Poly , tactic variations

Using the methods of amorphous halo separation from the narrow reflex, Figure lib shows d(T) dependences for poly(decamethylcyclohexasiloxane) of different tacticity. Figure lib indicates that tacticity variation affects the values and shape of d(T) dependence of amorphous halo. At 20°C, the difference of studied samples in d is insignificant, but at 197 -... 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different values of Tg, depending on whether it is isotactic, syndiotactic, or atactic. See Table 3 for a collection of literature data (1) on the effects of tacticity on Tg. A theoretical analysis of the effects of tacticity variations on Tg has been provided (52). 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg = — 108°C) and trans(Tg = — 18°C) or polyisoprene cis Tg= —73°C) and trans T = —53°C). An example of tacticity variation is poly(methyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

Figure 4-22. Variation of the reciprocal molar optical rotation [0] with tacticity jCf and for poly[(S)-4-methyl-l-hexene] polymerized under various conditions from a monomer of 93% optical purity. Tacticity was determined by IR. (After P. Pino et al.)...

On the basis of our prior work on PO polymerization, with this same chelate catalyst , TOO polymerizes about 10 times more slowly than PO does under the same conditions. Also, the TOO homopolymer is much less soluble than poly(propylene oxide), since the latter polymer is soluble in heptane and methanol, both nonsolvents for poly(trimethylene oxide). The TMO homopolymer is, of course, crystalline because of its very regular structure. On the other hand, the poly(propylene oxide) prepared with the chelate catalyst is largely amorphous because of tacticity and head-to-tail variations in structure which are not possible in poly(tri-methylene oxide). 

The invariance of relaxation frequency with molecular wei t may not be apparent at finite concentrations of polymer, because cofl — c< interactions can effect the relaxation of internal modes as well as the first order, rotational mode. There have been a number of studies of the concentration dependence of dielectric loss processes some of which show weU the continuous trend in behawour from dilute solution to the bulk state. Temperature variation can provide useful information on the enthalpies of activation of local mode motions. Finally, since such local modes are very structure sensitive, differences in chain tacticity would be expected, and do cause changes in loss behaviour. This prediction has been authenticated for such polymers as poly(methyl methacrylate) and poly(ethyl acrylate). ... 

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