Tacticity effects transition

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

The effect of tacticity of the polymeric backbone on properties of POCT-4 has been searched for [26], Figure 4 shows curves for PMCS-4 with different backbone structure, obtained by the diffe-rential scanning calorimetry (DSC) method (curves 1 - 4). For atactic polymer the only transition, corresponded to Tg, is observed on curve 1, above which, in accordance with the data of X-ray dif-fraction analysis, the polymer is amorphous (two amorphous haloes at 20 = 8-11° and 20-35°). As the polymeric backbone is enriched with... 

The effect of chain tacticity on the phase states of POCS-6 has been studied. On the example of two POCS-6 polymers, the influence of configuration sequences on thermal transitions has been investigated [52], For this purpose, cis- and trans-isomers of 1,7-dihydro-xydecamethylcyclohexasiloxane, 1,7-dichloro-1,7-dipheny 1-3,3,5,5,9,9,11,11 -octamethyl-... 

The glass transition temperature of amorphous polymers is a function of the chemical structure of the polymer chain. It varies widely with the types of skeletal atoms present, with the t T)es of side groups, and with the tacticity of side groups along the polymer backbone. Table 14.11 demonstrates the effects of structural variations on the crystalline melting temperature and glass transition temperature for several polymers. 

The beauty of the tactic is that it can be used effectively against enzyme reactions involving two substrates. With such enzymes, inhibitors based on one substrate or the other could be designed, but neither will be as good as an inhibitor based on a transition-state analogue where the two are linked together. The latter is bound to have more bonding interactions. 

The former pointed out a stereocontrol by hydrogen bond in the transition state as the cause of the solvent effect. Recently, Yamamoto et al.90 investigated the solvent effect on the tacticity of poly (vinyl acetate) by 13C-NMR in various solvents, indicating that there was no solvent effect on the stereoregularity. 

Grohens, Y., Hamon, L., Carriere, R, HoU, Y., and Schultz, J., Tacticity and surface chemistry effects on the glass transition temperature of thin supported PMMA films. Mater. Res. Soc. Symp., 629, FF171-FF177 (2000). 

Dielectric relaxation spectra of poly(methyl acrylate) (IfA) and poly(t-butyl acrylate) (tBA) were measured at temperatures above and below Tg, and both a- and 3-relaxation processes were observed. As for the 3-relaxation process, in order to clarify the quantitative relationship between the relaxation mechanism and the polymer structure, the effective dipole moment(Pg) was estimated by a method according to the 2-state transition theory. In the estimation, the average local configuration of the main chain was assumed to be in isotactic form or syndiotactic form. Since samples used were atactic polymers, the authors assume that Pg(atact) = Xi Pe(i) + (1 - X ) Pe(s)> where X denotes the tacticity, i represents isotactic form, and s, sytidiotactic form, respectively. And, the activation energy for the atactic form sample is examined in a similar way. From the results, it can be concluded that the 3-relaxation of samples is attributed to the restricted rotation of the side chain, especially, to the rotation of the first bond-axis connecting the side chain and main chain. 

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