Highly syndiotactic copolymer

Highly syndiotactic copolymer prepared with t-C4H9Li-R3Al Preparation of highly... 

For the syndiotactic copolymer, highly extended conformations with c values of 7.5-7.6 A can be obtained for a glide plane symmetry when... 

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

The rationale for this approach further involved the hypothesis that swelling in water occurs by virtue of water uptake in the amorphous regions of the polymers with network structure provided by relatively water-insensitive crystallites in the polymers. In this approach, vinyl trifluoroacetate was polymerized or copolymerized to provide highly syndiotactic thermoplastic polymeric intermediates that were converted to the corresponding syndiotactic PVA homopolymers or copolymers by solvolysis with mild nucleophilic reagents in media that were not solvents for both starting and end-product polymers. 

Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed. One of them is /-propylene(ri -cyclopentadienyl-r fluorenyl)zirconium dichloride [291]. Initial disclosures of metallocene catalysts were followed by numerous publications in the literature that described similar materials for the polymerizations of either ethylene or propylene, or both, and for formation of various copolymers. Thus, for instance, Kaminsky et al. [292], reported preparaticm of a zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic olefins are cyclopentene, norbomene, and the hindered cyclopentadiene adducts of norbomene [292]. The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane ... 

Group-transfer polymerizations yield very narrow molecular weight distribution polymers. When mixtures of monomers are used, random copolymers form. The polymerization reaction is very tolerant of other functional groups in the monomer. Thus, for instance, p-vinylbenzyl methacrylate is converted to poly(p-vinylbenzyl methacrylate) without the polymerization of the vinyl group [397]. In addition, it is possible to form polymers with high syndiotactic content. 

When carried out using a syndioselective catalyst system, such as CpTiCb/MAO, the copolymerization of the styrene-terminated macromonomer, M4, with styrene can lead to the formation of graft copolymers with highly syndiotactic polystyrene main chains. The melting point of the graft copolymers decreases with an increasing number of grafts per main polymer chain. 

N.m.r. has been used to probe various aggregation phenomena. Heatley and Begum observed that mieellization of styrene-butadiene-styrene tri-block copolymers in non-solvents for butadiene is accompanied by marked hindrance to butadiene mobility. Bovey et a/. have shown that C Ti s for poly(vinylidene fluoride) and PMMA are the same in a mixed solution as in single solutions. The unusual compatibility of these two polyn rs therefore does not arise from complex formation. Self-association of stereo-r ular PMMA s in dilute solution has been studied. In toluene, isotactic PMMA is 10—17% associated at 27 C whereas a highly syndiotactic sample is 76% associated. 

Highly syndiol ctic and living polymers of various alkyl methacrylates, such as ethyl, isopropyl, butyl and isobutyl methacrylates, could also be obtained by the polymerization in toluene with t-C4HgLi-(n-C4Hg)3Al at -78 0 [2]. Thus the highly syndiotactic random and block copolymers of various methacrylates can be prepared easily in toluene with -C4HgLi-(/j-C4Hg)3Al at -78 >C. Polymerization of EMA with... 

Complex 6.21 represents a class of precatalysts that are active for both ethylene and propylene polymerization reactions. When Ar is phenyl, highly syndiotactic PP of low molecular weight is obtained. However, when Ar is pentafluro phenyl, highly syndiotactic living PP is the product. The same catalyst can also produce ethylene-propylene block copolymers. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral ... 

---