Free Radical Polymerization Photoinitiated by Riboflavin/Amines. Effect of the Amine Structure

Abstract

Polymerization rates of 2-hydroxyethyl methacrylate (HEMA) photoinitiated by riboflavin were investigated using as co-initiator several amines of different structure. The photoinitiation efficiency is highly dependent on the structure of the amine. The photochemical behavior of these systems was investigated under the polymerization conditions. The results are explained in terms of the dependence of the amino radicals production and the reactivity of these radicals toward monomer double bonds with the structural features of the amines. IntroductionThe application of photoinitiated polymerization is extensive. Among the most commonly utilized photoinitiators for free radical polymerization stand those where radicals result from a bimolecular process comprising an excited chromophore and a co-initiator that behaves as electron donor. 1 Tertiary amines are the most frequently employed co-initiators in these processes. It is generally accepted that the R-amino radicals generated in these reactions are the species that add to the monomer. 2 In these reactions the yield of the primary photochemical processes is strongly dependent on the structure of the electron donor. Therefore, a strong dependence of the polymerization efficiency with the amine structural features can be expected.Dependence of the polymerization rate with the structure of the amine has been reported for the acrylate polymerization using camphorquinone 3,4 and thioxanthones 5 as photoinitiators. However, there is not a general mechanistic explanation of the experimentally found dependence of the efficiency of the amine as coinitiator with its chemical structure. More recently, Davidson et al. 6 have proposed that for a particular aromatic ketone the efficiency of photoinitiation is determined by the structure and the molecular geometry of the R-aminoalkyl radical produced. On the other hand, Paczkowski et al. 7 have suggested that the initial electron-transfer reaction from an aromatic amines to the excited state of xanthene dyes, is the responsible of the variation of the photoinitiation efficiency.In a previous work, 8 we have shown that riboflavin acts as an efficient photoinitiator of vinyl polymerization in the presence of triethanolamine (TEOHA). Radicals are produced with high yield by a photoinduced electrontransfer process. Therefore, the riboflavin-amine appears as an interesting system to study the effect of the amine structure on the photoinitiation efficiency. Here we report the polymerization of 2-hydroxyethyl methacrylate (HEMA) photoinitiated by riboflavin in the presence of several amines (Chart 1), with the aim of relating the efficiency of generation of radicals and their ability to initiate the polymerization with the structure of the amine.

Visible light induced dextran-methacrylate hydrogel formation using (−)-riboflavin vitamin B2 as a photoinitiator and L-arginine as a co-initiator

“…Amine has been mostly used as an electron donor, so Type II photoinitiator is also called a ketone/amine system [20,21]. Bertolotti et al studied riboflavin/triethanol amine system for the photopolymerization of vinyl monomers, such as 2-hydroxyethyl methacrylate (HEMA) [22][23][24]. They studied how the amine structure affected the photopolymerization reaction upon the UV irradiation and found that triethanol amine was the most effective co-initiator among other amines in their studies.…”

Synthesis and Characterization of Injectable Sulfonate-Containing Hydrogels

“…Three different cross-linking methods were used (Table 5): thiol–ene reaction with redox initiators APS and TEMED; Michael addition with an organic base, TEA; and thiol–ene reaction with photoinitiators, riboflavin, and arginine, a well-known combination for photoinitiation. 47 All three methods resulted in the formation of gels. However, other possible side reactions could also form gels, which include a cross-link between the copolymer and the 4-arm PEG, free radical polymerization of the acrylamide, and oxidation of the thiol groups.…”

The Interaction of Ground and Excited States of Lumichrome with Aliphatic and Aromatic Amines in Methanol

“…Alloxazines unsubstituted at N(1) can undergo an excited-state proton transfer from N (1) to N (10) to form the corresponding isoalloxazine in the presence of pyridine, carboxylic acids, and other compounds bearing proton donor and acceptor functions, such as primary amines [4] [5]. This reaction has been postulated to proceed via a cyclic complex where H-bonding takes place at both N (1) and N(10) simultaneously, and involves a double-proton transfer.…”

“…The mechanism was explained in terms of a one-electron-transfer process from the amine to the flavin. Other aliphatic amines have also been shown to act as effective co-initiators of vinyl polymerization with riboflavin as photosensitizers [10]. However, in the presence of aliphatic amines of higher basicity, lumichrome suffers a proton-transfer process instead of the electron-transfer reaction of the isoalloxazine ring.…”