Poly syndiotactic content

In a few instances, poly(methylmethacrylate) has been prepared exceeding the syndiotactic content attainable through a free ion pair or free radical mechanism at the same temperature [20]. A possible mechanism for homogeneous syndiotactic propagation has been proposed. However, none of these highly syndiotactic systems has been reproducible [10], and it appears to be no real need for such a mechanism. Coordination-directed stereospecific polymerization of methyl methacrylate seems to be limited to isotactic propagation. 

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. 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

Poly(vinyl chloride) Tg increased with syndiotactic triad content (28% to 43%), and decreased with isotactic triad content (13% to 21%), with lowest and highest values of 352K and 370K, respectively, observed for the complete set of 19 samples [150], ... 

In the present study, syndiotactic polypropylene was supplied by FINA Chemical and Oil Company, having the reported weight-average and number-average molecular weights of sPP of 174,000 and 74,700 gmol , respectively, with a poly-dispersity (Mw/Mn) of 2.3 and 92.6% content of syndiotacticity. Poly(ethylene-octene) copolymer was provided by the Dow Chemical Company (Mw = 41,800 g mol , Mw/Mn = 2.26, and octene content = 10% by weight). 

Poly(methyl methacrylate) which has a low heterotactic content compared with isotactic and syndiotactic one was first called a stereoblock polymer, but afterwards it was found that the pdymer often was a mixture of isotactic and ndiotactic polsmaers. The two tactic isomers in the mixture could be recog nized by thin layer chromatography on silica-gel, and recently separated by a competitive adsorption method. ... 

Boyer also reported a transition at still higher temperature, T p Tll + 50 1.47 g, which he named the intramolecular relaxation transition, which separates structured polymer melts from true liquids [Boyer, 1977, 1980a,b, 1985, 1987]. This high-temperature transition may be related to melting. Formally, liquids remain supercooled below the melting point, 1.5Tg [van Krevelen, 1997]. Detection of this temperature is contingent on the polymer achieving sulHcient crystalline content. For example, poly(vinyl chloride), usually treated as an amorphous polymer, shows Tm = 444 to 452 K when the crystalline content of the syndiotactic isomer is about 2wt% [Marshall, 1994]. 

Figure 4-24. Dependence of the reciprocal molar optical rotaton,, with tacticity content of isotactic, x, and syndiotactic, Xs, diads for poly (S)-4-methyl-l-hexene. Optical purity of initial monomer was 93% (according to measurements by P. Pino et al).

Figure 6-11. Negative specific enthalpy for the stereocomplex formation from st- and it-poly(methyl methacrylates) in o-xylol (O) or dimethyl formamide ( ) at 25°C as a function of the mass fraction of the syndiotactic polymer. The dashed line gives the relationship for a plot against the syndiotactic heptad content (after data from W. Siemens and G. Rehage).

Chlorination of poly(vinyl chloride) destroys the regularity of the tacticity. Dor-restijn et al. [123] executed chlorination in two different ways. First, pol>tvinyl chloride) was chlorinated in solution so that no preference existed for the kind of hydrogen atom which was replaced by chlorine atoms the tacticity of the polymer molecules is destroyed more or less completely. Second, the polymer was chlorinated in powder form in this case there is a large preference for the amorphous parts, whereas the crystalline parts are mudi less available for chlorination and remain more or less intact Even if the polymer is attacked prrferentially at the isotactic sequences, the tendency to form gels will decrease, as the [nresence of an even number of isotactic sequences in between syndiotactic sequences causes an enormous increase in the crystallinity of polyfviny chloride) [82,84]. Dorrestijn et al. showed that chlorination had a dramatic influence on the tendency to form gels in the case of solution chlorination, whereas powder chlorination had only a minor effect despite much higher chlorine contents. 

Syndiotactic poly-1,2- More than 88% 1,2 content amorphous hydrogenation produced [148]... 

Thus, it was not until 1990 that the group of Kaminsky and Arndt-Rosenau took a more detailed look at the homopolymerization of norbornene and the structures of the resulting polymers. Driven by the growing interest in copolymers with high norbornene contents and high glass transition (Tg) temperatures, as well as the unusual properties of PNBs, Arndt-Rosenau et al. used the hydrooligomerization technique to produce saturated model norbornene dimers and trimers with metallocene catalysts known to produce atactic, isotactic, and syndiotactic poly(a-olefins) (1-3, Table 16.1). 

The large majority of stereoselective acrylate polymerizations afford either isotactic or syndiotactic-biased material and since heterotactic polymers have only been prepared on rare occasions, they are not covered in depth in this chapter. The principal initiating system for the production of heterotactic poly(alkyl methacrylates) is a stoichiometric 1 1 mixture of tcrt-butyllithium and bis(2,6-di-tert-butylphenoxy)methylaluminum. The heterotactic content is particularly pronounced at low temperatures and increases in the order methyl < ethyl < propyl < allyl methacrylates. For example, at -95 °C, poly(allyl methacrylate) with 95.8% mr triad content is obtained after reduction to poly(methacrylic acid) and subsequent methylation, highly heterotactic PMMA results. ... 

Although the free-radical polymerization of MMA typically exhibits a syndiotactic bias (rr triad content = 60%-70%), it has long been known that the stereochemical interactions between the chain-end radical and vinyl monomers in free-radical polymerization can be modified by using chiral protecting groups. For example, the 80 °C AIBN-initiated polymerization (AIBN = 2,2 -azobisisobutyronitrile) of oxazolidine acrylamides based on valine and t rt-leucine ultimately yields highly isotactic (92% m dyad content) poly(acrylic acid) and PMA after chemical modification (Scheme 23.23). " ... 

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