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

Bshena, Osama (2007-12)

Thesis (MSc) -- University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: In this study six Reversible Addition-Fragmentation chain Transfer (RAFT) agents with different leaving groups and different activating moieties were prepared. Three are novel thiophene-based RAFT agents having a thiophene substituent as the activating moiety, namely: (2-thienyl thiocarbonyl)disulfide (BTD), 1-cyano-1-methylethyl 2-thiophene dithiocarboxylate (CPDT), and benzyl thiophene-2-dithiocarboxylate (BDTT). The other three are model RAFT agents bearing a phenyl group as the activating moiety, namely: bis(thiobenzoyl) disulfide (BBD), benzyl dithiobenzoate (BDTB), 2-cyano-2-yl dithiobenzoate (CPDB). These agents were characterized by Nuclear Magnetic Resonance spectroscopy (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 bulk polymerization of styrene, self-initiated thermally at 100 °C. The novel thiophene-based mediated systems were compared with the phenyl-based ones in terms of polymerization kinetics, molecular weight and polydispersity index (PDI). The polymerization results showed that the novel CPDT aiid !:!!)TT thiophene-based compounds were effective RAFT agents for the RAFT polymerization of styrene with the characteristics of "living"/controlled free radical polymerization. The BTD mediated system showed the poorest control, as the PDI progressively broadened with monomer to polymer conversion, and it had the lowest reaction rate. In general, the thiophene-based mediated systems had slower reaction rates (higher retardation) when compared to the analogous phenyl-based mediated systems. The RAFT technique was then used to synthesize stereo-controlled poly (methyl methacrylates) in the presence of CPDB (since the thiophene-based RAFT agents showed unfavorable rate retardation) in different solvents, namely: toluene, 2-propanol and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at different temperatures, namely: 60, 30, 4, and -18 °C. The prepared polymers were characterized by 1 H-NMR spectroscopy in order to monitor the tacticity and SEC for the determination of the controlled/living behavior of the polymerization system. Results showed that the simultaneous control of the molecular weight and stereochemistry of PMMA was accomplished via RAFT-mediated polymerization, especially in HFIP at -18 °C, where the syndiotactic content of the polymer was the· highest and low PDI values (< 1.4) were achieved.

AFRIKAANSE OPSOMMING: Hierdie navorsing behels die studie van ses omkeerbare addisie fragmentasie kettingoordrag (OAFO) (Eng. reversible addition fragmentation chain transfer, RAFT) verbindings met verskillende verlatende groepe en verskillende aktiverende groepe. Drie van die verbindings is nuwe tiofeenbevattende RAFT-verbindings met 'n tiofeen groep as die aktiverende gedeelte (moiety), naamlik (2-tieniel tiokarboniel)disulfied (BTD), 1-siano-1-metieletiel 2-tiofeen ditiokarboksilaat (CPDT), en bensiel tiofeen-2-ditiokarboksilaat (BDTT). Die ander drie verbindings is 'model' RAFT-verbindings met fenielgroepe as die aktiverende groepe, naamlik bis(tiobensoiel) disulfied (BBD), bensiel ditiobensoaat (BDTB), en 2-siano-2-iel ditiobensoaat (CPDB). Hierdie verbindings is met behulp van kernmagnetieseresonansiespektroskopie (KMR), Fourier-Transforminfrarooispektroskopie (FT-IR) en ultraviolet-sigbare lig spektroskopie (UVvis) gekarakteriseer. Die gebruik van bogenoemde verbindings as RAFT-verbindings in die massapolimerisasie van stireen, self-afsettend by 100°C, is bestudeer. Die nuwe tiofeenbevattende RAFT-verbindings is met die fenielbevattende RAFT-verbindings in terme van polimerisasiekinetika, molekulere massa en die poliverspreiding (PDI) vergelyk. Die nuwe tiofeen-bemiddelde (Eng. mediated) verbindings was goe1e RAFT-verbindings vir gebruik m die RAFT-polimerisasie van stireen. Die reaksie het kenmerke van "lewende"/gekontroleerde vryeradikaalpolimerisasie getoon. Die BTD-bemiddelde sisteem het die swakste kontrole gelewer. Die PDI-waarde het toegeneem namate die monomeer na polimeer omgeskakel is. Dit het boonop die laagste reaksietempo gehad. In die algemeen het die tiofeenbevattende sisteme die laagste reaksietempos gehad (hoer vertraging) in vergelyking met die fenielbevattende sisteme. Daama is die RAFT tegniek gebruik om stereogereguleerde poli(metielmetakrilate) in die teenwoordigheid van CPDB te berei (aangesien die tiofeenbevattende OAFO verbindings swak tempovertraging getoon het). Verskillende reaksiekondisies is gebruik: die oplosmiddels was tolueen, 2-propanol en 1, 1, 1,3,3,3-heksafluoro-2-propanol (HFIP) en die temperature was 60, 30, 4, en -18 °C. Die polimere wat berei is is met behulp van 1 H-KMR geanaliseer om die taktisiteit te bepaal. Gelpermeasiechromatografie is gebruik om die gekontrolleerde/"lewende" aard van die polimerisasiesisteem te bepaal. Sukses is behaal met die gelyktydige beheer van die molekulere massa en stereochernie van die PMMA, veral in HFIP by -18 °C. Onder hierdie reaksiekondisies was die sindiotaktiese inhoud van die polimeer die hoogste en die PDI-waardes die laagste (< 1.4).

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