Selected aspects of RAFT agents : the use of thiophene as an activating group in the RAFT mediated polymerization of styrene and the stereo-controlled polymerization of MMA via RAFT

[摘要] ENGLISH ABSTRACT: In this study six Reversible Addition-Fragmentation chain Transfer (RAFT) agents withdifferent leaving groups and different activating moieties were prepared. Three are novelthiophene-based RAFT agents having a thiophene substituent as the activating moiety,namely: (2-thienyl thiocarbonyl)disulfide (BTD), 1-cyano-1-methylethyl 2-thiophenedithiocarboxylate (CPDT), and benzyl thiophene-2-dithiocarboxylate (BDTT). The otherthree are model RAFT agents bearing a phenyl group as the activating moiety, namely:bis(thiobenzoyl) disulfide (BBD), benzyl dithiobenzoate (BDTB), 2-cyano-2-yldithiobenzoate (CPDB). These agents were characterized by Nuclear Magnetic Resonancespectroscopy (NMR), Fourier-Transform Infrared spectroscopy (FT-IR) and Ultraviolet -visible spectroscopy (UV/vis).These compounds were studied as RAFT agents, and used as mediators in the bulkpolymerization of styrene, self-initiated thermally at 100 °C. The novel thiophene-basedmediated systems were compared with the phenyl-based ones in terms of polymerizationkinetics, molecular weight and polydispersity index (PDI).The polymerization results showed that the novel CPDT aiid !:!!)TT thiophene-basedcompounds were effective RAFT agents for the RAFT polymerization of styrene with thecharacteristics of living/controlled free radical polymerization. The BTD mediatedsystem showed the poorest control, as the PDI progressively broadened with monomer topolymer conversion, and it had the lowest reaction rate. In general, the thiophene-basedmediated systems had slower reaction rates (higher retardation) when compared to theanalogous phenyl-based mediated systems.The RAFT technique was then used to synthesize stereo-controlled poly (methylmethacrylates) in the presence of CPDB (since the thiophene-based RAFT agents showedunfavorable rate retardation) in different solvents, namely: toluene, 2-propanol and1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at different temperatures, namely: 60, 30, 4, and-18 °C. The prepared polymers were characterized by 1H-NMR spectroscopy in order tomonitor the tacticity and SEC for the determination of the controlled/living behavior ofthe polymerization system. Results showed that the simultaneous control of the molecularweight and stereochemistry of PMMA was accomplished via RAFT-mediatedpolymerization, especially in HFIP at -18 °C, where the syndiotactic content of thepolymer was the· highest and low PDI values (< 1.4) were achieved.

[发布机构] Stellenbosch University