Stereoregular copolymer

Stereoregular copolymers. We shall restrict our discussion to stereoregular homopolymers. 

The cocrystallization of different units in stereoregular copolymers of a-olefins is of particular interest. In these cases the distribution of the different monomer units is generally random, although both the steric configuration of the chain backbone and its helical conformation keep regular. 

Duradene [Firestone]. TM for a stereoregular copolymer of butadiene and styrene with exceptional purity, soluble in aromatics and ali-phatics. 

One of these structures is a stereoregular copolymer of the type shown in Figure 2-1, consisting of triads with AAB substituents along the chain. 

However, the examination of the steric defects alone in the homopolymerization cannot be considered as fully demonstrative. In fact, it is difficult to deduce probative data on the stereospecific polymerization by examining just those units that are inserted in the chain in a disordered way. In order to obviate this difficulty we started research on stereoregular copolymers of propylene with a low amount of 60% enriched 1 ethylene. 

Scheme 21. Synthesis of a "partially stereoregular copolymer by radical copolymerizatio n of methyl methacrylate with an optically active comonomer.

Partially stereoregular copolymers have been obtained by asymmetric polymer synthesis using maleic anhydride as one comonomer and optically active a-methylbenzyl methacrylate (216) or a-methylbenzyl vinyl ether (217) as the other comonomer. These copolymers were optically active even after removal of their a-methylbenzyl groups (Scheme 25). Analogous results have been obtained by Minoura s group (218, 219) on copolymerizing optically active a,3-disubstituted olefins with achiral vinyl monomers. 

If a stereoregular copolymer containing A and B monomer imits is considered, then some of its structural features that can be measured by NMR spectroscopy can be listed as follows. 

The various structural features outlined above can give rise to a large number of resonance lines in the nmr spectrum of a stereoregular copolymer. It is not a simple matter to make assignments for these resonances since there are ten possible tetrads and twenty possible pentads, and since the various tetrad or pentad resonances are usually overlapped to a considerable extent. The case can be even more complicated if all of the nuclei in a given environment do not have the same chemical shift, as for example, the methylene protons in isotactic copolymers. 

Once assignments have been made for the resonances of stereoregular copolymers, the spectra of copolymers prepared voider varying conditions can be analyzed to obtain information about stereoregular copolymerization... 

The microstructural features of stereoregular copolymers prepared directly from monomers can be related to conditional monomer placement probabilities, which are, in turn, related to monomer reactivity ratios and monomer feed ratios. Thus, the conditional probability that an A unit follows a B unit in a copolymer chain, P(a/b), can be calculated from BA and BB dyad distributions or from the monomer reactivity ratio for B,r, and the ratio of, monomers A and B in the feed (A /B ), according to the following equations. 

P(A/b) = P(A/AA) = P(a/BA) = P(A/anything). This is the situation commonly encoxmtered in stereoregular copolymers. In some cases, however, first order Markoffian statistics are needed to characterize the copolymers. In such cases P(a) P(A/a) P(A/b). ... 

STUDIES ON STEREOREGULAR COPOLYMERS OBTAINED BY CHEMICAL MODIFICATION... 

STEREOREGULAR COPOLyMERS PREPARED FROM ACHIRAL MONOMERS AND CHIRAL SPECIFIC CATALYSTS... 

Procedures are avail le for preparing isotactic-cis-, isotactic trans- and syndiotactic-cis- varieties of poly-l,3-pentadiene. Hydrogenation of these polymers yields polymers that may be considered to be stereoregular alternating copolymers of ethylene and propylene. The cmr spectra of such copolymers have been investigated [14,15,18]. Differences observed in the methyl and methylene carbon resonances were observed for the isotactic and syndiotactic polymers. The parent polymers may be considered to be alternating stereoregular copolymers of acetylene with propylene. Their spectra also revealed stereochemical differences. 

Such polymers can be considered to be stereoregular copolymers, and their nmr spectra can be analyzed to obtain information about the relative amoxmts of cis and trans inits present and their arrangement along the polymer chain. The structures of polymers derived from cyclopentene [87-89], cyclooctadiene [71,90] and bicyclo-2,2-1-octene [89,91] have been studied by nmr spectroscopy, for example. The relative amounts of cis and trans units present vary with reaction conditions, but the cis and trans units are generally distributed randomly along the polymer chains. Ivin and coworkers [91] report that a tendency toward blocki-ness develops at long reaction times, however. 

It should be noted that there are two ways of incorporating bicyclo-2,2,l-octene units into polymers. It is necessary to consider the distributions of the asymmetric centers present in polymers derived from this monomer in addition to considering the cis and trans structures present, to completely interpret the spectra of polymers derived from bicyclo-2,2,1-octane. Polymers derived from unsymmetrical cyclic olefins, in general, should not be e3q>ected to be stereoregular. However, cmr and pmr studies of polymers derived from 1-methyl cyclobutene and 1-methyl-trans-cyclooctene have predominantly head-tail structures [93,94]. Since they contain both cis- and trans- inits, they can be considered to be stereoregular copolymers. 

STEREOREGULAR COPOLYMERS PREPARED FROM CHIRAL MONOMERS... 

When copolymers are prepared by ring opening polymerization of chiral monomers such as cyclic ethers and thioethers, lactones, lactams, N-carboxy anhydrides, etc., the chirality of the monomers is often retained in the repeating groups incorporated into the copolymers. A few stereoregular copolymers prepared this way have been analyzed by nmr spectroscopy, but much remains to be done in this area. 

The concept of the optically active aromatic chromophore as conformational probe in stereoregular copolymers can be further extended by employing optically active aromatic monomers. Thus the coisotactic copolymers of (R)-U-phenyl-l-hexene with U-methyl-l-pentene [29] (Figure 7) can be used for correlating the sign of CD of the aromatic Il-electrons system with the macromol-ecular conformation. Both the above copolymer and poly[(R)-U-phenyl-l-hexenej show negative CD between 270 and 250 nm ( Lj -band)... 

With the asymmetric site in the position to the double bond (monomer LXVIb) the polymerization occurs easily with good yield but without any stereoselectivity in contrast the polymerization of monomer LXVIa yields crystalline polymers, with a low yield due to steric interactions between active sites and the side chain as for jS-branched-a-olefins [168]. The chiroptical properties of these stereoregular copolymers compared to those of OA phenylalkanes (LXVlc, d) [169b] show that the homopolymer of (LXVl) as well as its copolymer contain block sequences in helical conformation with a predominant direction of the screw, which seems to be the right handed one. 

This contrasts with the results published by Chiellini etal [173] about stereoregular copolymers of racemic 5-methyl-1-heptene with OA 2-methylbutylvinylether. From adsorption chromatography data on poly(L)-lactide which leads to optically inactive fractions, it seems doubtless that the polymerization is stereoselective, even if at least partially isospecific that means able to give prevailingly isotactic polymers. 

Up to now, for stereoregular copolymers, non-linear variation has been considered as resulting from conformational effect as in the case of copolymers of chiral and achiral units [59, 60]. As shown above, for polymers with very low probabilities for regular secondary structures in solution, no macromolecular effects can be reasonably taken into account and another origin must be suggested. 

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