CONFORMATIONAL CONTROL BY QUATERNARY CENTERS - THEORY, DATABASE EVIDENCE AND APPLICATION TO POLYMERS

The conformational effects of the quaternary centre in (RCH2CH2)(4)X-q species are studied by molecular mechanics calculations on hydrocarbo ns, ammonium ions, and related species, and the results verified by da ta on quaternary ammonium ions from the Cambridge Structural Database. (Pr4C)-C-n and (Pr4N+)-N-n have just two low-energy structures, with D-2d or S-4 symmetry All other conformations suffer from g(+)g(-) non- bonding interactions and will be populated to the extent of <5% at amb ient temperatures. These nonbonding interactions affect both the inner torsion angles, CH2-CH2-X-q-CH2 and the next set of torsions, -CH2-CH 2-CH2-X-q, but the third set of torsions away from the quaternary cent re is unaffected, Two competitive mechanisms for interconversion of th e D-2d and S-4 conformations are proposed on the basis of molecular me chanics calculations. Polymers [(RCH2CH2)(2)C(CH2)(2)](n) and [(RCH2CH 2)(2)C(CH2)(3)](n) are strain-free with controlled conformations for t he -(CH2)(2)- and -(CH2)(3)- segments. Ln polymers containing simple a lkyl side chains, there are two energetically similar conformations as sociated with the D-2d local structure which have aaaa and ag(+/-)g(+/ -)a torsion angle sequences for the polymer chain, and two comparable sequences ag(+/-)aa and ag(+/-)g(+/-)a associated with S-4 local symme try. Poly[(1,1-di-R)butane-1,4-diyl]s (R = Et, Pr, Bu and PhCH2CH2) ha ve been prepared by ring opening metathesis polymerisation of 3,3-dial kylcyclobutenes, followed by diimide reduction, and their physical pro perties are in accord with these predictions. More highly structured s ide groups can act as extra conformational control elements, and the p reparation of monomers and some polymers with these extra features is reported. Planar;aromatic side groups like fluorene favour aaaa confor mations, cyclohexyl side groups disfavour aaaa, but do not strongly di scriminate between ag(+/-)aa and ag(+/-)g(+/-)a, adamantane side group s strongly favour ag(+/-)g(+/-)a, and a chiral ag(+)aa sequence is fav oured for a polymer from R)-2,8,8-trimethyltricyclo[5.1.1.0(2,5)]non-3 -ene, itself derived from a pinene, Studies directed at preparing some structurally-related ketal polymers and a potential covalent ketal ne twork are also described.