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Controlled Radical Copolymerization of Styrene and Maleic Anhydride

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Controlled Radical Copolymerization of Styrene and Maleic Anhydride Controlled Radical Copolymerization of Styrene and Maleic Anhydride and the Synthesis of Novel Polyolefin-Based Block Copolymers by Reversible Addition–Fragmentation Chain-Transfer (RAFT) Polymerization HANS DE BROUWER, MIKE A. J. SCHELLEKENS, BERT KLUMPERMAN, MICHAEL J. MONTEIRO, ANTON L. GERMAN Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands Received 8 May 2000; accepted 10 July 2000 ABSTRACT: Reversible addition–fragmentation chain transfer (RAFT) was applied to the copolymerization of styrene and maleic anhydride. The product had a low polydis- persity and a predetermined molar mass. Novel, well-defined polyolefin-based block copolymers were prepared with a macromolecular RAFT agent prepared from a com- mercially available polyolefin (Kraton L-1203). The second block consisted of either polystyrene or poly(styrene-co-maleic anhydride). Furthermore, the colored, labile di- thioester moiety in the product of the RAFT polymerizations could be removed from the polymer chain by UV irradiation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3596–3603, 2000 Keywords: controlled radical polymerization; RAFT; block copolymers; polyolefin; styrene; maleic anhydride; addition–fragmentation; degenerative transfer INTRODUCTION poly(styrene-co-maleic anhydride) block. This type of polymer may prove useful as a blend compatibi- Several controlled radical polymerization tech- lizer or as an adhesion promoter for polyolefin coat- niques that have emerged in the past decade have ings on more polar substrates such as metals.7,8 put free-radical polymerization in a new perspec- 1–6 Although controlled radical polymerizations tive. The robustness of radical chemistry can are unable to polymerize olefins, several methods now be combined with a more sophisticated de- have been reported to overcome this problem in sign of the polymer chain architecture. Block co- the preparation of polyolefins containing block polymers are an example of such a class of mate- and graft copolymers. It has been shown that an rials for which careful tailoring of the block alkene functionalized with an alkoxyamine moi- lengths and monomer composition yields materi- ety could be copolymerized with propene to yield a als with unique properties that are not solely of polyolefin, which could be used as a macroinitia- academic significance but are of commercial in- 9 terest as well (e.g., surfactants, blend compatibi- tor in nitroxide-mediated polymerizations. An al- lizers, surface modifiers, and pigment stabilizers). ternative approach is the transformation of a The aim of this work is the preparation of block ready-made polyolefin into a suitable dormant copolymers containing both a polyolefin block and a species by organic procedures. This approach has been reported to be successful in atom transfer radical polymerization (ATRP).10–12 Correspondence to: M. J. Monteiro The application of the ATRP technique to reac- Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 3596–3603 (2000) © 2000 John Wiley & Sons, Inc. tive monomers, however, is not straightforward. 3596 CONTROLLED RADICAL COPOLYMERIZATION BY RAFT 3597 Scheme 1. Scheme of the RAFT polymerization, in which Z is an activating group for the addition–fragmentation process and R is a good homolytic leaving group that can reinitiate polymerization: (a) the transformation of the RAFT agent into dormant polymer species and (b) the degenerative transfer reaction responsible for the exchange of the radical over a large number of dormant polymer species. Some functional monomers cause problems be- L-1203; PEB) to prepare block copolymers in cause of their interaction with the applied copper which the second block consists of either polysty- complex. Block copolymers containing para-hy- rene (PS) or poly(styrene-co-maleic anhydride). droxy styrene11 and methacrylic acid12 could be Furthermore, the labile dithioester moiety (at- obtained exclusively in an indirect way with pro- tached to the dormant chains) could be removed tecting groups (or the deprotonated form of the by UV photolysis. The prepared PEB–PS block acid13). All attempts to obtain a controlled co- copolymers were irradiated to produce triblock polymerization of styrene (STY) and maleic anhy- and unusual star-shaped polymers. dride (MAh), both in the literature14,15 and in our own laboratory, have been fruitless. Although the copolymerization of STY and EXPERIMENTAL MAh can proceed in a controlled fashion with 14 nitroxide-mediated polymerization, it requires General a specially designed alkoxyamine to be used at high temperatures (120 °C). Kraton L-1203 (PEB; 3) was obtained from Shell Reversible addition–fragmentation chain-transfer Chemicals (number-average molecular weight 3 (RAFT) polymerization16–18 appears to be the Ϸ 3.8 ⅐ 10 Da; weight-average molecular weight/ best choice for this type of work because it does number-average molecular weight Ϸ 1.04) and not require more stringent polymerization condi- was dried under reduced pressure for several tions than conventional free-radical polymeriza- days before use. 4-Cyano-4-[(thiobenzoyl)sulfa- tion. In addition, its application allows the con- nyl]pentanoic acid (2) and 2-cyanoprop-2-yl di- trolled polymerization of monomers containing a thiobenzoate (1) were prepared according to 19,20 variety of functional groups. In this process, ef- the literature. Anhydrous dichloromethane fective transitions between growing free-radical (DCM) was prepared by distillation over lithium chains and dormant end-capped chains are en- aluminum hydride and was stored over molecular sured with an efficient degenerative transfer re- sieves. All other chemicals were obtained from action to a dithioester moiety (Scheme 1). If the Aldrich Chemical Co. and used without further exchange reaction [Scheme 1(b)] is fast compared purification. with propagation and the equilibrium of the transformation reaction [Scheme 1(a)] is shifted Synthesis of the Kraton Macromolecular Transfer toward the right by an appropriate choice of the R Agent (4) group, the application of a dithioester compound will result in a pseudoliving system. Kraton L-1203 (29.5 g, 8 mmol), p-toluenesulfonic This work utilized a synthetically modified acid (0.30 g, 1.6 mmol), 4-(dimethylamino)pyri- polyolefin RAFT agent (synthesized from Kraton dine (0.29 g, 2.4 mmol), and 1,3-dicyclohexylcar- 3598 DE BROUWER ET AL. trile (AIBN)] in a 100-mL, three-necked, round- bottom flask equipped with a magnetic stirrer. Copolymerizations contained an equal molar ra- tio of STY and MAh. The initiator concentration was always one-fifth of the RAFT-agent concen- tration. The mixture was degassed with three freeze–evacuate–thaw cycles and polymerized under argon at 60 °C. Periodically, samples were taken for analysis. Gel Permeation Chromatography (GPC) Analyses GPC analyses of the STY polymerizations were performed on a Waters system equipped with two PL-gel mixed-C columns, a UV detector, and a refractive index (RI) detector. The analyses of the STY/MAh copolymers were carried out on an HP1090M1 with both UV-DAD and Viscotec RI/DV 200 detectors. All molecular weights re- ported in this article are PS equivalents, unless Scheme 2. Both 2-cyanoprop-2-yl dithiobenzoate (1) otherwise stated. and the macromolecular RAFT agent (4) were applied in polymerizations. The latter was synthesized from a hydroxyl-terminated ethylene butylene copolymer (Kraton L1203) and an acid-functional dithioester. High Pressure Liquid Chomatography (HPLC) Analyses The HPLC analyses were performed with an Al- bodiimide (3.9 g, 19 mmol) were dissolved in an- liance Waters 2690 separation module. Detection hydrous DCM in a 1-L, three-necked, round-bot- was done with a PL-EMD 960 evaporative light tom flask equipped with a magnetic stirrer. scattering detector (ELSD) detector (Polymer 4-Cyano-4-[(thiobenzoyl)sulfanyl]pentanoic acid Laboratories) and with a 2487 Waters dual UV (2.5 g, 9 mmol) was dissolved in anhydrous DCM detector at wavelengths of 254 and 320 nm. All and added dropwise to the reaction mixture at samples were analyzed by the injection of 10 ␮Lof room temperature. Upon completion, the reaction a DCM solution of the dried polymer with a con- mixture was heated to 30 °C and allowed to stir centration of 5 mg/mL. Columns were thermo- for 48 h. The mixture was then filtered and stated at 35 °C. The PEB-block-PS copolymers washed with water. The solution was dried with were analyzed on a NovaPak௡ silica column (Wa- anhydrous magnesium sulfate and filtered, and ters; 3.9 ϫ 150 mm) with a gradient going from DCM was removed under reduced pressure. The pure heptane to pure tetrahydrofuran (THF) in 50 crude product was purified by column chromatog- min. The system was stepwise reset to initial raphy (silica; 9/1 hexane/ethyl acetate) to give a conditions via MeOH, THF, and then DCM, after purplish-red, viscous liquid (29.4 g, 92% yield, which the column was re-equilibrated in 30 min based on Kraton; see Scheme 2). The 1HNMR with heptane. PEB-block-PS/MAh and PS/MAh spectrum indicated a quantitative yield based on copolymers were analyzed on a NovaPak௡ CN the number of hydroxyl groups. The chemical column (Waters; 3.9 ϫ 150 mm) by the applica- shift of the set of protons in the Kraton situated tion of the following gradient (heptane/THFϩ5% next to the hydroxyl group changed from 3.6 to 4.2 v/v acetic acid/MeOH): 100/0/0 to 0/100/0 in 25 ppm upon esterification. min and then 0/0/100 from 25 to 35 min. After each run, the
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