Photoiniferters bifunctional

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. 

Nakagawa and co-workers [52] have reported that iV-ethyl-dithiocar-bamate ester groups in polystyrene can be hydrolyzed by nucleophiles such as sodium hydroxide and dimethylamine to the thiol group, which is then oxidized to form chain extension reactions. It can also form a chelate bond with divalent metal ions such as a Cu(II) ion. If these reactions are applied to monofunctional and bifunctional monomers obtained by BEDC and XEDC photoiniferters [47], the following chain extension reactions will occur ... 

In previous articles [45,47], it has been reported that AB and ABA block copolymers are easily synthesized through living radical polymerization using polymeric photoiniferters obtained from BDC and XDC as monofunctional and bifunctional photoiniferters. This technique was applied to the synthesis of a variety of block copolymers consisting of three- or four-component systems [53]. The use of a tetrafunctional photoiniferter [i.e., 1,2,4,5-tetrakis (iV,iV-diethyl dithiocarbamyl (methyl)benzene (DDC)] has been reported [49] to synthesize star polymers of MMA. The structure of DDC is given in the following scheme. 

The tetrafunctional photoiniferter (DDC) was prepared by the reaction 1,2,4,5-tetrachloromethylbenzene with sodium JV,AT-diethyl-dithiocarbamate in ethanol at room temperature for about 24 hr and recrystallized from a benzene//i-hexane mixture (—1 1 v/v) the melting point is 125.5-126.3°C. Photopolymerization of MMA with the tetrafunctional iniferter (DDC) was compared with the bifunctional iniferter (XDC) and the monofunctional in-... 

Thiuram disulfides, described above as photoiniferters, can also act as thermal iniferters. The mechanism of the polymerization is the same and polymer chains are invariably end-capped at both ends with iniferter segments. The use of thiurams as thermal or photoiniferters for the preparation of block copolymers greatly depends on the quantum yield of dissociation. The synthesis of dithiocarbamate functional polymers by direct photolysis of the iniferters is limited due to the low quantum yield of dissociation, especially in the case of the thiuram disulfide (e.g., the quantum yield of dissociation (rf) of TD is 0.0025 in cyclohexane [81]). This very low value makes the photochemical dissociation much less attractive than the thermal one. It was suggested that better dithiocarbamate functionalization can be achieved by either thermal initiation with TD at 95°C or polymerization in the presence of AIBN as a thermal initiator and TD as a chain transfer agent. In the latter case, monofunctional or bifunctional TD-PSt were formed, depending on the mole ratio AIBN/TD. Interestingly, the quantum yield of BDC was found to be 0.06, which is 24 times higher than that of TD. Thus, BDC can be used both as a thermal initiator and as a photoiniferter [81]. 

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