Highly isotactic copolymers

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]. 

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. 

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. 

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. 

Structural analysis of highly isotactic copolymers by NMR spectroscopy ... 

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. 

It is interesting that some heterogeneous superhigh-activity Ziegler-Natta catalysts such as MgC /TiCU/LB—AlEt3 also yield random ethylene/propylene copolymers. These copolymers, however, exhibit a blocky nature and highly isotactic propylene sequences (with no 2,1-inserted propylene units) that contribute to undesired crystallinity [68,456]. 

The copolymerisation of ketene and acetaldehyde was carried out with ethyl-zinc diphenylamide as the catalyst. It was found to yield an alternating copolymer [scheme (43)], the respective polyester of highly isotactic structure (Table 9.3) [279] ... 

Present-day Ziegler-Natta catalysts are supremely suitable for the production of linear polyethylene and of highly isotactic polypropylene. They are also used to produce the softer ethylene-propylene copolymers, used for packaging and related purposes. Due to the presence of distinct catalyst sites in typical Ziegler-Natta catalysts, these copolymers suffer from non-uniformity however, and copolymers which contain increased amounts of higher ot-olefins, desirable for certain applications, cannot easily be made with these catalysts. 

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. 

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... 

Recently, a new technology that manipulates the propylene sequence in EP copolymers has been proposed using a Cr(acac)3/MgCl2-Et2AlCl-ethylbenzoate catalyst system (36). The novel EP random copolymers with high isotacticity in propylene sequence show essentially different properties from those of conventional EPRs (36). This section deals with morphology and dynamic mechanical properties in binary blends of the novel EP copolymers (referred as EP) with various propylene contents. 

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