Copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms

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1 European Polymer Journal ) 151±156 Copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms Harald Braun a, Yusuf Yagci b, Oskar Nuyken a, * a Technische Universitat Munchen, Lehrstuhl fur Makromolekulare Sto e, Lichtenbergstrasse 4, D Garching Bei Munchen, Germany b Department of Chemistry, Istanbul Technical University, Maslak, Istanbul TR-80626, Turkey Received 15 August 2000; received in revised form 26 January 2001; accepted 27 June 2001 Abstract Butyl vinyl ether BVE) and methyl methacrylate MMA) mixtures were polymerized by using free radical initiators in conjunction with a cationic initiator such as diphenyl iodonium salt. Polymerization mechanism involves free radical polymerization of MMA which is switched to cationic polymerization of BVE by addition of growing poly MMA) radicals to BVE and subsequent oxidation of electron donating polymeric radicals to the corresponding cations by iodonium ions. Two representative bifunctional monomers, ethylene glycol divinyl ether EGDVE) and ethylene glycol dimethacrylate EGDMA) were also used together with MMA and BVE, respectively, in photo and thermal crosslinking polymerizations. Vinyl ether and methacrylate type monomers can successfully be copolymerized by this double-mode polymerization under photochemical conditions. Ó 2001Elsevier Science Ltd. All rights reserved. Keywords: Copolymerization; Butyl vinyl ether; Methyl methacrylate; Free radical polymerization; Free radical promoted cationic polymerization 1. Introduction Vinyl ethers [1] and derivatives have received revitalized interest due to their readiness to undergo complete polymerization by a cationic mechanism induced by photochemical [2], thermal and chemical methods and excellent properties of their corresponding polymers. They can also be polymerized in a controlled/living [3] manner to yield polymers with well de ned and * Corresponding author. Tel.: ; fax: addresses: bzw.harald.braun@ch.tum.de H. Braun), yusuf@itu.edu.tr Y. Yagci), oskar.nuyken@ch.tum.de O. Nuyken). predetermined structures. Despite their high reactivity towards cationic initiators, vinyl ethers, however, do not undergo free radical homopolymerization because of the highly nucleophilic nature of the double bond caused by the alkoxy group present in the structure [4]. In the literature, it is often claimed that these monomers e ciently copolymerize with monomers that polymerize by a free radical mechanism. However, recent detailed studies [5] on the copolymerization activity of vinyl ethers revealed that these monomers exhibit very little tendency to undergo polymerization with acrylates and even less with styrenic monomers. On the other hand, vinyl ethers can undergo e cient copolymerization with electron-de cient vinyl monomers such as maleates and fumarates [6]. In this case, spontaneous thermal and photoinduced polymerizations is initiated via donor± acceptor complexes formed between the corresponding monomer components /01/$ - see front matter Ó 2001Elsevier Science Ltd. All rights reserved. PII: S )

2 152 H. Braun et al. / European Polymer Journal ) 151±156 Vinyl ethers are also important monomers for UV curable coatings as they o er environment friendly formulations alternative to commonly used acrylate monomers which possess strong odor and skin irritating properties [7]. Vinyl ether/acrylate hybrid systems were also found to polymerize upon irradiation in the presence of both radical and cationic photoinitiators. With a cationic photoinitiator such as triphenyl sulfonium salt, photochemically generated sul nium radical cations or protonic acids react with vinyl ether while free radicals formed concomitantly polymerize acrylates according to the following reactions [7]: 2. Experimental 2.1. Materials All monomers and solvents were puri ed with conventional drying and distillation procedures. 2,2 0 -Azobis isobutyronitrile) AIBN) Aldrich) was recrystallized from ethanol. 2,4,6-trimethyl benzoyl-diphenylphosphine oxide TMPDO) BASF) was used as received. N-ethoxy-2-methyl-pyridinium hexa uoro antimonate [8] EMP SbF 6 ) and diphenyl iodonium hexa uorophosphate [9,10] Ph 2 I PF 6 ) were synthesized according to the literature Polymerization For thermal polymerizations, the monomer mixtures, the salt and AIBN were dissolved in methylene chloride in Pyrex tubes that were closed with a Te on stopcock after bubbling through with nitrogen. These tubes were immersed for a given time in an oil bath kept at 70 C. Photopolymerizations were performed in a similar manner. The mixtures containing TMDPO photoinitiator instead of AIBN were irradiated from Ushio UXM 200H Hg±Xe high pressure lamp using Schott 340 nm interference lter to avoid direct absorption by the salts. At the end of polymerization, the solutions were poured into ten-fold excess of methanol and the polymer was ltered o. The polymers were then dried at 60 C for 24 h at 10 mbar and analyzed by IR and H-NMR spectroscopy and GPC. When using bifunctional monomers, basically the same procedure was applied but at the end of reaction crosslinked polymers were formed Characterization 4 Interpenetrating networks were thus formed from the bifunctional monomers indicating that the two polymerizations proceeded independently and chemical bonding of the two types of polymeric chains is not achieved. Irradiation of the same mixture in the presence of a free radical photoinitiator resulted in the formation of a cured polymer containing large amount of unreacted vinyl ether groups. In this paper, on the basis of the above information, we inquire whether vinyl ethers can be copolymerized with acrylates by the combination of radical and radical promoted cationic polymerization. The IR spectra were recorded with a Bruker IFS 55 FT-IR spectrophotometer, while the NMR spectra were obtained with a Bruker ARX 300 d in ppm relative to TMS). The molecular weights of the linear polymers were determined with GPC Waters 510) equipped with RI and UV detectors. The eluent was chloroform. Molecular weights were determined by using polystyrene standards. 3. Results and discussion Free radical promoted cationic polymerization is an elegant method to initiate cationic polymerization by the use of free radical initiators [11±13]. This process is based on the oxidation of electron-donor radicals, generated by photochemical or thermal means from onium

3 H. Braun et al. / European Polymer Journal ) 151± salts On X ) such as diphenyl iodonium salts, to the corresponding cations capable of initiating cationic polymerization. 7 5 A similar mechanism involving the induced decomposition of diaryliodonium iodonium salts by free radicals generated by electron beam radiation has been proposed by Crivello et al. [14]. In the case of vinyl ether monomers, electron-donor radicals are obtained by the addition of the radicals to the monomer and subsequent oxidation then allows the initiation of cationic polymerization. 8 When the addition of the growing poly MMA) radical to vinyl ether occurs, the polymeric alkoxy alkyl radical thus formed is converted to the cation and polymerization proceeds further via a cationic mechanism. Probability of occurrence of back electron transfer is excluded since Ph 2 I radicals rapidly decomposes to phenyl radical and iodobenzene [11,14]. Notably, addition of MMA to the growing cation is also not possible since it polymerizes only by anionic and radical mechanism. 6 This prospect is particularly attractive since a wide range of free radical photoinitiators having absorbancies at wavelengths greater than 300 nm is commercially available and cationic polymerization can be activated by free radical initiators. This expands the use of cationic polymerization considerably in applications where high wavelength irradiation is required Free radical and radical promoted cationic polymerizations can be performed both thermally and photochemically. In thermal polymerizations, radicals derived from the thermal decomposition of AIBN initiate the free radical polymerization of methyl methacrylate MMA). The cationic initiation arising from the addition of low-molar mass radicals to vinyl ether monomer and subsequent oxidation can be neglected since vinyl ethers are much less reactive than MMA towards the 2-cyano-2-propyl radical. The corresponding rate constants for vinyl ethers was found [12] to be smaller by a factor of 12. The results on the polymerization of several monomer combinations are listed in Table 1. It should also be noted that the success of the free radical promoted cationic polymerization is related to the acceptor strengths of the cationic initiators employed in the system, as known from their reduction potentials. Iodonium salts appeared to be the most e ective oxidant while sulfonium salts do not participate in this redox process due to unfavorable redox potential Table 2). Compositions of the polymers were calculated from 1 H-NMR spectra using ratios of spectral area of signals near 3.6 ppm corresponding to the protons of OCH 3

4 154 H. Braun et al. / European Polymer Journal ) 151±156 Table 1 Polymerization of MMA and BVE at 60 C in methylene chloride, AIBNŠ ˆ mol l 1, Ph 2 I PF 6 Šˆ mol l 1 Run MMA/BVE in feed mmol/mmol) Conversion a %) M b n g mol ) M w =M n Composition c MMA mol%) / / / / / a Overall conversion. b Determined by GPC according to polystyrene standards. c Determined by 1 H-NMR. Table 2 Polymerization of MMA 4.68 mmol) and BVE 3.88 mmol) by using di erent cationic salts at 60 C; AIBNŠ ˆ mol l 1, OniumsaltŠ ˆ mol l 1 Run Initiator Redox potential V SCE) Conversion %) M n g mol 1 ) 6 Ph 2 I PF 6 0: EMP SbF 6 0:7 1 ± 8 Ph 3 S SbF 6 1:0 0 ± MMA) to the area of signals between 3.5 and 3.4 ppm corresponding to the protons of OCH 2 and OCH BVE). Fig. 1shows typical 1 H-NMR spectra of the related homopolymers and the products obtained from mixtures of the monomers Run 3). The GPC traces of the copolymers Fig. 2) usually exhibited a monomodal molecular weight distribution. However, a shoulder appeared in the polymer formed from the monomer mixture with equal volume ratio Run 3). The two homopolymers have similar solubility properties. Therefore, extraction could not be used to evaluate if the process resulted in copolymer formation. In order to obtain more convincing evidence for the successful copolymerization, experiments using acrylate and vinyl ether type bifunctional monomers as components in the system were performed. As can be seen from Table 3, in the presence of a bifunctional monomer, regardless of the type, crosslinked polymers are readily formed. In photopolymerization experiments, an acyl phosphineoxide type photoinitiator namely TMDPO was used as the free radical source for several reasons. First, it absorbs light at rather long wavelengths where the iodonium salt is transparent. Secondly, the radicals formed according to the following reaction, particularly phosponyl radicals, are quite reactive towards ole nic monomers [13] and do not undergo electron-transfer reactions with iodonium ions [14]. Thus direct cationic initiation leading to homopolymer formation can be excluded. 11 The IR spectra of the crosslinked copolymers exhibited characteristics of the both components. As can be seen from Fig. 3, both ester carbonyl and ether groups resonating at 1732 and 1092 cm 1, respectively, were detected. The treatment of the networks with methylene chloride removed uncrosslinked poly BVE) probably formed by H -transfer reactions taking place in the vinyl ether polymerization However, the networks formed by photopolymerization Table 4) were not soluble in methylene chloride. The complete insolubility suggests the formation of chemical bondings between vinyl ether and methacrylate segments in the network which was not achieved by using either cationic polymerization or free radical polymerization independently.

5 H. Braun et al. / European Polymer Journal ) 151± Fig. 1. a) H-NMR spectra of homopoly MMA) Table 1, Run 1), b) homopoly BVE) Table 1, Run 5) and c) polymer obtained by the polymerization of BVE and MMA 50=50; v/v) Table 1, Run 3). Fig. 2. GPC traces of the polymers Table 1, Run 1±5 a±e)). a) PMMA; b) P MMA co BVE) 71=29); c) P MMA co BVE) 31=69); d) P MMA co BVE) 12=88); e) PBVE. Fig. 3. IR spectrum of the crosslinked polymer Table 3, Run 9).

6 156 H. Braun et al. / European Polymer Journal ) 151±156 Table 3 Crosslinking polymerization of mono and bifunctional monomers a in methylene chloride Run Method Radical source) mol l 1 ) Monofunctional monomer Bifunctional monomer 9 Thermal c AIBN BVE EGDMA Thermal c AIBN MMA EGDVE Photo d TMDPO BVE EGDMA Photo d TMDPO MMA EGDVE 58.0 a 50=50 v/v mixtures, Ph 2 I PF 6 Šˆ310 2 mol l 1. b Soluble part <1%. c At 60 C. d k > 340 nm. Conversion b %) Table 4 Photocrosslinking a of MMA and EGDVE in methylenechloride, k > 340 nm, TMDPOŠ ˆ Ph 2 I PF 6 Šˆ310 2 mol l 1 Run MMA mmol) EGDVE mmol) Conversion %) a No soluble portion was detected. Acknowledgements One of the authors Y.Y.) is very grateful to Alexander von Humboldt-Stiftung for nancial support. References [1] Nuyken O, Crivello JV, Kricheldorf HR. Handbook of synthesis of polymer synthesis part A). New York: Marcel Decker; p [2] Crivello JV, Canlon DA. J Rad Curing 1983;1:6. [3] Sawamato M. Prog Polym Sci 1991;16:111. [4] Matsumato A, Nakane T, Oiawa M. Makromol Chem Rapid Commun 1983;4:277. [5] Bevington JC, Huckerby TN, Jenkins AD. J Macromol Sci Pure Appl Chem 1999;A36:1907. [6] Miller CW, Hoyle CE, Howard C. Polym Prepr 1996; 37 2):346. [7] Decker C, Decker D. J Macromol Sci Pure Appl Chem 1997;A34:605. [8] Botcher A, Hasebe K, Hizal G, Steberg P, Yagci Y, Schnabel W. Polymer 1991;32:2289. [9] Crivello JV, Lam JHW. J Polym Sci Polym Chem Ed 1980;18:2677. [10] Crivello JV, Lam JHW. J Polym Sci Polym Chem Ed 1980;18:2697. [11] Yagci Y, Reetz I. Prog Polym Sci 1998;23:1485. [12] Crivello JV, Liu S. J Polym Sci Polym Chem Ed 1999;37:1199. [13] Bi Y, Neckers DC. Macromolecules 1994;27:3633. [14] Crivello JV, Walton TC, Malik R. In: Yagci Y, Mishra MK, Nuyken O, Ito K, Wnek G, editors. Tailored polymers and applications. Utrecht: VSP; p. 265± 86.