Influence of chain length and temperature on UV-Vis absorption and degradation behavior of poly(diethyl dipropargylmalonate) with an alternating cis-trans-1,2-(cyclopent-1-enylene)vinylene structure

Abstract

Cyclopolymerization of diethyl dipropargylmalonate (DEDPM) with Mo(N-2,6-i-Pr2-C6H3) (CHCMe2Ph)(OC(CH3)3)2 in the presence of quinuclidine yields poly(DEDPM) with an alternating cis-trans-1,2-(cyclopent-1-enylene)vinylene structure. Poly(DEDPM) n with different degrees of polymerization (n = 5–90) and PDI < 1.4 was prepared. A linear plot of number of monomers (N) added versus molecular weight as determined by light scattering (data were collected at λ = 690 nm) suggests that the cyclopolymerization of DEDPM proceeds in a living manner. A degree of polymerization (DP)-dependent UV-Vis absorption is found. At 25°C, the absorption maximum λmax for poly(DEDPM) n for n ≥ 50 is 591 nm. Thus, a maximum effective conjugation length N eff of 52 can be calculated. Increasing chain length results in a highly resolved fine structure in the UV-Vis spectrum with absorption maxima at 550 and 590 nm and a shoulder at around 513 nm for poly(DEDPM)70. In the solid state, absorption maxima were in the range of 478-574 nm for poly(DEDPM)5–90. Poly(DEDPM)10 and poly(DEPM)90 showed glass transition temperatures of 25.8 and 26.5°C, respectively. The measurement of λmax of poly(DEDPM)50 solutions in chloroform within a temperature range of 30°C reveals a reversible thermoresponsive behavior with changes in λmax of 8 nm. Poly(DEDPM) is stable under air over months in the solid state, as well as in solution using non-acidic solvents (e.g. CH2Cl2). In contrast, values for λmax for poly(DEDPM) solutions in CHCl3 decrease, if the solutions are exposed to air and light, yet remain constant in the absence of light and air. Changes in molar mass of poly(DEDPM) solutions in CHCl3 and CH2Cl2 were investigated by SEC measurements using light scattering detection and correlated with their time-dependent UV-Vis absorption behavior. Finally, poly(DEDPM)10–90 is thermally stable up to 185°C under a helium atmosphere.