Ketone constructed by alkyl bridge strategy for LED-sensitive free radical photopolymerization

Abstract

To enhance the photosensitivity of ketones, which are prone to side isomerization reactions during free radical photopolymerization, a novel strategy involving the use of an alkyl bridge was introduced. This approach was assessed by comparing the performance of two ketone photoinitiators: (E)-2-((1H-pyrrol-2-yl)methylene)-4-methyl-3,4-dihydronaphthalen-1(2H)-one (PDN) and (2E,2′E)-2,2′-((octane-1,8-diylbis(1H-pyrrole-1,2-diyl))bis(methaneylylidene))bis(4-methyl-3,4-dihydronaphthalen-1(2H)-one) (bis-PDN). The synthesis of bis-PDN involved linking two PDN molecules via a long alkyl chain derived from 1,8-dibromooctane. It was shown that this alkyl bridge not only provides steric hindrance that limits the isomerization of PDN but also imparts a significant increase in the molar extinction coefficient of the newly synthesized bis-PDN to 43,779 M⁻¹cm⁻¹ by following the strategy of multiple chromophores. Consequently, bis-PDN demonstrated a notably high initiating efficiency for LED-sensitive polymerization. The efficacy of this alkyl bridge modification was further validated through DFT calculations, steady-state photolysis, NMR testing, polymerization kinetics analysis, and 3D printing experiments. The alkyl bridge technique offers considerable promise for the development of ketone compounds with enhanced photo-activity.