Abstract:
A zwitterionic Group VIII transition metal complex containing the simple and relatively small 3-(arylimino)-but-1-en-2-olato ligand that catalyzes the formation of polypropylene and high molecular weight polyethylene. A novel feature of this catalyst is that the active species is stabilized by a chelated olefin adduct. The present invention also provides methods of polymerizing olefin monomers using zwitterionic catalysts, particularly polypropylene and high molecular weight polyethylene.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under Grant No. DE-FG03098ER14910 from the United States Department of Energy. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to the polymerization of olefin monomers and in particular to polymerization by zwitterionic metal catalysts. 
       BACKGROUND OF THE INVENTION 
       [0003]    Nickel-based olefin oligomerization and polymerization initiators are of interest in industrial and academic laboratories [1, 2]. Cationic versions are the most commonly encountered [3]. Neutral species, although less active, are under investigation because of their higher tolerance toward functionalities [4, 5, 6, 7]. Zwitterionic counterparts are the least common and provide an intermediate range of reactivities [8]. 
         [0004]    While developing tandem catalytic processes [9, 10, 11, 12], one of the inventors discovered that the reactivity of SHOP-type catalysts such as [(C 6 H 5 ) 2 PC 6 H 4 C(O)O-κ 2 P,O]Ni(η 3 —CH 2 CMeCH 2 ) [13] increases upon addition of B(C 6 F 5 ) 3 [ 14, 15]. Carbonyl coordination to the borane gives zwitterionic [(C 6 H 5 ) 2 PC 6 H 4 C(O—B(C 6 F 5 ) 3 )O-κ 2 P, O]Ni(η 3 —CH 2 CMeCH 2 ) and removes electron density from nickel. Examination of ligand/reactivity relationships for cationic Ni(diimine) initiators [3, 16] led to the design and synthesis of {(H 3 C)C[═NAr]C[O—B(C 6 F 5 ) 3 ][═NAr]-κ 2 N,N′}Ni(η 3 —CH 2 C 6 H 5 ), with which high molecular weight polyethylene (PE) can be produced for bulky aromatic (Ar) substituents [17]. Several other ligand types have been reported that are amenable to the concept of Lewis acid attachment on a ligand site to redistribute electron density, including 3-(1-arylimino-ethyl)-acetylacetonato [18], 2-diphenylphosphinylbenzamido [19], N-(2-benzoylphenyl)benzamido [20], α-iminoenamido [21], 2-(alkyldeneamino)benzoato [22] and iminoamido pyridine [23]. However, none of these systems has been able to produce high molecular weight PE, in the absence of bulky Ar substituents, or polypropylene. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a zwitterionic Group VIII transition metal complex containing the simple and relatively small 3-(arylimino)-but-1-en-2-olato ligand that initiates the polymerization or co-polymerization of olefins. A novel feature of this catalyst is that the active species is stabilized by a chelated olefin adduct. 
         [0006]    The present invention also provides a process of polymerizing olefin monomers using zwitterionic catalysts. In particular, the olefin, preferably a monomer, is selected from the group consisting essentially of: (1) olefins having formula R v CH═CH 2 , wherein R v  is hydrogen, a substituted or unsubstituted hydrocarbyl group, or a substituted or unsubstituted hydrocarbyl bearing functional group; (2) a substituted or unsubstituted cyclopentene, (3) a substituted or unsubstituted styrene, (4) a substituted or unsubstituted norbornene derivative bearing functional group, and (5) a polar olefin. Preferred olefins are polypropylene and ethylene for the formation of polypropylene and high density polyethylene. 
         [0007]    The zwitterionic Group VIII transition metal complex acts as an initiator in the formation of polymers and co-polymers, but in this specification will be referred to either as an initiator or catalyst. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an Oak Ridge Thermal Ellipsoid Program (ORTEP) molecular modeling drawing of an intermediate compound in the production of an initiator of this invention, at the 50% probability level, with hydrogen atoms omitted for clarity; and 
           [0009]      FIG. 2  is an ORTEP molecular modeling drawing of the initiator of this invention, at the 50% probability level, with hydrogen atoms omitted for clarity. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The initiator/catalyst of the present invention is a zwitterionic Ni complex containing a 3-(arylimino)-but-1-en-2-olato ligand stabilized by a chelated olefin adduct. In particular, the present invention provides a catalyst of the following formula 1: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein M is a Group VIII transition metal such as Ni, Pd, Pt;
 
R′ is a one electron hydrocarbyl, such as methyl, ethyl, substituted or unsubstituted R′ is→benzyl (which is a combination of a sigma bond and a pi-bond from the bound aromatic unit), and the like;
 
L′ is a two electron ligand, such as a phosphine, amine, olefin, ether, and the like, or is a link to R′ when R′ is R′ is→benzyl;
 
X′ is a two electron donor, such as imine, phosphine, carbonyl, and the like;
 
each R, R″, R″′, R iv  is independently hydrogen, an alkyl or a substituted or unsubstituted aryl group;
 
(L) n  is a tether group, such as a series of methylene carbons, oxygen, amine, and the like, wherein n is 0 or 1;
 
X″ is a component having two electron donor functionality, such as oxygen, amine, phosphine, and the like;
 
LA is a Lewis Acid, such as a trisubstituted boron, trisubstituted aluminum or a proton; and
 
the olefin bearing X″ is bound to the metal center.
 
         [0011]    In preferred embodiments: 
         [0012]    M is nickel; 
         [0013]    R′ is→benzyl; 
         [0014]    L′ is linked to R′ 
         [0015]    X′ is an imine; 
         [0016]    R is an aryl group; 
         [0017]    R″ is an alkyl group; 
         [0018]    R iv  is an alkyl group; 
         [0019]    R″′ is hydrogen; 
         [0020]    n is 0; 
         [0021]    X″ is oxygen; and 
         [0022]    LA is a trisubstituted boron. 
         [0023]    A particularly preferred catalyst is compound 2 shown in the scheme 1 below, having the structure: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0024]    Preparation of the zwitterionic Ni complex catalysts of this invention is exemplified by the specific reactions shown in Scheme 1: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0025]    As shown in Scheme 1, synthetic access begins with 3-(2,6-diisopropylphenylimino)-butan-2-one [24], which is obtained by condensation of 2,6-diisopropylaniline with 2,3-butanedione. Deprotonation with NaH in THF provides the sodium salt in 80% yield. Subsequent reaction of the salt with Ni(PMe 3 ) 2 (η 1 —CH 2 C 6 H 6 )Cl [25] provides a new compound, which contains the ligand fragment ( 1 H NMR in C 6 D 6 : 4.91, 4.63 ppm), an η 1 -benzyl group ( 1 H NMR: 7.56 ppm) and PMe 3 . Single crystals of the product were obtained from a solution of pentane and the results are shown in  FIG. 1 . The product is thus (3-(2,6-diisopropylphenylimino)-but-1-en-2-olato)(η 1 -benzyl)-(trimethylphosphine)-nickel (compound 1 as shown in Scheme 1). 
         [0026]    Structural characterization of compound 1 reveals a distorted square-planar geometry with a cis relationship between the benzyl ligand and the imine nitrogen. The 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato ligand coordinates to the nickel via the nitrogen and oxygen atoms with Ni—O and Ni—N bond lengths of 1.9031(16) and 1.9555(18) Å, respectively. The C1-C2 (1.344(3) Å) and C3-N (1.294(3) Å) distances are consistent with double bond character, whereas the C2-C3 (1.486(3) Å), C3-C4 (1.497(3) Å) and C2-O (1.315(3) Å) distances reveal single bond character. These data indicate localized bonding in the chelating ring. 
         [0027]    The addition of 2 equivalents B(C 6 F 5 ) 3  to compound 1 in toluene results in the formation of Me 3 P—B(C 6 F 5 ) 3 , which precipitates out of solution, and a new organometallic product.  1 H NMR spectra of the product show the formation of two isomers in a 9:1 ratio. The upfield shift of the aromatic signals from —CH 2 C 6 H 5  from 6.9 to 7.6 to 5.9 to 6.6 ppm indicates □ 3 -coordination. The vinyl protons are observed as a pair of doublets in the range of 3.25 to 5.02 ppm. The  11 B NMR spectrum shows a signal at −2.6 ppm, consistent with boron-oxygen coordination.  1 H-NOE spectroscopy between the vinyl group and the benzyl group indicates that the sets of isomers arise from pseudorotamers of the benzyl ligand [23] and that the major isomer contains a cis relationship between the vinyl group and the benzyl group, as shown in Scheme 1. 
         [0028]    Single crystals of compound 2 formed from a toluene solution and the structure is shown in  FIG. 2 . Isomer 2(A) was obtained preferentially. The oxygen binds to the Lewis acidic boron and by doing so, forces coordination of the olefin to the nickel center. The distances from Ni to C1 and C2, are 2.109 (6) Å and 2.455(6) Å, respectively. The rotation of the C2-C3 bond and the C1-C2-C3-N torsional angle) (41.6(8)° optimize overlap between the π-orbital of the —C═CH 2  group and Ni. The C2-O distance (1.319(6) Å) is characteristic of a single bond, while the C1-C2 (1.371(7) Å) distance is more indicative of a double bond. These observations are consistent with the charge distribution as shown for structures of compound 2(A) and 2(B) in Scheme 1. The C3-N distance (1.300(6) Å) is close to that observed in compound 1 (1.294(3) Å). The N—Ni bond length (1.900(4) Å) is 0.055 Å is shorter than that of compound 1 (1.9555(18) Å), revealing a more electron deficient metal center in the zwitterionic compound. 
         [0029]    The zwitterionic initiators/catalysts enable the polymerization of olefins. In particular, the olefin, preferably a monomer, is selected from the group consisting essentially of: (1) olefins having formula R v CH═CH 2 , wherein R v  is hydrogen, a substituted or unsubstituted hydrocarbyl group, or a substituted or unsubstituted hydrocarbyl bearing functional group; (2) a substituted or unsubstituted cyclopentene, (3) a substituted or unsubstituted styrene, (4) a substituted or unsubstituted norbornene derivative bearing functional group, and (5) a polar olefin. Preferred olefins are polypropylene and ethylene for the formation of polypropylene and high density polyethylene. The zwitterionic catalysts of this invention catalyze the formation of polypropylene and high molecular weight polyethylene, e.g., having a molecular weight from 50 K to over a million Daltons 
       EXAMPLES 1-8 
       [0030]    Ethylene polymerization reactions with compound for 2 were studied, and the results of these studies are listed in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Polymerization of ethylene  a  and  
               
               
                 propylene  b  with compound 1 or 2. 
               
             
          
           
               
                   
                 Com- 
                 Addi- 
                   
                   
                   
                   
                   
                   
               
               
                 Ex- 
                 pound 
                 tive 
                 T 
                 Time 
                 Activ- 
                   
                   
                 T m   
               
               
                 ample 
                 (μmol) 
                 (equiv.) 
                 (° C.) 
                 (min) 
                 ity c   
                 Mw 
                 PDI 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 1 
                 2(6) a   
                 0 
                 30 
                 10 
                 453 
                 135 500 
                 5.86 
                 135 
               
               
                 2 
                 2(3) a   
                 BCF(6) 
                 30 
                 8 
                 2190 
                 134 800 
                 2.59 
                 134 
               
               
                 3 
                 2(3) a   
                 BCF(6) 
                 75 
                 8 
                 4050 
                 149 700 
                 6.90 
                 117 
               
               
                 4 
                 2(2) a   
                 BCF(6) 
                 75 
                 5 
                 6900 
                 132 300 
                 6.86 
                 118 
               
               
                 5 
                 1(6) a   
                 0 
                 30 
                 10 
                 0 
                   
                   
                   
               
               
                 6 
                 2(20) b   
                 BCF(2) 
                 17 
                 90 
                 18 
                 207 400 
                 2.05 
                   
               
               
                 7 
                 2(20) b   
                 BCF(3) 
                 17 
                 90 
                 34 
                 205 800 
                 2.07 
                 70 
               
               
                 8 
                 2(20) b   
                 BCF(3) 
                 17 
                 240 
                 17 
                 242 700 
                 2.23 
               
               
                   
               
               
                   a Polymerization conditions: 28 g toluene, 100 psi ethylene. 
               
               
                   b Polymerization conditions: 20 g toluene, 26 mL propylene. 
               
               
                   c Activity in kg polymer/(mol cat · h). 
               
             
          
         
       
     
         [0031]    Addition of 100 psi ethylene to a solution of compound 2 in toluene at 30° C. results in the quick consumption of the monomer and polymer formation (Example 1). Use of an additional 6 equivalents of B(C 6 F 5 ) 3  increases the activity of the catalyst and decreases the polydispersity index (PDI) of the polymer (Example 2). The PDI is a measure of the distribution of molecular weights in a given polymer sample and is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular weight number average molecular weights in a batch of polymers. The PDI has a value always greater than 1, but as the polymer chains approach uniform chain length, the PDI approaches unity. 
         [0032]    Thermal analysis indicates the polymer has high melting point, consistent with the highly linear polymer structure (7 branches/1000 carbons) revealed by  1 H NMR spectroscopy. The activity increases with temperature and at 75° C. (entries 3 and 4) it is comparable to those of cationic diimine nickel initiators [3]. The polymers formed at high temperature have broad PDI, more branching in the backbone (23 branches/1000 carbons) and lower melting points. No reaction occurs by using compound 1 (Example 5). 
         [0033]    Compound 2 also initiates propylene polymerization to produce high molecular weight PP (Table 1, entries 6-8). The product is purified by precipitation from toluene using acetone.  13 C NMR spectroscopy indicates the PP is isotactic rich with an mmmm pentad fraction of 0.6. In contrast, diimine nickel initiators provide atactic or syndiotactic structures [3, 26]. Chain-end control by using compound 2 is suggested by the observation of mmrm sequence and absence of mmrr and mrrm sequences [27]. Signals at 36.7, and 30.2 ppm in the  13 C NMR spectra reveal the presence of polyethylene segments, which were observed previously in the PP formed with cationic Ni(diimine) initiators. It has been previously proposed that 2,1 monomer insertion and subsequent chain walking to give 1,ω-enchainments results in these linear segments [1]. The PP obtained has a broad Tm at 70° C. and a Tg at −23° C. 
       Example 9 
       [0034]    Example 4 can be repeated with a catalyst of formula 1 in which M is Pd, L′ is phosphine, R′ is methyl, X′ is carbonyl, R is methyl, R″ is ethyl, R″′ is phenyl, R iv  is hydrogen, n is 1 and L is oxygen, X″ is amine, and LA is Al(C 6 F 5 ) 3 . 
       Example 10 
       [0035]    Example 4 can be repeated with a catalyst of formula 1 in which M is Pt, L′ is amine, R′ is ethyl, X′ is imine, R is ethyl, R″ is propyl, R″′ is hydrogen, R iv  is hydrogen, n is 0, X″ is phosphine, and LA is a proton. 
       Example 11 
       [0036]    Example 4 can be repeated with a catalyst of formula 1 in which M is Pd, L′ is ether, R′ is propyl, X′ is phosphine, R is ethyl, R″ is propyl, R″′ is n-methyl phenyl, R iv  is methyl, n is 1 and L is amine, X″ is oxygen, and LA is BF 3 . 
       Example 12 
       [0037]    Example 4 can be repeated using an olefin having formula R v CH═CH 2 , wherein R v  is a hydrocarbyl group. 
       Example 13 
       [0038]    Example 4 can be repeated using an olefin having formula R v CH═CH 2 , wherein R v  is styrene as the olefin. 
       Example 14 
       [0039]    Example 4 can be repeated using cyclopentene as the olefin. 
       Example 15 
       [0040]    Example 4 can be repeated using a norbornene bearing functional group as the olefin. 
         [0041]    In summary, we have demonstrated that a zwitterionic nickel complex supported by the 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato ligand can be used to prepare high molecular weight PE and PP. The most noteworthy feature of the active site is that it is supported by an olefinic fragment, which lacks substantial steric hindrance. Such a structure departs from previous structure/reactivity relationships and theoretical predictions for cationic Ni(diimine) complexes, which require sterically demanding substituents to produce high molecular weight PE. Bonding from an olefin is considerably different from that of a nitrogen donor in that on-back bonding is a possibility. Whether this electronic feature is responsible for the increasing the ratio of the rate of chain propagation, relative to chain transfer or termination rates is unknown at this stage and requires theoretical analysis for elucidation. 
         [0042]    Although the present invention has been described in connection with the preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the principles and scope of the invention, as those skilled in the art will readily understand. Accordingly, such modifications may be practiced within the scope of the following claims. 
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