1-(Arylmethyloxy)anthracenes: how substituents affect their photoreactivity and ability to initiate radical and cationic polymerizations

Abstract

This study sought to aid in the development of an efficient, novel hybrid photoinitiator by examining the bond cleavage of 1-(arylmethyloxy)anthracenes 1 performing as a bichromophoric photoinitiator in the polymerization of styrene (St) and cyclohexene oxide (CHO) and classifying the cleavage as heterolytic or homolytic. The results indicated that the bichromophoric photoinitiator 1, containing no halogen or metal, induces radical and cationic polymerization reactions of St and CHO, respectively, to generate high yields of the corresponding polymers. In addition, an analysis of the effects of substituents on the photoreactivity and polymer yield of 1 indicated that the ability of the resultant radical or ion pair intermediate that was generated by the photolysis of 1 to initiate polymerization increased with the enhanced stability of the reactive intermediate. An analysis of the polymer end groups and photoproducts that were derived from specific initiators demonstrated that the arylmethyloxyl radical and arylmethyl carbocation are the reactive species involved in the initiation steps of the polymerization reactions for St and CHO, respectively. Polymer Journal (2012) 1 Much of the recent research has focused on novel photoinitiators that promote the initiation of both radical and cationic polymerizations, 2-8 because a combination of these two polymerizations could produce a hybrid polymer that is composed of radical-and cation-derived polymer units. For example, interpenetrating network polymers 9-11 combine the properties of these two polymer units and represent an attractive research area with many practical applications.Although some salt-type photoinitiators can initiate both cationic and radical polymerizations, [2][3][4][5]8,12,13 they have limited industrial application because they have low monomer solubility and require metal-or halogen-containing inorganic initiators. To overcome these limitations, nonsalt photoinitiators have been developed. Unfortunately, these photoinitiators are limited in their ability to induce hybrid polymerization. 6,7,14 A systematic study of the photoreactivity of O-substituted anthracene-9-methanol derivatives in methanol revealed that in the singlet excited state, an intramolecular charge-transfer interaction assisted in the heterolytic CH 2 -O bond cleavage to generate ion and radical pairs. 15 Therefore, these derivatives are good candidates for a novel hybrid-type photoinitiator that does not require halogen or metal. Although O-acyl anthracene-9-methanol functioned as a photoinitiator for

Nonsalt 1‐(arylmethyloxy)pyrene photoinitiators capable of initiating cationic polymerization

“…This value for 1a (0.19 in toluene and 0.15 in 1,2‐dimethoxyethane) was greater than that for 1b (0.014 and 0.015, respectively), indicating that the 2‐methoxy‐substituted pyrene initiator 1a was more reactive than its isomer 1b by a factor of ∼10 in these two solvents. The order of photoreactivity for 1a and 1b is consistent with that for the corresponding anthracene photoinitiators …”

“…Approximately same product composition was obtained from photolysis in 1,2‐dimethoxyethane: 2a (15%), 3 (4%), 4a (25%), and 5 (25%) at 50% conversion. On the basis of the criteria for the bond cleavage mode established in our previous studies, it can be concluded that heterolysis of the CH 2 –OAr bond (leading to 4a and 5 ) occurs more readily than the homolysis of the CH 2 O–Ar bond (leading to 2a , 3 , and 4a ), even in the nonpolar solvent, toluene. The addition of 10 vol% methanol in toluene and 1,2‐dimethoxyethane led to the formation of 2‐methoxy‐1‐methoxymethylnaphthalene ( 6a ) in 2% HPLC yield in the former solvent and 3% yield in the latter (Scheme ).…”

“…Previously, we found that photoproduct analysis can reliably distinguish the mode of bond cleavage for bichromophoric photoinitiators . Therefore, in this study, pyrene initiator 1a was chosen as the model, and the 1a ‐derived product distribution was examined by comparing the HPLC data for the photoproducts with those for the commercially available and independently synthesized authentic samples.…”

“…This result suggests the involvement of the pyrene ring in the initiation step of these polymerization reactions. In addition, the unexpected finding that both 1a and 1b induced the photopolymerization of St to produce PSt with considerably lower number average molecular weight ( M n ) and much lower yield than the corresponding anthracene initiators, which induce the exclusive radical polymerization of St, led us to propose an ionic mechanism for this photopolymerization . Thus, to ascertain whether the St polymerization proceeded via an ionic or radical mechanism, the effect of 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) on polymer yield after irradiation at 60°C for 8 h was examined.…”

“…A novel approach based on the intramolecular charge separation in the singlet excited state of O ‐substituted aryl alcohols, which assists the heterolytic and homolytic cleavage of specific covalent bonds, led to the design of arylmethyloxyanthraquinone and arylmethyloxyanthracene initiators as novel, high‐performance, and hybrid‐type photoinitiators . These initiators exhibited intense absorption bands near the visible region and underwent rapid photodecomposition to induce bulk polymerization of both styrene (St) and cyclohexene oxide (CHO).…”