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Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization

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Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009 Volume-5, Issue-4, Oct.-2017 http://iraj.in SYNTHESIS OF LINEAR HIGH MOLECULAR WEIGHT POLYVINYL ACETATE BY RAFT POLYMERIZATION 1HAMID REZA HEDAYATI, 2MANOUCHEHR KHORASANI, 3MOSTAFA AHMADI 1,2,3Department of Polymer Engineering and Color Technology, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran E-mail: [email protected] Abstract- Polyvinyl acetate has wide range of application in fibers, films and protective colloids. Because of the very high reactivity of the propagating radicals makes VAc one of the most challenging monomers for RAFT polymerization. Use of a proper RAFT agent allows synthesis of polymers with low polydispersity index (PDI) and high functionality, but high molecular weight polymers cannot be obtained because of reduced propagating rate. In this paper, some of the most important methods for obtaining linear high molecular polyvinyl acetate via RAFT method is investigated. Index terms- Polyvinyl acetate, RAFT polymerization, High Molecular Weight, Low Temperature Initiators I. INTRODUCTION include thiocarbonylthio compounds (ZC(=S)S–R) such as dithioesters, dithiocarbamates, Poly (vinyl acetate) (PVAc) is produced by free- trithiocarbonates, and xanthates (Fig.1), which radical polymerization and it has found application as mediate the polymerization via a reversible chain- adhesives for porous substrates such as wood, paper transfer process. Good control over polymerization of and cloth, emulsion paints and as powder additives a less active monomer such as vinyl acetate is for construction materials. PVAc is also known as a observed with dithiocarbamates (Z=NR′2) and precursor for poly (vinyl alcohol) (PVA) which is xanthates (Z=OR′) [3]. important industrial polymers in the coatings area. Use of a proper RAFT agent allows synthesis of This broad range of applications could be explained polymers with low polydispersity index (PDI) and by the large variety of commercially available vinyl high functionality as shown below in Figure 2, but ester monomers. On the other hand, vinyl acetate high molecular weight polymers cannot be obtained (VAc) is a because of reduced propagating rate [2]. widely used and relatively inexpensive monomer that can only be polymerized through radical polymerization [1]. In vinyl acetate polymerization, the very different reactivity of the growing radicals (high) and the monomer (low) result in a high level of chain transfer reactions and main chain irregularities, which make the production of uniform polymers rather difficult. In addition to a high propensity for chain transfer to solvent in comparison with other monomers, a significant contribution of chain transfer to monomer and polymer resulting in the formation of branched polymer has been revealed in VAc polymerization [2]. However, the very high reactivity of the propagating radicals makes VAc one of the most challenging Fig. 1: Different type of RAFT agents monomers for controlled radical polymerization, which usually results in relatively broad molecular weight distributions and limitation of controlled molecular weights for the resultant PVAcs. Up to now, various LRP methods have been applied for the polymerization of VAc, such as atom transfer radical polymerization (ATRP), cobalt-mediated radical polymerization, and reversible addition-fragmentation chain transfer (RAFT) polymerization [1]. The RAFT process involves conventional free radical polymerization of a substituted monomer in the presence of a suitable chain transfer agent (RAFT agent or CTA). Commonly used RAFT agents Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization 1 International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009 Volume-5, Issue-4, Oct.-2017 http://iraj.in VAc due to its high demand. Zhao et al. [1] synthesized a novel water-soluble N,N-dialkyl dithiocarbamate RAFT agent and employed in the emulsion polymerization of vinyl acetate. Molecular weight analysis demonstrated that the participation of the water-soluble RAFT agent in the polymerization restrict chain transfer reactions. This was determined by examining the evolution of polymer chain length and attaining higher molecular weights up to 126000 g mol-1. B. Photo-initiation Conventional radical initiators need high temperature to decomposing and producing radicals. Higher Fig. 2: General comparison of polymers made with temperature influences initiation and termination rate conventional radical polymerization against those made using which means large number of radicals and lower RAFT process. degree of polymerization. Therefore, polymerization at lower temperature leads to fewer radicals and II. METHODS higher molecular weight. It is generally difficult to apply a RAFT Because of controlled condition during RAFT process at low temperatures because conventional polymerization of vinyl acetate, synthesize of high RAFT molecular weight PVAc still remains as a great processes use a thermal initiator, such as Azobis challenge. Many approaches have been suggested in isobutyronitrile (AIBN) or benzoyl peroxide (BPO), recent years which focus on change in initiation or and polymerization usually proceeds at elevated propagation rates of polymerization. In the following, temperatures. High temperatures unfortunately some of the most important approaches will be disfavour the preparation of high molecular weight issued. polymers. One way to achieve rapid polymerization A. Heterogeneous Polymerization under mild conditions is to photochemically generate Most of the successful RAFT mediated techniques radicals. Cai et al. employed (2,4,6-trimethylbenzoyl) that have been carried out through bulk and solution diphenylphosphine oxide as a photoinitiator in a polymerization of VAc, the molecular weights of RAFT process involving styrene and acrylates along homopolymer or block copolymers are usually less with trithiocarbonate CTA. Polymerization proceeded than 60,000 g/mol with a relative narrow rapidly, even at room temperature, and the polydispersity index (PDI less than 1.5; Mw/Mn). initialization period was short [7]. However, an investigation into the evolution of Ham et al. developed a rapid and well-controlled molecular weight during the polymerization has been RAFT polymerization method applied to VAc using neglected and one can rarely find reports on methyl (ethoxycarbonothioyl) sulfanyl acetate producing high molecular weight PVAc via RAFT (MESA) and bis(2,4,6-trimethylbenzoyl)- polymerization. To solve the problem of the evolution phenylphosphine oxide (BAPO) as the RAFT agent of molecular weight, this approach (i.e., conducting and photoinitiator, respectively. MESA was selected the polymerization reaction in the dispersion system) as the photochemically inert RAFT agent to minimize would be effective. Schork et al. demonstrated that in photolysis of the thiocarbonylthio groups during the RAFT miniemulsion polymerization of VAc, the polymerization. PVAc with a prespecified well- conversion is inversely correlated to the RAFT agent controlled molecular weight and a narrow molecular concentration and the maximum Mn was almost weight distribution was successfully synthesized. The 80,000 g/mol (PDI of 2.6) [4]. Cunningham and co- polymerization reaction proceeded as a living workers were able to prepare PVAc homopolymers polymerization and was remarkably rapid compared via RAFT miniemulsion polymerization with Mn of with approaches that use thermally initiated processes about 160,000 g/mol and PDI of about 2.0 [5]. It was with a very short induction period. interesting to note that the experimental deviation C. High Pressure Polymerization was approximately 50% and was higher in value than High pressure can facilitate to increase the the theoretically calculated Mn. However, there are propagation rate coefficient of the polymerization kp only a few reports on RAFT emulsion by several orders of magnitude and to decrease the polymerization, and for VAc monomer, the results overall activation volumes. Under high pressure, high only show a weak control of the living radical molecular weight polymer can be obtained with polymerization as inferred from a PDI value that is accessible industrial polymerization process while the greater more than 3.0 (Mn less than 100,000 g/mol) propagation of free radicals was enhanced and [6]. We believe that any progress in this area will be termination was suppressed. As reported by Penelle et of great importance and can ensure the future al. [9], poly(methyl methacrylate) (PMMA) has applicability of RAFT emulsion polymerization of extremely high molecular weight (>1,000,000 g/mol) Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization 2 International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009 Volume-5, Issue-4, Oct.-2017 http://iraj.in and narrow molecular weight distribution (Mw/Mn = his coworker investigated the effect of these initiators 1.03) under high pressure (5 kbar). Recently, on kinetic of vinyl acetate polymerization in bulk, Matyjaszewski and coworkers [10] applied activators solution, suspension, and emulsion media. The results generated by electron transfer for ATRP to high show that ADMVN was more effective in preparing polymerization system and obtained high molecular high molecular PVAc with less branches than AIBN. weight polystyrene with Mn more than 1,000,000 Through a series of calculation using initial rate
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