Conformational control by quaternary centres: theory, database evidence and application to polymers

Abstract

The conformational effects of the quaternary centre in (RCH₂CH₂)₄X_q species are studied by molecular mechanics calculations on hydrocarbons, ammonium ions, and related species, and the results verified by data on quaternary ammonium ions from the Cambridge Structural Database. Prⁿ₄C and Prⁿ₄N⁺ have just two low-energy structures, with D_2d or S₄ symmetry. All other conformations suffer from g⁺g^– non-bonding interactions and will be populated to the extent of <5% at ambient temperatures. These non-bonding interactions affect both the inner torsion angles, CH₂–CH₂–X_q–CH₂ and the next set of torsions, -CH₂–CH₂–CH₂–X_q, but the third set of torsions away from the quaternary centre is unaffected. Two competitive mechanisms for interconversion of the D_2d and S₄ conformations are proposed on the basis of molecular mechanics calculations. Polymers [(RCH₂CH₂)₂C(CH₂)₂]_n and [(RCH₂CH₂)₂C(CH₂)₃]_n are strain-free with controlled conformations for the -(CH₂)₂- and -(CH₂)₃- segments. In polymers containing simple alkyl side chains, there are two energetically similar conformations associated 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 S₄ local symmetry. Poly[(1,1-di-R)butane-1,4-diyl]s (R = Et, Pr, Bu and PhCH₂CH₂) have been prepared by ring opening metathesis polymerisation of 3,3-dialkylcyclobutenes, followed by diimide reduction, and their physical properties are in accord with these predictions. More highly structured side groups can act as extra conformational control elements, and the preparation of monomers and some polymers with these extra features is reported. Planar aromatic side groups like fluorene favour aaaa conformations, cyclohexyl side groups disfavour aaaa, but do not strongly discriminate between ag^±aa and ag^±g^±a, adamantane side groups strongly favour ag^±g^±a, and a chiral ag⁺aa sequence is favoured for a polymer from (1R,2R,5R,7R)-2,8,8-trimethyltricyclo[5.1.1.0^2,5]non-3-ene, itself derived from α-pinene. Studies directed at preparing some structurally-related ketal polymers and a potential covalent ketal network are also described.

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