PROCESS FOR PREPARING SEMICRYSTALLINE SYNDIOTACTIC VINYLAROMATIC POLYMERS

Abstract: Semicrystalline copolymers of styrene and one or more ring alkyl substituted styrene compounds possessing a crystalline melting point from 180 DEG C to 254 DEG C and a styrene content from 50 to 94 mole percent uniquely may be prepared in a solution polymerization process.

Description: PROCESS FOR PREPARING SEMICRYSTALLINE SYNDIOTACTIC VINYLAROMATIC POLYMERS The present invention relates to a process for preparing polymers of vinylaromatic monomers that are semicrystalline due to the presence of intentional chain defects which slows the rate of crystal formation. In addition, the present invention relates to a solution process for preparing such polymers. It is previously known in the art to prepare polymers including copolymers of vinylaromatic monomers that are highly syndiotactic. Typically such polymers are prepared by the use of certain Group 4 metal complex containing catalysts. Such polymers rapidly crystallize from the melt due to their uniform polymer structure, resulting in highly crystalline solids. Due to such high crystallinity these polymers are generally insoluble in commonly available solvents. While solvent resistance is desirable in a molded article under certain use conditions this same property makes such polymers difficult to prepare. In particular, when a solution polymerization process is used to prepare such polymers, a gel phase and then a biphasic mixture generally occurs. This results in poor heat dissipation from the reaction mixture, difficulty in mixing and ineffective mass transport during the polymerization process. Fouling of the reactor surfaces is a constant and significant problem. Attempts to overcome the problems associated with such a bulk or solution polymerization process for preparing syndiotactic vinylaromatic polymers include the use of wiped surface reactors, use of powder bed reactors wherein the liquid phase is maintained on the surface of the particulated product, and use of suspension polymerization processes. These techniques have been previously disclosed in EP-A-389,939, in US-A-5,484,862 and US-A-4,950,724 and elsewhere. Despite the advances achieved in the polymerization of vinylaromatic monomers to prepare highly crystalline syndiotactic polymers, the previously mentioned difficulties continue to limit the ease of manufacturing syndiotactic vinylaromatic polymers. It would be desirable if there were provided polymers possessing a sufficient degree of syndiotacticity to possess a crystalline melting point thereby imparting improved heat resistance to such polymers, compared to atactic polymers, but wherein the degree of crystallinity thereof is sufficiently low and the crystallization rate is sufficiently slow so as to render such polymers soluble in aromatic solvents, especially at elevated temperatures. Such desirable polymers are referred to herein as being "semicrystalline". In addition it would be desirable to provide a process for preparing such semicrystalline syndiotactic vinylaromatic polymers under solution polymerization conditions. Brief Description of the Drawings Figure 1 indicates the physical state (solution, gel or solid) of various experimentally determined styrene/p-methylstyrene polymerization mixtures defined by mole percent paramethylstyrene in the copolymer and volume fraction copolymer in the polymerization mixture at 70°C with toluene diluent. The operating conditions to assure that solution polymerization conditions are maintained (determined by fitting the data to a theoretical model) are also provided. Figure 2 gives the crystalline melting points of various styrene/p- methylstyrene copolymer films as a function of p-methylstyrene content. Figure 3 provides a graph of crystallinity as a function of comonomer content for styrene/p-methylstyrene copolymers. Both the crystallinity as measured from the melt and the crystallinity measured after stretching and heat setting are provided. According to the present invention there is provided a solution process for the preparation of semicrystalline polymers of vinylaromatic monomers having a syndiotactic structure. The present inventors have discovered that certain syndiotactic copolymers of styrene and ring alkyl substituted styrene compounds possessing a crystalline melting point from 180°C to 254°C (thereby imparting increased heat distortion temperature properties compared to atactic polyvinylaromatic polymers) and a styrene content from 50 to 94 mole percent also are soluble in aromatic solvents under polymerization conditions due to a low degree of crystallization and also to a slow crystallization rate. Moreover, the crystallinity of these same polymers may be increased in molded articles or objects by the use of nonquenching molding conditions or by the use of strain induced crystallization techniques, thereby imparting acceptable solvent resistance in articles and films formed from such copolymers. Specifically, articles such as films can be obtained having a high degree of crystallinity both by tentering and blown film techniques after a heat setting step or through proper control of the cooling rate of the polymer. Accordingly, the invention lies in the discovery of the above unique species of semicrystalline syndiotactic vinylaromatic polymers and in the solution polymerization conditions suitable for use in the preparation thereof. Moreover, certain of the above semicrystalline copolymers of styrene and one or more ring alkyl substituted styrene compounds possessing a crystalline melting point from 180°C to 234 C C and a styrene content from 50 to 85 mole percent are particularly adapted for preparation by means of the foregoing polymerization process and accordingly are highly desirable as compositions of matter. Suitable semicrystalline polymers for preparation and use hereunder are interpolymers of styrene and one or more ring alkyl substituted styrene monomers, containing from 50 to 94 mole percent styrene, preferably from 55 to 85 mole percent styrene. As used herein the term "interpolymer" is used interchangeably with the term "copolymer" and refers to a polymer comprising one or more comonomers. At styrene contents near to the range herein specified crystallinity cannot be introduced into the solid polymer by ordinary techniques. Such noncrystalline polymers are not sufficiently resistant to solvents, especially aromatic liquids, as is desired, nor do they possess a crystalline melting point which imparts a higher use temperature. At styrene contents greater than 85 percent, especially greater than 94 percent, the comonomer content is insufficient to introduce a significant number of chain defects into the polymer so that the polymers have a high rate of crystallization and, consequently, become insoluble in the polymerization mixture or liquid diluent at low polymer concentration thereby limiting the total conversion in a polymerization process. Such high styrene content copolymers are undesirable according to the present invention since they are unsuited for manufacture according to the present solution polymerization technique. According to the present invention, the above semicrystalline syndiotactic vinylaromatic copolymers are prepared by contacting styrene and one or more polymerizable ring alkyl substituted vinylaromatic monomers with a catalyst comprising a Group 4 metal complex at temperature and under polymerization conditions such that the resulting polymer is retained in solution. Optionally an inert liquid diluent, preferably an aromatic solvent may also be present during the polymerization. By the use of such liquid diluents, the polymerization generally can be conducted to a higher degree of conversion than is possible under bulk polymerization conditions while retaining the polymer in solution. By the term "bulk" polymerization conditions is meant that the polymerization is conducted in the substantial absence of liquid diluent other than vinylaromatic monomers. Preferred ring alkyl substituted vinyl aromatic monomers for use herein correspond to the formula: wherein R is C1.10 alkyl. Highly preferred ring alkyl substituted vinyl aromatic monomers are the various isomers of vinyltoluene, especially p-vinyl toluene. A most preferred interpolymer is a copolymer of styrene and p-vinyltoluene. Suitable inert liquid diluents for use in the polymerization include aromatic compounds, especially toluene, xylene, ethylbenzene, and mixtures thereof. A preferred diluent is toluene. Typically the amount of diluent utilized is from 30 weight percent to 90 weight percent of the total polymerization mixture. Preferred quantities of diluent utilized are from 35 to 75 weight percent of the total polymerization mixture. Suitable polymerization temperatures are from 40 to 90 °C, preferably from 50 to 85 °C. The polymerization is preferably conducted under conditions to provide a total conversion of vinylaromatic monomer that is determined by the formula: )) wherein: X is the total monomer conversion; φ, is the volume fraction of solvent; Φ 2 is the volume fraction of the copolymer; p 2 is the density of the copolymer; p c is the density of the comonomer; p S iy is the density of styrene; and n c is the mole fraction of comonomer in the copolymer. The maximum volume fraction of copolymer that is soluble in the reaction mixture can be obtained by solving for φ 2 in the quadratic equation: RT ^ [(l-^)-^,(l- 02 ) 2 ] = ΔH„ [l-^ wherein: R is the universal gas constant; T is the temperature of the polymerization; Vj is the molar volume of the diluent; V c is the molar volume of the monomer unit; Xi is the Flory-Hugggins interaction parameter between the diluent and copolymer; φ 2 is the volume fraction of the copolymer; ΔH u is the heat of fusion for the repeating unit; and T° m is the melting temperature of the copolymer. By operating the polymerization such that conversion does not exceed the maximum volume fraction of copolymer determined by this formula, preferably such that the conversion does not exceed 0.95 times such maximum volume fraction, the polymerization will not depart from solution polymerization conditions. The polymers desirably have a crystallinity measured by Differential Scanning Calorimetry (DSC) from annealing the melt of from 2 to 42 weight percent, more preferably from 10 to 40 weight percent. In the above technique of determining crystallinity the melting endotherm is divided by 54 Joules/g to provide the unitless measure of percent crystallinity. The skilled artisan will appreciate that the invention disclosed herein may be practiced in the absence of any component which has not been specifically disclosed. The following examples are provided as further illustration of the invention and are not to be construed as limiting. Unless stated to the contrary all parts and percentages are expressed on a weight basis. Example A 20 ml glass ampoule was dried and purged with a nitrogen atmosphere. All monomers and solvents were purified by passing through a 3 A (0.3 nm) molecular sieve, an alumina bed, and finally contacting with hydrogen in the presence of a palladium catalyst to hydrogenate acetylenic impurities. 2.34 g of purified styrene and 0.89 g of purified p-vinyltoluene were added under an inert, nitrogen atmosphere. 5.58 g of dry toluene was then added also under nitrogen atmosphere. The ampoule was sealed and placed in a water bath at 70°C for 10 minutes. At that time 363 μL of a 0.003 M toluene solution of pentamethylcyclopentadienyltitanium trimethoxide catalyst was added. (The catalyst solution was prepared by combining 3.0 x 10 '5 moles of titanium complex, 2.25 x10 3 moles of methylalumoxane cocatalyst and 7.5 xlO "4 moles of triisobutyl aluminum in 10 ml dry toluene). The reaction was allowed to proceed until the catalyst was no longer active (approximately 16 hours). After this time the copolymer remained completely soluble in the reaction mixture, No crystallization or precipitation was observed. The polymer was recovered by precipitation with methanol, filtering through filter paper and washing with fresh methanol. Residual solvent and monomers were removed by reduced pressure at 150°C for 12 hours. Final polymer weight was 2.08 g. Total monomer conversion was 64 percent by weight. Copolymer composition was 75 mole percent styrene and 25 mole percent p-vinyltoluene. The number average molecular weight and weight average molecular weight of the polymer sample as determined by high temperature size exclusion chromatography using atactic polystyrene standards were: Mn = 191 ,000, Mw = 780,000, Mw/Mn = 4.1. Samples of the copolymer were extruded into a 4" (100 mm) wide web of 5-10 mil (0.13-0.25 mm) thickness. The sample was quenched by using a chill roll at the exit of the die. The web was punch cut into a 2" x 2" (50 mm x 50 mm) square and stretched on a biaxial T.M. Long brand film stretcher to various lengths at a deformation rate of 0.4 inch/sec (10 mm/sec) at 120°C. The degree of crystallization of the sample depends on the amount of orientation or stretching. This was quantified by DSC analysis of both the unstretched, stretched, and stretched & heat set material. Percent crystallinity was calculated by determining the integrated energy from the melting peak and subtracting the energy from the cold crystallization peak and dividing by 54 Joules per gram. The unstretched film had 0 percent crystallinity. The stretched film had 23 weight percent crystallinity, thereby indicating strain induced crystallization had occurred. The stretched film after heat setting at 180°C for 40 seconds to relax the amorphous regions and allow the aligned molecules to fully crystallize had a crystallinity of 28 weight percent. Accordingly, the polymer was semicrystalline. Additional copolymer samples were prepared by substantially repeating the above procedure using different comonomer concentrations in the reactor and differing degrees of monomer conversion. The polymerization conditions and physical state of the copolymer so formed were observed. The crystalline melting points and crystallinity of several representative samples of the copolymers so formed were measured by DSC before and after stretching. Results of the polymerization are plotted in Figure 1. Results of the melting point and crystallinity measurements are contained in Figures 2 and 3 respectively.

Claims: CLAIMS: 1. A semicrystalline copolymer comprising styrene and one or more ring alkyl substituted styrene compounds possessing a crystalline melting point from 180°C to 234°C and a styrene content from 50 to 85 mole percent. 2. A semicrystalline copolymer according to claim 1 wherein the alkyl substituted styrene compound is p-vinyltoluene. 3. A semicrystalline copolymer according to claim 1 having crystallinity of from 2 to 42 weight percent. 4. A process for preparing a copolymer of styrene and one or more ring alkyl substituted styrene compounds possessing a crystalline melting point from 180°C to 254°C and a styrene content from 50 to 94 mole percent comprising contacting styrene and one or more polymerizable ring alkyl substituted vinylaromatic monomers with a catalyst comprising a Group 4 metal complex under polymerization conditions such that the resulting polymer is retained in solution. 5. A process according to claim 4 wherein the degree of conversion of such process is maintained at a level less than or equal to that determined by the formula: )) wherein: X is the total monomer conversion; φi is the volume fraction of solvent; Φ 2 is the volume fraction of the copolymer; p 2 is the density of the copolymer; p c is the density of the comonomer; ps t y is the density of styrene; and n c is the mole fraction of comonomer in the copolymer.

Inventors: CALLAGHAN THOMAS A, MALANGA MICHAEL T, CALLAGHAN, THOMAS, A., MALANGA, MICHAEL, T.
Applicants: CALLAGHAN THOMAS A, DOW CHEMICAL CO, MALANGA MICHAEL T, CALLAGHAN, THOMAS, A., MALANGA, MICHAEL, T., THE DOW CHEMICAL COMPANY
Application year: 1997
Main classification: C08F12
IPC codes: C08F2, C08F4, C08F212