed adjusted to ph 7 with hci , 244 mg tmed freebase / ml ). all mixtures were mixed for 15 seconds and then placed in the syringe template , with each solution having a final solid volume of about 850 μl . the gels were then allowed to polymerize for 4 hours after which time they were extruded and cut into slices . the gels were weighed , attached to wires and were placed in separate vials for the release studies . the dye labeled gels were incubated in water for two days to allow any unreacted monomer to diffuse out of the network . the gels were placed in 15 ml of buffer solutions at ph 4 , 7 , and 9 . all solutions were incubated at 37 ° c . on a temperature - regulated orbital - shaking bath at 30 rpm . gels were suspended in buffers of different ph &# 39 ; s . all buffers were adjusted to the same ionic strength . the release of the rhodamine labeled albumin and the rhodamine labeled hpma was monitored at 550 nm . 750 μl of the sample was removed from the vial and periodically measured on a spectrophotometer . release values were normalized to the maximum amount released . the examination of the release of macromolecules entrained in the polymer network provides another way to study the performance of these materials . in this section , the degradation of the network is ascertained by analysis spectrophotometrically through the release of the hpma polymer backbone itself by labeling it with the polymerizable dye tmrahmam . moreover , the release of a model macromolecular solute ( tmra - albumin , molecular weight of ˜ 66 , 000 da .) from the network is measured spectrophotometrically . [ 0140 ] fig6 displays a photograph of three different gels in ph 7 . 3 buffer made with hpmasuc , hpmaglyglysuc ( 4 days ) and hpmaglyglysuc ( 8 days ), which were co - polymerized with the chromophoric label and hpma . fig6 displays not only the different degrees of swelling but also the release of rhodamine labeled hpma into the solution at a given time versus control . fig7 shows the release curve for rhodamine labeled hpma polymer backbone as well as the corresponding swelling data . the release of rhodamine labeled hpma largely occurs to the greatest extent at the onset of complete degradation of the polymer . in contrast to the release of the polymer backbone , the release of the globular macromolecule bs albumin more closely follows the swelling of the network . the gels in this section were made at a mole feed ratio of 1 . 45 mole percent cross - linker as a copolymer with hpma 95 . 4 mole % and methacrylic acid sodium salt 3 . 18 mol %. before the gels were polymerized , three 1 . 0 ml plastic syringes to be used as a slab gel template were silylanized with sigmacote by briefly incubating them in the heptane solution and oven drying at 90 ° c . ( see 1 - 1 ). also , three 8 cm lengths of 25 gauge tungsten wire . the procedure to form gels is as follows : to a 7 ml test tube is charged hpma ( 564 . 1 mg , 0 . 00394 mol , [ hpma ] final ˜ 5 m ), the cross - linker ( 6 . 00e - 05 mol , [ xl ] final = ˜ 0 . 075m ) and the sodium salt of methacrylic acid ( 42 mg 3 . 89e - 04 mol ) are charged into the same vial with the hpma and 0 . 4 ml of di water is added to the mixture . the components of the mixture are dissolved by agitation and gentle bath sonication at 15 ° c . the dissolution of the hpma is retarded by its negative heat of solution but the mixture should not be warmed above room temperature . to this solution is added 44 . 4 μl of a 2 . 63 m solution of ( aps ) in water ( 27 mg , 1 . 17e - 04 mol , [ aps ] final = 0 . 143m ). this is again agitated until homogeneous . to this mixture is added 55 μl of a 2 . 10 m solution of tmed to initiate the polymerization ( 13 . 3 mg , 1 . 14e - 04 mol , tmed solution adjusted to ph 7 ). it is important to control the ph of the tmed because tmed solutions in water are basic enough to cause significant degradation of the hydrolytically reactive cross - linker . immediately after the tmed is added the mixture of monomers and aps is vigorously mixed on a vortexer for 15 seconds and then drawn into the 1 . 0 ml plastic syringes by plunger aspiration which acts as a mold for the forming gel ( see above for a description of gel processing ). for gels of the three different compositions synthesized the following amounts of cross - linkers were used in addition to the materials described above . compound mw mole fraction moles mass ( mg ) hpmasuc 366 . 38 0 . 0136 0 . 00006 22 hpmaglysuc 484 . 51 0 . 0136 0 . 00006 29 hpmaglyglysuc 600 . 58 0 . 0136 0 . 00006 36 the gels were cut into approximately equal volumes ( 100 μl , ˜ 100 mg ) and the unloaded masses were determined for the gels in the rubbery state . each gel was placed in 1 . 9 ml of a 2 . 0 mg / ml solution of doxorubicin hydrochloride which was buffered to ph 7 . 4 with 5 mm tris buffer . the gels were agitated with the solution for 4 days at room temperature on a temperature - regulated orbital - shaking bath at 30 rpm . as the red doxorubicin was taken up into the gels the gels became red . the solution around the gels became depleted of doxorubicin due to the ion exchange of doxorubicin for the sodium counterions . as discussed earlier the preferred embodiment of a network polymer for use as a degradable water absorbent will include ionomeric monomers which bring ions and water into the gel network . below is a description of the method to make a highly charged gel of these cross - linkers . to a 5 ml test tube was charged acrylic acid ( 675 μl , 9 . 85 mmol ), water ( 1120 μl ) and hpmaglysuc ( 73 mg , 0 . 15 mmol ) ( 5b ). the cross - linker was weighed into the mixture as described earlier . the mixture was homogenized and aps was added ( 66 . 6 μl , 0 . 175 mmol ) from a 2 . 43 m solution in water . this solution was again mixed . to this solution was added tmed ( 137 μl , 0 . 287 mmol ) from a 2 . 10 m ph 7 . 0 solution in water . the mixture was vortexed rapidly for 15 seconds and the polymerizing solution was charged into two 1 . 0 ml syringes that acted as a mold for the polymerization . the syringes were allowed to sit for four hours . the gel was removed from the syringe and cut into pieces (˜ 100 μl cylinders ). the mass of the cylinder was recorded and placed in a 20 ml vial containing 18 ml of pbs at ph 7 . 4 . the gels were incubated overnight with buffer . the next day the buffer was changed twice in order to keep a constant external ph as the gel was charged . after incubating in pbs for 3 days the gel has swollen with water to approximately 20 times the total initial polymer volume . the salt form of the gel will be synthesized and the gel material processed into smaller pieces either before or after drying . the dry gel pieces would then be incorporated as one component in an absorbable layer of the absorbent device . generally the pieces should to small so as to increase the surface area of the gel and therefore to increase the rate at which water would be absorbed by the gel material . those skilled in the art of organic synthesis will be aware of the general considerations in designing cross - linkers of this class . generally if any alcohol groups are present in the poly - acids used they must be protected unless it desired that they react with the activated acids to be used in the formation of the oligo - ester . generally the synthesis must be performed under anhydrous condition except when performing acid or base washes of water immiscible organic solvents where the cross - linker or intermediate largely partitions into the organic phase . if the materials are to be used in an aqueous environment it is generally best to keep the acid in the anionic form only a few units away from its pka . this is due to the well - known effect of inhibition of attack of hydroxide by negatively charged electrophiles . in the most preferred cases the cross - linkers are constructed by adding a protected degradable piece to a polyacid . in a preferred embodiment the degradable piece contains a nucleophilic moiety and a protected acidic moiety , e . g . benzyl lactate . the protecting groups are removed under appropriate conditions known to those skilled in the art . the activation and reaction with a protected bifunctional degradable molecule can be repeated on the molecule as many times as desired . alternatively , the final step of the synthesis can be accomplished by terminating the molecule with reactive groups that are later used to cross - link polymer filaments . the preferred embodiment of the protecting group are groups that can be removed under neutral anhydrous conditions such as the benzyl protecting group . the next preferred protecting groups are ones that can be removed with anhydrous acids or bases such as the boc or mem protecting groups . preparation of di [ n - carbobenzoxy - n ′- hydrazidooxycarbonyl ] ethyl butane - 1 , 4 - dioate ( bnhydlacsuc ). to a 25 ml round - bottomed flask was charged 4a ( 262 mg , 1 . 00 mmol ), thf ( 2 . 0 ml ), and pyridine ( 162 μl , 2 . 00 mmol ). the flask was placed on an ice bath and to the reaction was added isobutyl chloroformate ( 260 μl , 2 . 0 mmol ). the reaction was allowed to stir and carbobenzoxyhydrazide was added ( 380 mg , 2 . 3 mmol ). the reaction was allowed to stir overnight . the white solid was dissolved in ethyl acetate and washed with 1m hcl ( 2 - 5 ml ), water ( 5 ml ) and saturated nahco3 ( 5 ml ). the organic layer was dried over mgso4 . the solvent was removed in vacuo resulting in a white solid . yield of bnhydlacsuc : 392 mg ( 62 %). preparation of di [ n - hydrazidooxycarbonyl ] ethyl butane - 1 , 4 - dioate ( hydlacsuc ) ( 8 ). to a 5 ml pressure tube was bnhydlacsuc ( 279 mg , 0 . 5 mmol ), pd - c ( degussa type , 10 % pd , 50 % h20 ) ( 600 mg ) and cyclohexene ( 1 . 25 ml , 12 . 5 mmol ) and meoh / dmf ( 1 : 1 , 1 . 25 ml ). the reaction was heated to 60 ° c . for 3 hours . evolution of co2 was observed . the pd - c was removed by filtration and the solvent was removed in vacuo resulting in an oil ( 8 ). preparation of di [ benzyloxycarbonyl ] methyloctane - 1 , 8 - dioate ( bnglyadp ). compound bnglyadp was synthesized by methods similar to those described for bnglysuc , compound 3b , by dissolving benzyl glycolate ( 9 . 08 g . 54 . 6 mmoles ) and pyridine ( 4 . 42 ml ., 54 . 6 mmoles ) in 150 ml ch2ch1 at 0 ° c . and adding adipoyl chloride ( 5 . 00 g , 27 . 3 mmoles ) via a syringe while stirring under nitrogen atmosphere . the reaction was allowed to warm to room temperature and stir for 5 hours . after 5 hours , tlc ( 5 : 95 methanol / ch2cl2 rf = 0 . 63 ) indicated almost complete conversion , and 0 . 1 ml of adipoyl chloride was added . the reaction was allowed 12 more hours . the medium was then cooled to 0 ° c . in a freezer for 2 hours to facilitate precipitation of pyridinium chloride salt ( pycl ). after 2 hours , the medium was filtered through a medium porous frit funnel and the filtrate was washed with 3 - 100 ml water washings . the organic layer was dried over na2s04 for 2 hours . the ch2cl2 was stripped on a roto - evaporator to concentrate the cnglyadp . the material was purified by recrystallization ( from 1 : 1 ethyl acetate / hexane ). yield of bnglyadp : 7 . 72g ( 64 . 0 %). 1 h nmr ( d 7 dmf ): δ1 . 67 ( s , 4h ), 2 . 46 ( s , 4h ), 4 . 80 ( s , 4h ), 5 . 23 ( s , 4h ), 7 . 44 ( m , 10h ). preparation of 2 , 3 -[( carboxymethyl ) oxycarbonyl ] octanoyloxyacetic acid ( hoglyadp ). compound hoglyadp was synthesized by methods similar to those described for hoglysuc , compound 4b , by dissolving bnglyadp ( 5 . 01 g , 11 . 3 mmoles ) in 250 ml 2 - proponal at room temperature in the presence of 1 . 51 g pd / c ( degussa type ). an air stone was immersed in the medium through rubber septum at the top of the flask . the medium was sparged with hydrogen gas at 1 atm . the system was isolated from air using a closed system bubbler . the medium was sparged with hydrogen gas for 12 hours . after 12 hours , the reaction mixture was filtered through celite to remove the catalyst and the reaction product was concentrated in vacuo resulting in a white solid . the white product was triturated with 1 : 1 diethyl ether / hexane . the white product was recovered by filtration through a medium porous filter funnel and then dried under vacuum in a desiccator . yield of hoglyadp : 1 . 74 g ( 60 . 0 %). preparation of di { 1 - methyl - 2 -( 2 - ethyl ) oyloxycarbonyl } methyloctane - 1 , 8 - dioate ( hemaglyadp ). compound hemaglyadp was synthesized by methods similar to those described for hpmaglysuc , compound 5b . the cross - linker hemaglyadp was prepared by adding hoglyadp ( 500 mg , 1 . 92 mmoles ) and cdi ( 622 mg , 3 . 83 mmoles ) to a 50 ml boiling flask . the flask was evacuated 3 times while iteratively purging with nitrogen . the temperature of the reaction vessel was reduced from room temperature to 0 ° c . with an ice bath and dry dmf ( 5 ml ) was rapidly added to the vessel under pressure with vigorous stirring via a magnetic stir bar . addition was accompanied with frothing and the formation of a white slurry , the intermediate precursor , the diimidazolide of glyadp . the slurry was allowed to come to room temperature and hydroxyethyl methacrylate ( hema , 466 μl , 3 . 84 mmoles ) was added via a syringe . the vessel was covered with aluminum foil to shield it from light and the reaction mixture was stirred under nitrogen atmosphere for 15 hours over which time the slurry completely dissolved . tlc of the reaction mixture showed the presence of both unreacted hema and hemaglyadp ( 5 : 95 methanol / ch 2 cl 2 rf = 0 . 80 ). the reaction was diluted with 100 ml ch 2 cl 2 and washed with 1m nah 2 po 4 ( ph 4 . 5 , 2 - 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