Poly side-chain torsions

Farmer et al. evaluated the conformations and dynamics of poly(di- -hexylsilane). " - " The lowest energy conformer for a polymer containing eight silicon atoms was a helical arrangement with 30° torsional angles in the silane backbone. The authors also monitored different backbone and side chain torsions during dynamics simulations and concluded that the conformation present in the crystalline solid is controlled by intermolecular effects. 

Side-Chain Torsions in Poly(methylethylstloxane). Three bond rotations can be considered for the side chains of poly(methylethylsiloxane) (PMES). 

Side-Chain Torsions in Poly(meihylphenykiloxane). Two bond torsions are possible for the side chains in poly(methylphenylsiloxane) (PMPS). 

The torsional movement of the side chains is characterized by an activation energy that is strongly dependent on the type and size of the side chains. The bonds adjacent to the side groups belong to the backbone chain. At temperatures below Tg, the activation energy for backbone relaxation is very high (75 kcal/mol for poly(dimethylsiloxane) [PDMS i]). Whether local and limited relaxation occurs for the backbone at temperatures below Tg is uncertain. At temperatures above Tg, the activation energy for backbone relaxation is much lower. For PDMS, a value of 2.75 kcal/mol was determined experimentally (i). 

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

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