Effect of chirality on pi-stacking in styrene and maleimide alternating copolymers

The effect of different chiral sequences in alternating copolymers of styrene and maleimide, poly(styrene-alt-dimethyl-N,N-propylamide) (SMI) was studied. The methods used for the SMI also applied to the general class of alternating copolymers of styrene and various maleimides. Only racemo-diisotactic polymers can associate into complexes because of the symmetrically distributed phenyl groups and maleimide monomers. This polymer is isotactic with respect to the chiral site of the styrene monomers (R, S) and isotactic with respect to the chirality of the maleimide racemo (RR, SS)]. The maleimide, racemo (SS or RR) chirality, and the complementary S or R chirality in the styrene moiety form the repeating unit of racemo-diisotactic SMI. Structural deviations from the isotactic configurations induces an asymmetry in the distribution of styrene monomers and introduce bends in the polymer backbone, which prevent association through pi-stacking interactions. Semiempirical PM3 calculations on the SMI polymer have shown that this pi-stacking occurs when the polymers are separated by a distance about 10 A, and each pi-stacking pair formed from the styrene monomers has a stabilization energy of about 12 kJ/mol.