Macromol. Symp. zyxwvut 146, 251-257 (1999) zyxwvu 251 zy New Conformations and New Types of Helix Sense Reversals and Defects in the Chains of Nonchiral Poly(alky1 isocyanates). zyxw Natalia V. Lukasheva*, Satu Niemela**, Igor M. Neelov*,Anatolii A. Darinskii*, Franciska Sundholm**, Robert Cook*** *Institute of MacromolecularCompounds of Russian Academy of Sciences 199004 Saint Petersburg, Bolshoy pr., 31 zyxw , Russia, e-mail luk@imc.macro.ru **Polymer Chemistry Department, Helsinki University, PB55, A.I. Virtasen Aukio 1, FIN-00014, Helsinki, Finland ***LawrenceLivermore National Laboratory, Livermore, California, USA SUMMARY: Molecular mechanics studies of helix sense reversals and defects of the regularity in the helices of the same sense in the chains of poly(alky1 isocyanates) were performed. The effect of medium was studied by using different values of the dielectric constant. Additionally to the reversals between the helix mirror images (MI) considered in the literature new types of reversals have been investigated. These reversals connect the helices having backbone torsion angles not only with opposite signs but also with inverse angle values (MII). For the MI reversals we report in addition to the small angle break conformations known from literature the discovery of new conformations of similar energy but with large angle breaks. Large angle reversals can be responsible for up to 50% of the chain persistence length in non-polar solvents. In vacuum MI1 reversals and defects have energies higher than MI reversals because of the strong electrostatic repulsion. At E =3.5 some defects have energy even lower that that of MI reversals. Introduction Polyisocyanates (PIC) with n-alkyl pendant groups known to have a relatively stiff, helical backbone structure due to a competition between electronic and steric factors The helices may be either left-handed (L) or right-handed (R). For nonchiral PICs (with symmetrical side groups) L and R helices have equal probabilities. For chiral PICs with a chirotropic carbon in side groups helices of one sign are more preferable energetically, leading to interesting optical property 7-9). PICs in solution 2-4) show a significant flexibility at high molecular weights, consistent with worm-like persistence lengths of from 20 to 60 nm, depending upon solvent. Two possible causes for the flexibility of the PIC'S chains have been discussed 'O-'') : (1) cumulative effect of torsion and angular librations and (2) the existence of helix reversal "breaks" along the chain backbone which might be considered as defining a freely rotating chain. Every break changes the helix direction on a definite angle (0). The contributions of the both mechanisms to the experimental value of the persistence length a can be evaluated by the equation 13) : zyxw 0 WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 CCC 1022-1360/99/$ 17.50+.50/0