Nonmonotonic dependence of intramolecular charge-transfer sidechain interactions for triazole containing phenylene-ethynylene grafted Co-Polyoxetane brushes

Abstract

This research explores effects of introduction of the rigid phenylene ethynylene (PEy) moiety as a side chain in polyoxetanes. The synthesis of PEy-grafted polyoxetanes was carried out via Huisgen copper (I) catalyzed alkyne-azide cycloaddition. These polymers have a flexible main chain (low Tg polyoxetane) and rigid side chains (phenylene ethynylene). Compositions for copolyoxetanes with PEy and alkynyl side chains were determined by 1H-NMR spectroscopy. Mole percents PEy, designated P-%, were P-21, P-44, P-69 and P-100. UV-VIS spectra for P-21, P-44, P-69, P-100 at 330 nm showed differentiation based on assignments to intramolecular charge-transfer (ICT) interactions between triazole and phenylene ethylene moieties. Interestingly, instead of a monotonic increase, absorption intensity is in the order P-69 ~ P-44 > P-100 > P-21. Low substitution and mainchain flexibility and account for P-21 having the lowest UV-VIS absorption at 330 nm. P-44 and P-69 have similar strong absorptions at 330 nm with overlapping curves. Steric effects culminate in P-100, which has the highest PEy content but a lower absorption at 330 nm than P-69 but higher than P-21. A model for optical absorptions for PEy copolyoxetanes is described that includes the effect of the rigid PEy side chain in decreasing molecular mobility.