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The peculiar conformational characteristics of cis ‐poly( tert ‐butlacetylene)
Author(s) -
Ferro Dino R.,
Provasoli Augusto,
Costa Giovanna
Publication year - 1996
Publication title -
macromolecular theory and simulations
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.37
H-Index - 56
eISSN - 1521-3919
pISSN - 1022-1344
DOI - 10.1002/mats.1996.040050206
Subject(s) - conformational isomerism , chain (unit) , crystallography , chemistry , polymer , helix (gastropod) , force field (fiction) , materials science , physics , molecule , ecology , organic chemistry , quantum mechanics , astronomy , snail , biology
Abstract Molecular mechanics was applied to investigate the conformational structure of cis ‐poly( tert ‐butylacetylene) ( c ‐PTBA). The program CHAMP, adopting a MM2‐based force field, was used for a thorough search of the minimum‐energy conformers of oligomers, helices and long segments of the c ‐PTBA chain. The results show that short oligomers are not good models of the polymer, as their preferred conformations are not allowed inside the chain. Segments of 8 3 and 8 5 helices appear as the most probable feature. Junctions between right‐ and left‐handed helices can occur at the cost of 1,7 kcal/mol: such defects may run through the chain, the barrier to their shift being 7,8 kcal/mol. Other conformational defects, having energies in the range of 1–3 kcal/mol above the minimum, form knuckle‐joints in the helix, changing the axial direction by 70–80°. The present calculations suggest a disordered model of c ‐PTBA in solution, made of rather stiff helical segments. Such a picture corresponds to a chain less rigid than proposed by previous computations on substituted polyacetylenes, and seems consistent with the observed physical aging of c ‐PTBA films. Preliminary packing calculations of right‐ and left‐handed helices lead to density values not much higher than observed, indicating rather low degrees of disorder and free volume in the solid state.