Significantly improve the photoinitiation ability of hydroxyalkyl-derived polymerizable α-hydroxyalkylacetophenone photoinitiators by blocking hyperconjugation

Abstract

Five ester derivatives from hydroxyalkyl of 2-hydroxy-2-methyl-1-phenyl propanone, one of the most successfully applied α-hydroxyalkyl acetophenone photoinitiators, are synthesized. From the spectroscopy results and theory calculation, it is proved that the much lower photocleavage rates of the directly linked unsaturated ester derivatives are due to that the strong electron withdrawing effect of carbonyl increase the bond strength between the benzoyl and quaternary carbon and the effective energy transfer from benzoyl to double bond because of the existence of p-σ-π hyperconjugation, which consequently decreased their initiating ability. Excitingly, simple introduction of two-methylene spacer between the carbonyl and alkoxyl can effectively block the hyperconjugation and significantly increase the photocleavage rate up to the same level of their mother commercial photoinitiator, which can open a gate for design of new high efficiency and polymerizable type I photoinitiators.