Unsymmetrically substituted, atactic

In sterically random propagation, the probability of meso diad generation Pm in each addition is equal to 0.5, and the ratio of triads in the chain is i h s = = 1 2 1. In an atactic polymer, the diads m and r are statistically distributed in the chain. Natta [81] called the simplest stereoregular chains isotactic —mmmm— and syndiotactic —rrrr—. Monomers with both a and / carbons unsymmetrically substituted can form diisotactic polymers... 

The double bonds which form part of the main chain may be either cis or trans, and in the case of the prochiral monomers norboraene, norboroadiene, their symmetrically substituted derivatives and their chiral unsymmetrically substituted derivatives the residues may be enchained in such a way as to yield tactic or, more commonly, atactic polymers [54]. A representation of atactic poly(norbomene) is shown in Scheme 9, where cis and trans double bonds are associated with r or m dyad units respectively. 

Unsymmetrical Alkyl-Substituted Polysilylenes A detailed comparison of theoretical predictions and experimental results for the atactic polysil-ylenes is more diflScult for several reasons (1) the observed transitions are much broader, (2) the effects of random substitutional disorder are not included in the theory, and (3) the magnitudes of the consequences of stereochemical disorder are expected to vary for different atactic polymers. Nevertheless, for all the asymmetrically substituted polysilylenes studied, except poly(n-dodecylmethylsilylene), the predictions discussed earlier... 

When a,a substitution is unsymmetric and the catalyst produces an optically neutral polymer, a drastic drop in melting point is observed as a function of the ratio of the number of carbons in the substituents (37). However since the materials studied must have been nearly atactic the true crystalline melting point of these materials must be considered unknown. 

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