“Push–push and push–pull” polystannanes

Abstract

The synthesis and characterization of polystannanes with “push” or “pull” moieties attached to the tin backbone are described. Precursor tetra aryl- (1, 2) stannanes were converted to mono- (3) and dichloro- (4, 5) stannanes by either sequential chlorination or by redistribution reactions with SnCl₄. Compounds 4 and 5 were transformed to polymerisable tin dihydride monomers 6 and 7 using a large excess (10×) of NaBH₄. Homopolymer 8 with electron donating aryl substituents (p-MeOC₆H₄-) was synthesized by dehydrogenative polymerization using Wilkinson's catalyst. Attempts to prepare the homopolymer 9 with electron withdrawing aryl substituents (p-CF₃C₆H₄-) from the dehydrocoupling of 7 using similar conditions led only to the formation of low molecular weight oligomeric species. Two alternating polymers, 10 and 11, were synthesized by condensation polymerization of (n-Bu)₂Sn(NEt₂)₂ with monomers 6 or 7. The first was a “push–push” alternating polymer, 10, comprised of a repeating unit consisting of two different electron donating groups (p-MeOC₆H₄-, n-Bu) at neighboring tin centres. The second was a “push–pull” alternating polymer, 11, bearing both an electron donating group (n-Bu) and a strongly electron withdrawing substituent (p-CF₃C₆H₄-) at neighboring tin atoms. All small molecule stannanes and tin-containing polymers were characterized by NMR (¹H, ¹³C, ¹¹⁹Sn, and where required ¹⁹F) spectroscopy, MS or EA. The absolute molecular weights of tin polymers (8, 10, 11) were determined by triple detection GPC and in the range of 1.07 × 10⁴ to 1.95 × 10⁴ Da. Rapid photodegradation of polymers was observed by UV-Vis spectroscopy, with a slower degradation observed for the “push–pull” polymer, 11, compared to the “push–push” polymer, 10.