Direct and sensitized photolysis of phosphine oxide polymerization photoinitiators in the presence and absence of a model acrylate monomer: A time resolved EPR, cure monitor, and PhotoDSC study

The effect of triplet sensitizers on the photoinitiated polymerization (cur e) of a model acrylate monomer, isobornyl acrylate (IBOA), has been investi gated. Time-resolved electron paramagnetic resonance (TR EPR) spectroscopy was employed to investigate the initiation of polymerization. Cure monitori ng and photodifferential scanning calorimetry (photoDSC) were employed to f ollow the course of the polymerization. Thioxanthen-9-one (TX) and 2-isopro pylthioxanthen-9-one (ITX) were found to be effective sensitizers of the ph otopolymerization, which was initiated by radicals produced from (2,4,6-bim ethylbenzoyl)diphenylphosphine oxide (TMDPO) and bis(2,4,6-trimethylbenzoyl )phenylphosphine oxide (BAPO). TR EPR experiments demonstrated that the mec hanism of sensitization involves T-T energy transfer from TX (or ITX) to TM DPO or BAPO followed by formation of radicals by cc-cleavage of the photoin itiators. Direct photolysis of TMDPO and BAPO results in an absorptive chem ically induced dynamic electron polarization (CIDEP) pattern due to the tri plet mechanism (TM) of polarization of the substituted benzoyl and P-center ed radicals produced by ct-cleavage of the photoinitiators. TR EPR demonstr ates that the same radicals were produced during direct and sensitized phot olysis. However, a different CIDEP pattern is produced by photosensitizatio n, namely an emissive/absorptive (E*/A) pattern. A TR EPR study of solution s containing phosphine oxide initiators and IBOA under direct and photosens itized conditions demonstrated that the polarized primary P-centered radica ls add to the double bond of IBOA with the formation of polarized secondary radical adducts. Both primary and secondary radicals exhibit the same pola rization pattern as the primary radical precursors, i.e., A in direct photo lysis and E*/A in the presence of a sensitizer. The rate of polymerization of neat IBOA was followed by cure monitoring. In the presence of ITX the ra te of cure increases significantly compared to direct photolysis of same. T he heat evolved in the polymerization of IBOA photoinitiated (direct and se nsitized) with TMDPO was monitored by photoDSC, and at early times was foun d to be higher in the sensitized photopolymerization. Time-intermittent UV irradiation allowed an estimation of the ratio of termination to propagatio n rate constants (k(t)/k(p)) during dark periods of polymerization. The obs erved decrease of k(t)/k(p), with the progress of polymerization is discuss ed. The results suggest that photosensitization may provide a means of mani pulating and controlling the parameters of photocuring of acrylates.