A case of conformational isomorphism in polymers. The crystal structure of isotactic poly-(S)-3-methylpentene-1

Conformational analysis of chiral polymers in solution

“…In case of vinyl aromatic copolymers calculations were performed by assuming for the chromophores the same geometry as in the helical chain found for the corresponding homopolymers in the crystalline state with left or right handness depending on the (S) or (R) absolute configuration of the 1-olef in enantiomer used as comonomer,respectively. Indeed according to previous deductions (S)-side chains induce a lefthanded helicity, whereas the right handness is generated by (R)-olefins (8). The aromatic chromophore, in the region of the electrically allowed -'B transition, was described by two oscillators, placed in the middle of the aromatic ring and directed as in the corresponding methylsubstituted aromatic hydrocarbons.…”

“…It is important to point out that the few conformations allowed (the ones with the lowest internal energy) for the "conformational model" (7) correspond to those of the tepeating unit when assuning for the isotactic macromolecule an one screw sense helical chain conformation, the selected screw sense corresponding to that experimentally observed in the crystalline state for the same monomer enantiomer (8). The validity of the assumption, that the predominant screw sense foreseen on the basis of "a priori" conformational analysis (6) and experimentally observed by X-ray is the sane in solution, was nicely confirmed by measuring the rotatory power in solid state which resulted practically the same as in the solution (9), at least when the macromolecules assume in the lattice helical conformations of one single handness.…”

“…Clearly the conformation in solution is characterized by helical blocks with shorter average length than in the crystals and connected through confornational reversals (8,10). Thus: it can be considered as consisting of a succession of chain sections spiralled in both screw senses; the average length of the sections in the favored screw sense markedly prevailingly.…”

“…Further support to such hypothesis derived from the calculation of the rotatori power of low molecular weight models and polymers; in case of these last agreement between experiments and calculations was indeed observed only when assuming the predominant screw sense for the: isotactic chains(6) (Tabl& 1). The driving force responsible for the prevalence of a single screw sense was proposed to be the cooperative interaction among chiral side chains disposed along the isotactic backbone (8,11). Accordingly the rotatory power absolute value decreases when going to less stereoregUlar fractions (6,8) and by random insertion of ethylene units in the poly-1-olef in chain (11); both reducing the average length of isotactic blocks.…”

“…The driving force responsible for the prevalence of a single screw sense was proposed to be the cooperative interaction among chiral side chains disposed along the isotactic backbone (8,11). Accordingly the rotatory power absolute value decreases when going to less stereoregUlar fractions (6,8) and by random insertion of ethylene units in the poly-1-olef in chain (11); both reducing the average length of isotactic blocks. Interestingly, the above cooperative effect is maintained by random insertion of units deriving from a nonchiral 1-olefin, in the isotactic chain derived from the chiral 1-olef in, the effect being more evident if the former has similar conformational properties as the chiral comonomer.…”