Isotactic copolymers

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

The experimental results of the two methods a) and b) applied to copolymers bearing COCI SC CH and COd SC frKCH-j) 2 8rouPs are in fairly good agreement 7 as shown in Figure 2 for isotactic copolymers the distribution of B units is nearly bernouillian, as expected from the kinetics kQ=kj for syndiotactic copolymers B units tend to be isolated between A blocks, and their distribution is quite compatible with that calculated taken into account the corresponding autoretarded kinetics (ko>k] k - 0). 

Figure 12.25. Cs meso ligands leading to isotactic copolymer...

By the formation of copolymers. Alternate isotactic copolymers. . . M,M2M M2. .. of monomers CHA=CHB with CH2=CH2, or of CHA=CHA or CHA=CHB with CHD=CH2 or CHD=CHE are chiral (38, 268, 269). Several examples arc reported in 82-89 (Scheme 18). Chiral copolymers with more complex structure such as. .. M,M,M2M MiM2. . . may be obtained even from two vinyl monomers, as recently demonstrated by WulfF and Hohn (see 53) (93). The copolymers formed by monomer units, which (vide supra) themselves produce chiral homopolymers (270) are also chiral, unless accidental compensation occurs. 

Based on the landmark studies of Jacobsen and coworkers, who employed chiral (salen)CoX complexes for the asymmetric ring opening and kinetic resolution of aliphatic epoxides [18-20], Lu and coworkers synthesized highly isotactic copolymer from rac-propylene oxide and carbon dioxide (Scheme 5) [21]. 

A number of linear isotactic copolymers of 4-methyl-l-pentene with 1-hexene and functionalized olefins, such as 5-(trialkylsiloxy)-1-pentene, could be prepared under similar conditions (9). 

D. Kisun ko, D. Lemenovskii, and A. Aladyshev, Synthesis of isotactic copolymers of 4-methyl-l-penteneby living polymerization catalyzed by zirconium non-metallocene complexes, Polym. Sci. Ser. A, 48(12) 1227-1231, 2006. 

This section summarizes recent advances in the asymmetric copolymerization of 1-alkene and carbon monoxide, giving the corresponding head-to-tail isotactic copolymer with high regio- and enantioselectivity as well as tacticity. 

Absolute configuration of the chiral centers in the isotactic copolymer main chain can be determined by comparing the CD spectrum of the copolymer that is a polyketone with that of (S)-3-methylpentan-2-one [126]. A recent model study has confirmed this assignment, showing that the copolymer with ( -configuration in the main chain exhibits plus optical rotation in (CF3)2CHOH and minus in CHC13 [128-130], The study has also revealed that the enantiose-lectivity for the propene insertion is at least 95% ee. 

It has been shown that the propene/CO isotactic copolymer has a y-polyketone structure in chloroform or (CF3)2CHOH solution, whereas the copolymer partially forms polyspiroketal structure in the solid state on the basis of solution and solid state NMR studies [132], The polyketone-polyspiroketal interconversion readily takes place in the presence of a Bronsted acid (Scheme 7.14). 

Helical conformations were also proposed for the isotactic copolymer derived from (/T)-3,7-d imethyl-1 -octene and styrene.48,49 The copolymer showed intense CD bands based on the styrene units incorporated into the polymer chain. The CD intensity was much larger than that of a model compound of an adduct of the chiral olefin and styrene. The helical structure of polyolefins has also been supported by force field calculations.50 The relationship of these considerations to isotactic vinyl polymers and more recent studies have recently been reviewed.41... 

The difference in behavior of fhe two diastereomeric ligands 84 and 85 (Scheme 8.16) was remarkable whereas ligand 84 produced a completely regioregular but atactic copolymer wifh very high catalytic activity, 85 gave a completely regular isotactic copolymer wifh very low catalytic activity. 

Despite fhe low activity of the system containing fhe dppe ligand (4, Scheme 8.6) for efhene copolymerization [7], the related Me-Duphos-modified catalyst 86 (Scheme 8.16) copolymerizes propene to isotactic copolymers with a fair catalytic... 

Fig. 20. 500-MHz 1H NMR spectra of highly isotactic copolymers of MMA and EMA prepared with f-C4H9MgBr in toluene at -60°C, measured in nitrobenzene- at 110°C. (A) Poly(MMA-raw-EMA) (B) PMMA-Wocfc-poly(EMA) (C) poly(EMA)-Mocfc-PMMA (D) PMMA + poly(EMA) (MMA/EMA = 1/1). (From Ref. 216.)...

One important application of Lewis acid to asymmetric radical reactions is in the control of tacticity in free radical polymerizations. Recently, Porter [38] showed that Sc(OTf)3 modulates the polymerization of oxazolidinone acrylamides to produce highly isotactic copolymers (Scheme 12). The same study described homopolymerizations in which the m/r dyad ratio was dependent on the reaction temperature. 

Glycidyl methacrylate High density polyethylene Isotactic copolymer of styrene and p-methyl styrene Isotactic poly(ethyl methacrylate) Isotactic poly(methyl methacrylate) Isotactic polystyrene Low density polyethylene Linear low density polyethylene Maleic anhydride Poly(4-methyl pentene) Random copolymer of phenyl ether and phenyl ketone... 

The asymmetric selectivity arises from the preferential formation of (S)-elective center at the beginning followed by the formation of (R)-elective center after the consumption of most of the (S)-monomer. The copolymerization of the (RS)-mono-mer and methyl methacrylate by this complex yielded a highly isotactic copolymer in which the (S>monomer predominantly incorporated over the (R)-monomer. On the other hand, in the copolymerization with a,a-dimethylbenzyl methacrylate only the homopolymer of a-methylbenzyl methacrylate was obtained with the same as-i mmetric selectivity as in the homopolymerization of this monomer. The results indicate that the steric interaction between the methyl group at the a- rosition of benzyl ester and the (-)-sparteine moiety of the catalyst plays an important role in the stereoelection of the polymerization. 

The synthesis of isotactic copolymers from carbon monoxide and propene has been more difficult to achieve. The stereoselective copolymerization of carbon monoxide and propene has been most selective with catalysts containing alkyl-substituted bisphosphines. The catalyst containing the dialkylphosphino biaryl ligand in Figure 17.16 has been shown... 

Nozaki has shown that the less symmetric BINAPHOS ligand (Equation 17.70) containing one phosphine and one phosphite donor also gives rise to highly isotactic copolymers from carbon monoxide and propylene. - NMR spectroscopic studies on is polymerization process have shown that the acyl group in one intermediate prefers to be located trans to the phosphine donor and the alkyl group of the other intermediate prefers to be located trans to the phosphite donor. Thus, olefin coordinates to the site trans to tiie phosphite prior to insertion, and CO coordinates trans to the phosphine donor prior to insertion. 

CPE may be incorporated into polypropylenes in order to lower their TmS and raise their TgS. Kaminsky et al. reported the copolymerization of propylene and CPE using the isoselective zirconocenes rac-Et(Ind)2ZrCl2 and rac-Me2Si(Ind)2ZrCl2. C NMR spectroscopy of the resulting isotactic copolymers (containing 2-5 mol% CPE) revealed a 1,2-enchainment mode for the CPE units no PCPE blocks were detected (Eigure 15.5). 

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