Photoiniferters

Kwon and coworkers have reported the use of diphenyl diselenide 2865"66 and a variety ofben/.ylic selenides (e.g. 29,67 615 30,69 3170 71 and 32K) as photoiniferters for polymerization of S, MM A and some derivatives. Very narrow dispersities were not obtained ( / Mn typically 2-2.5). However, it was possible to prepare... 

Such a two-component iniferter technique is also applied to the living radical polymerization of several DC photoiniferters for the design of block and graft copolymer synthesis (Sect. 5). 

In polymerization with the compounds having a photodissociable DC group as photoiniferters, the polymerization can be performed at low temperature, such as room temperature, in contrast with thermal iniferters. Moreover, we can readily prepare many kinds of DC derivatives with various structures, indicating that the functionalization and molecular structure design are easy [156]. 

Tetraethylthiuram disulfide (13) induces St polymerization by the photodissociation of its S-S bond to give the polymer with C-S bonds at both chain ends (15). The C-S bond further acts as a polymeric photoiniferter, resulting in living radical polymerization. Eventually, some di- or monosulfides, as well as 13, were also examined as photoiniferters and were found to induce polymerization via a living radical polymerization mechanism close to the model in Eq. (18), e.g., the polymerization of St with 35 and 36 [76,157]. These disulfides were used for block copolymer synthesis [75,157-161] ... 

Table 2. Radical Polymerization of St with Photoiniferters in Benzene at 30 °C [156] ...

Recently, Kondo and coworkers reported on the polymerization of St with diphenyl diselenides (37) as the photoiniferters (Eq. 39) [ 162]. In the photopolymerization of St in the presence of 37a and 37b, the polymer yield and the molecular weight of the polymers increased with reaction time. The chain-end structure of the resulting polymer 38 was characterized. Polymer 38 underwent the reductive elimination of terminal seleno groups by reaction with tri-n-butyltin hydride in the presence of AIBN (Eq. 40). It also afforded the poly(St) with double bonds at both chain ends when it was treated with hydrogen peroxide (Eq. 41). They also reported the polymerization of St with diphenyl ditelluride to afford well-controlled molecular weight and its distribution [163]. 

When 13 is used as a photoiniferter for the living radical polymerization, the thermal DC groups of both chain ends of 15 are not identical, i.e., the DC group bonds to the head and tail positions of the terminal St monomer units of the polymer, as shown in the structure of poly(St) 40. 

Otsu and Kuriyama designed photoiniferters which yield a highly reactive carbon radical and less reactive thiyl radical by photodissociation [169]. The former radical participates in propagation, and the latter acts only as a terminator. Bifunctional photoiniferters 8 as well as monofunctional 7 were prepared (see Eqs. 10 and 11 for the structures of 7 and 8). These photoiniferters dissociate only at the easily dissociable benzylic C-S bond to give a benzylic radical similar to the propagating poly(St) radical and the less reactive DC radical. 

Benzyl AT-ethyldithiocarbamate (44) and p-xylylene bis(N-ethyldithiocarba-mate) (45) were also prepared as mono- and difunctional photoiniferters, respectively [171,172], consisting of a structure similar to 7 and 8. The polymerization of St with 44 under ultrasonic irradiation was also reported [173]. 

The dissociation of model compounds for co-chain ends of polymers obtained using iniferters with the DC group was examined by the spin-trapping technique, similar to the disso dation of 7 and 8 previously mentioned [174,175]. From the results of the trapping experiments, it was concluded that 46,47, and 48 as model compounds for poly(MA), poly(MMA), and poly(VAc), respectively, dissociated at the appropriate position to produce a reactive carbon-centered radical and a stable DC radical. In fact, these compounds were found to induce the living radical polymerization of St when they were used as photoiniferters. 

As previously described, the polymers obtained by 7 and 8 further serve as mono- and difunctional photoiniferters, respectively. If the poly(St)s 42 and 43 are used for the polymerization of MMA as a second monomer, AB- and ABA-type block copolymers, respectively, would be synthesized, as shown in Eqs. (49) and (50) ... 

Polymeric Photoiniferter Monomer (M2) Time (h) Fraction Extracted (%) Unreacted M2 Homo- Block Iniferter polymer Copolymer ... 

Synthesis and application using polymeric photoiniferters based on poly(DMS) and polyurethanes are found in a review by Kumar et al. [ 184]. 

The synthesis of some multiblock copolymers was attempted by successive polymerization using this iniferter technique. However, pure tri- or tetrablock copolymers free from homopolymers were not isolated by solvent extraction because no suitable solvent was found for the separation. In 1963, Merrifield reported a brilliant solid-phase peptide synthesis using a reagent attached to the polymer support. If a similar idea can be applied to the iniferter technique, pure block copolymer could be synthesized by radical polymerization. The DC group attached to a polystyrene gel (PSG) through a hydrolyzable ester spacer was prepared and used as a PSG photoiniferter (Eq. 53) [186] ... 

If the DC photoiniferter having a polymerizable double bond, i.e., a monomer iniferter, is successively used as both monomer and iniferter, macromonomers and graft copolymers would be obtained according to Eq. (54) [188] ... 

Matsuda 71 Photoiniferter-Driven Precision Surface Graft Microarchitectures for Biomedical Applications. Vol. 197, pp. 67-106. 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Photoiniferter-Driven Precision Surface Graft Microarchitectures for Biomedical Applications... 

This article is intended to summarize new strategic and technological aspects of photoiniferter-based graft architectures in conjunction with biomedical applications as follows. 

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