Dithiocarbamate telechelic polymers: Synthesis and block copolymerization

Abstract

The iniferter method of Otsu was studied for the synthesis of polyvinyl block copolymers of relatively low molecular weight using tetramethylthiuram disulfide (TD) and benzyl NWdiethyldithiocarbamate (BDC) as initiator. Considering the low quantum yield of dissociation (@d) of TD (2,5 .in cyclohexane), TD was used as thermal initiator (95 "C) for the synthesis of dithiocarbamate-polystyrene (TD-PS) telechelics. 13C NMR analysis of this TD-PS shows two 13C=S signals corresponding to two different end-groups: Et2N-CS-S-CH(C6H5)-CH2and Et,N-CS-S-CH,-CH(C,H,)-.Several styrene polymerizations were also carried out in presence of azoisobutyronitrile (AIBN) as thermal initiator and TD as chain-transfer reagent. Depending on the mole ratio AIBN/TD, mono-and difunctional TD-PS's are formed, as evidenced by NMR analysis. These TD-PS's were used for the photochemical initiations of ethyl acrylate (EA), acrylic acid (AA) and methyl methacrylate (MMA). It is assumed that the quantum yield of dissociation of the dithiocarbamate end-group is equal to that of BDC (@d : 0,06). TD-PS nonfunctional polymers were also prepared, either photochemically by dissociation of BDC, or thermally in presence of AIBN and BDC as transfer agent. They were used for block copolymerization with MMA. Inversely, TD-PMMA's were prepared in a first step; in this case @d = 0,026. They were then used for the polymerization of EA. The block copolymers were fractionated; their composition and molecular weights were determined by 'H NMR and gelpermeation chromatography, respectively.

"Living" radical polymerization. 1. Possibilities and limitations

“…Pn +PI-R k)PI" + Pn-R ktr (17) In this case, the main requirement for a living system is that a growing radical Polydispersities in the systems discussed will depend on the relative rate of initiation and termination as well as on the rate of exchange between active and dormant species 13 .…”

Synthesis of block copolymers by radical polymerization and telomerization

“…This was confirmed by the inefficiency of "SCSNEt2 radical 58 (produced by its dimer or the end-group) to initiate the homopolymerization. In the same way, Van Kerckhoven et al [237] obtained from PS-S-C(S)NEt2 as precursor, different diblock copolymers by reacting ethyl acrylate (EA), acrylic acid or MMA. In these cases, the living character was evidenced by an increase in molecular weight of the copolymers with the conversion.…”

“…Furthermore, Turner and Blevins [236] and Van Kerckhoven et al [237] observed with dithiocarbamates that, if the photochemicatly induced telomerization is monitored by a living character, the thermal reaction does not exhibit a living behavior and that the number average molecular weight remains practically constant during the polymerization. They show that the end-group 56 is stable up to 100 °C in contrast to the starting thiuram disulfide, but under UV radiation, dissociation may occur:…”

“…Turner and Blevins [236] have shown that, during the process, the formation of CS2 is not negligible, unlike the polymerization of styrene, and they explained its production by the following scheme: This side reaction could explain the loss of activity of the PMMA end-group. Besides, Van Kerckhoven et al [237] have demonstrated that the quantum yield of the photodissociation of thiocarbamate-PMMA end-group is 2.5 times lower than that of thiocarbamate 51 and thiocarbamate-PS. Thus, all authors agree that side reactions occur with MMA-thiuram terminations, leading to a deactivation of the species.…”

“…To complete these studies, Van Kerckhoven et al [237] have investigated the behavior of styrene according to two key experiments: first, they showed by 13C NMR that when an excess of thiuram disulfide is present, a dicapped structure is effectively obtained: …”