sign in sign up
<<
Home , Polyvinyl acetate

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

1HAMID REZA HEDAYATI, 2MANOUCHEHR KHORASANI, 3MOSTAFA AHMADI

1,2,3Department of 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 for RAFT polymerization. Use of a proper RAFT agent allows synthesis of with low polydispersity index (PDI) and high functionality, but high molecular weight polymers cannot be obtained because of reduced propagating rate. In this , 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 () (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- for porous substrates such as , paper transfer process. Good control over polymerization of and cloth, emulsion and as powder additives a less active 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 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 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 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 g/mol and molecular weight distribution less than method, it was found that the bulk polymerization 1.25 at 6 kbar at room temperature. However, to the rate of VAc at 30°C was proportional to the exponent best of our knowledge, there have been no reports on 0.58 of ADMVN concentration. The degree of the polymerization of unconjugated monomers (e.g., branching for acetyl group at 30°C was calculated to VAc) under high pressure. Chen et al. [11] be 0.6-0.7. There is no systematic work on RAFT synthesized a series of well-defined PVAc with high polymerization of vinyl acetate using low temperature molecular weight (>123,000 g/mol) and narrow initiators [14]. molecular weight distribution (Mw/Mn = 1.28) can be obtained relatively fast by using RAFT CONCLUSION polymerization at 5 kbar. D. Gamma Irradiation Polyvinyl acetate propagating radicals makes are Although thermal chemical initiators, such as azo- highly activated, consequently it is prone to chain compounds, have been commonly employed to transfer reaction during polymerization and it leads to initiate RAFT polymerization other sources of preparation of a PVAc with a lot of branches. initiation for instance, gamma radiation, UV Reversible addition fragmentation chain transfer radiation, microwave or even visible light can also be polymerization is one of the best method for used. Gamma radiation is high-energy ionizing obtaining linear PVAc with narrower PDI, but rising radiation that has been extensively used in the molecular weight is still an important issue. In this modification of polymers due to its ability to generate study some methods for achieving high molecular radical species within a polymeric structure. In weight PVAc via RAFT polymerization including addition, gamma radiation is an attractive option to Heterogeneous Polymerization, photo-initiation, High initiate free radical polymerization because it offers Pressure Polymerization, Gamma irradiation and low clear advantages over chemical initiators, such as the temperature initiation were introduced. possibility to carry out the reactions at ambient temperature and with short reaction times, and it REFERENCES greatly simplifies work up conditions by obtaining polymers with high conversion yields and purity. [1] Zhao, Funian, et al. "RAFT-mediated emulsion polymerization of vinyl acetate: a challenge towards To illustrate the advantages of using gamma radiation producing high molecular weight poly (vinyl instead of other initiation sources, Segura et al. [12] acetate)." Colloid and 290.13 (2012): 1247- compared gamma radiation and AIBN as initiators in 1255. the polymerization mediated by xanthate. Compared [2] Harrisson, Simon, et al. "RAFT polymerization of vinyl : synthesis and applications." Polymers 6.5 (2014): with conventional miniemulsion polymerization, the 1437-1488. controlled polymerization process allowed obtaining [3] Moad, Graeme, Ezio Rizzardo, and San H. Thang. "RAFT monomodal polymerization and some of its applications." Chemistry–An molecular weight distributions and a significant Asian Journal 8.8 (2013): 1634-1644. [4] Russum, James P., et al. "Miniemulsion reversible addition control over the molecular weight distribution. fragmentation chain transfer polymerization of vinyl E. Low Temperature Initiators acetate." Journal of Polymer Science Part A: Polymer The free radical polymerization of VAc is prone to Chemistry 43.10 (2005): 2188-2193. bring about chain transfer (branching) reaction. [5] Simms, Ryan W., Thomas P. Davis, and Michael F. Cunningham. "Xanthate‐Mediated Living Radical However, a proper choice of polymerization Polymerization of Vinyl Acetate in conditions may suppress the branch formation Miniemulsion." Macromolecular rapid communications 26.8 reaction. As is well known, the polymerization (2005): 592-596. temperature, type and concentration of initiator have [6] Uzulina, I., S. Kanagasabapathy, and J. Claverie. "Reversible addition fragmentation transfer (RAFT) polymerization in a notable effect on the molecular weight of emulsion." Macromolecular Symposia. Vol. 150. No. 1. polymer. Although polymerization in low WILEY‐VCH Verlag, 2000. temperature occurs slowly, it could prevent chain [7] Shi, Yi, et al. "Effect of Mild visible light on rapid aqueous transfer reactions and also side branches. Low RAFT polymerization of water-soluble acrylic monomers at ambient temperature: initiation and temperature initiators could be categorized into water activation." Macromolecules 42.12 (2009): 3917-3926. soluble and oil soluble ones. 2,2'-Azobis (2,4- [8] Ham, Min‐kyoung, et al. "Photoinitiated RAFT dimethylvaleronitrile) (ADMVN), 2,2'-Azobis (4- polymerization of vinyl acetate." Journal of Polymer Science methoxy-2,4-dimethylvaleronitrile), 2,2'-Azobis(2- Part A: Polymer Chemistry 50.12 (2012): 2389-2397. amidinopropane) dihydrochloride (AAPH) and 4,4'- [9] Rzayev, Javid, and Jacques Penelle. "HP‐RAFT: A Free‐Radical Polymerization Technique for Obtaining Living Azobis (4-Cyanovaleric acid) (ACVA) are the most Polymers of Ultrahigh Molecular Weights." Angewandte important low temperature initiators [13]. Lyoo and Chemie International Edition 43.13 (2004): 1691-1694.

Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization

3 International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009 Volume-5, Issue-4, Oct.-2017 http://iraj.in [10] Mueller, Laura, et al. "Synthesis of high molecular weight [13] Lyoo, W., et al. "Low-temperature suspension polymerization polystyrene using AGET ATRP under high of vinyl pivalate for the preparation of syndiotacticity-rich pressure." European Polymer Journal 47.4 (2011): 730-734. ultrahigh molecular weight poly (vinyl alcohol) microfibrils [11] Chen, Jing, et al. "Reversible addition‐fragmentation chain with high yield." Colloid and Polymer Science 280.12 transfer polymerization of vinyl acetate under high (2002): 1075-1083. pressure." Journal of Polymer Science Part A: Polymer [14] Lyoo, Won Seok, et al. "Preparation and molecular structure Chemistry 53.12 (2015): 1430-1436. of poly (vinyl alcohol) by low temperature bulk [12] Segura, Tania, et al. "Synthesis of narrow molecular weight polymerization of vinyl acetate and distribution polyvinyl acetate by gamma–rays initiated saponification." International Journal of Polymeric RAFT/MADIX miniemulsion polymerization." Polymer 102 Materials 46.1-2 (2000): 181-194. (2016): 183-191.



Synthesis of Linear High Molecular Weight Polyvinyl Acetate by Raft Polymerization

4