Controlled Radical Copolymerization of Styrene and Maleic Anhydride

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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 polydispersity and a predetermined molar mass. Novel, well-defined polyolefin-based block copolymers were prepared with a macromolecular RAFT agent prepared from a commercially available polyolefin (Kraton L-1203). The second block consisted of either polystyrene or poly(styrene-co-maleic anhydride). Furthermore, the colored, labile dithioester 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 ratio of STY and MAh. The initiator concentration was always one-ﬁfth of the RAFT-agent concentration. 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 reported 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 system was stepwise reset to initial conditions via THF and then DCM, after which Polymerizations the column was re-equilibrated in 30 min with The RAFT agent, monomer, and solvent were added heptane. Data were acquired by Millennium 32 together with an initiator [2,2Ј-azobisisobutyroni- 3.05 software. CONTROLLED RADICAL COPOLYMERIZATION BY RAFT 3599

Table I. Experimental Details of the RAFT Polymerizations

Styrene MAh RAFT Agent Theoretical ៮ (Concentration (Concentration (Concentration Mn Conversion Mn ϫ 2 ϫ Ϫ3 ៮ ៮ ϫ Ϫ3 Experiment M) M) M 10 ) Solvent (Da 10 ) Mw/Mn (%) (Da 10 )

1 2.0 — 4 (1.0) Xylene 10 1.18 21 10 2 4.8 — 4 (1.0) Xylene 23 1.20 28 20 3 1.0 1.0 None BuAc Ͼ2000 — — — 4 1.0 1.0 1 (2.6) BuAc 4.1 1.06 57 4.4 5 0.50 0.50 4 (1.3) BuAc 11 1.12 62 12

͓ ͔ ⅐ ⅐ UV Irradiation ៮ m 0 M0 x Mn,th ϭ Mn,raft ϩ (1) For UV irradiation of the concentrated block- [RAFT]0 copolymer solutions (in heptane), a broadband high-pressure mercury lamp (Philips) was used where [m]0 and [RAFT]0 are the starting concen- with a maximum intensity at a wavelength of 360 trations of the monomer and the RAFT agent, x is the fractional conversion, and M is the molar nm. The spectrum of the emitted light had a sig- ៮ 0 nificant intensity as low as 290 nm. These exper- mass of the monomer. Mn,raft is the number-av- iments were carried out at 25 °C. erage molar mass of the RAFT agent as determined by GPC (in this case, 6.5 ⅐ 103 Da). All molar masses are in PS equivalents, as the cor- rection for the difference in hydrodynamic volume RESULTS AND DISCUSSION is inherently difficult with block copolymers of gradually changing compositions. The molar The polyolefin block was introduced in the poly- mass distributions of samples at different conver- merization in the form of a macromolecular trans- sions (Fig. 2) clearly show the growth of the PS- fer agent. This was achieved by the modification block-PEB chains. Besides the growth of these of Kraton L-1203, a commercially available copol- chains derived from the RAFT agent, a small ymer of ethylene and butylene (PEB) containing number of chains exist that originate from initi- one hydroxyl end group. The hydroxyl group was ation by the azo initiator. These chains do not esterified with an acid-functional dithioester contain a PEB chain and are clearly visible in the (Scheme 2) to yield a polyolefinic RAFT agent (4). first three samples as low molecular weight PS The addition of this RAFT agent to a radical poly- homopolymer (inset of Fig. 2). During the later merization allows the PEB chain to be activated stages of polymerization, these homopolymer (reversibly), after which it can incorporate monomer units to form a block copolymer. This course of reaction was first studied in several STY polymerizations to aid and facilitate the analyses of the more complex blocks prepared.

STY Polymerizations The polymerizations involving STY and the macromolecular RAFT agent (4; Table I, experiments 1 and 2) allowed veriﬁcation of the living character of the polymerization and conﬁrmed that the PS is attached to the PEB chain. The number- average molar mass is plotted against conversion ៮ ■ Figure 1. Experimentally determined Mn ( , left in Figure 1. A linear relationship was found that axis) compared with the theoretically expected values corresponds closely to the theoretical values, (– –, left axis) and polydispersities (E, right axis) for which can be obtained with the following formula: several samples taken from experiment 2. 3600 DE BROUWER ET AL.

Figure 2. Logarithmic molar mass distributions of samples taken from experiment 2. Figure 3. HPLC chromatograms of samples taken from experiment 2. The main peak is the growing block copolymer, and the secondary peak is the PS homopoly- chains are no longer separated from the main mer. peak but remain visible as a low molecular weight tail. All of the molecular weight distributions have a shoulder at the high molar mass side that thioester. This can be attributed to the fact that is due to bimolecular termination. In this case, the starting material does not consist of purely the block copolymer radicals recombine to form monofunctional material. HPLC analyses of the triblock copolymers, the middle block being PS. original material revealed that 2–3% of the chains Both the high molar mass shoulder and the low is unfunctionalized. During these analyses, no molar mass homopolymer broaden the molar other irregularities (e.g., multifunctional materi- mass distribution and reduce the living character als) were found. The disappearance of the corre- and purity of the block copolymer. Although the sponding UV signals indicates that the transfor- effect on the polydispersity is not dramatic (Table mation of the RAFT agent into growing block I and Fig. 1), narrower molar mass distributions copolymers is both rapid and quantitative. The can be obtained with a careful choice of reaction main peak that corresponds to the growing PS- conditions. Lowering the initiator concentration block-PEB block copolymer shifts toward longer will reduce the amount of termination events rel- elution times as the PS block becomes longer. ative to propagation. In addition, a reduction of This peak is followed by a secondary peak that the termination-derived shoulder will also elimi- corresponds to the PS homopolymer material. nate most of the low molar mass tail of PS ho- Figure 4 shows the ratio of the UV signal at mopolymer. However, the tradeoff in this case is a 320 nm over the ELSD signal for both the main reduction in the polymerization rate. peak and the secondary peak. An increase in HPLC analyses of the same samples, with a triple detection setup, confirmed the GPC obser- vations. This allows the various components of the polymerizing system to be followed sepa- rately. The ELSD detects all polymeric com- pounds, whereas the UV detector selectively ob- serves the dithiobenzoate moiety at a wavelength of 320 nm and detects both the dithiobenzoate group and PS at 254 nm. The signal of the macromolecular transfer agent, eluting at 7 min (Fig. 3), diminishes rapidly during the initial phase of the polymerization. Although it disappears com- pletely in the UV detection, a small ELSD signal, corresponding to a few percent of the starting material, remains visible during the entire poly- Figure 4. Ratio of the UV signal at a wavelength of merization. This signal is caused by unmodified 320 nm and the ELSD signal for both the block co- PEB that is not coupled to the UV-absorbing di- polymer (■) and the homopolymer (E). CONTROLLED RADICAL COPOLYMERIZATION BY RAFT 3601 chain length is confirmed by the decrease of the end-group-sensitive UV signal at 320 nm relative to the two other signals. Furthermore, the signal ratio for the secondary peak is consistently higher, indicating the lower molar mass for the PS homopolymer. The final product was precipitated in methanol to yield a pink solid. This indicates the formation of block copolymers because the starting material was originally a viscous liquid. The polymer was, however, fully soluble in heptane, in contrast to PS homopolymer of similar molar mass.

UV Irradiation Figure 5. Molar mass distributions of PEB-block-PS Low-conversion samples of experiment 2, essen- copolymers before and after UV irradiation. The second derivative of the distribution clearly shows the signal tially block copolymers with a small PS block at three times the original mass. length, were dissolved in heptane and subjected to UV irradiation for 5 h. A part of the resulting product was passed through a short silica column with 9/1 heptane/DCM as an eluent. Both the rence of the additional termination reaction (d) is crude product and the columned material were a possible explanation for the retardation that is analyzed with GPC, and their molar mass distri- usually observed in RAFT homopolymerizations. butions were compared with those of the samples This will be elucidated further in a future publi- before irradiation. Although the UV-irradiated cation. product still had the same red color as the poly- Although columning of the product removed mer before irradiation, the compound responsible the colored dithiobenzoate group, it did not for this color was no longer attached to the poly- change the molar mass distribution. The nature mer chain. The polymer collected after columning of the end groups after irradiation is currently was colorless, and the red color from the product under investigation. had turned into a brown component with a very low retention factor (R ). This color change is also f STY/MAh Copolymerization observed when, for example, dithiobenzoic acid or its dimer, bis(thiobenzyl)disulﬁde, comes into The copolymerization of STY with MAh copoly- contact with silica, and the brown color corre- mers exhibits some interesting features. MAh it- sponds to that of dithiobenzoate salts. This led us self does not homopolymerize, and its copolymer- to conclude that the dithioester group has been ization with STY has a strong tendency toward cleaved from the polymer chain and transformed alternation, indicated by the reported reactivity into a more labile species. An examination of the ratios.7 Convincing evidence was published a few molar mass distributions indicates that some of years ago indicating that the STY/MAh copoly- the material has been transformed to higher mo- merization obeys the penultimate unit model.21 lecular weight species of precisely twice and three On the basis of the copolymerization parameters, times the original molar mass (as clearly shown it can easily be estimated that the vast majority of in the second derivative in Fig. 5). We expect this propagating radicals carry a terminal STY unit. to be attributed to the reactions depicted in For this reason, the copolymerization of STY with Scheme 3, which would mean that the diblock MAh was expected to proceed in a controlled fash- copolymer has been transformed into triblock ma- ion similar to the STY homopolymerization. terial (6) free of the labile dithiogroup. The triple As can be seen in Table I (experiments 3–5), molar mass shoulder may be explained by the three STY/MAh copolymerizations were carried combination of a block copolymer radical (5) with out under similar conditions but were different intermediate species (7) to yield a star-shaped with respect to the RAFT agent that was em- block copolymer with three arms (8). Both termi- ployed and the monomer concentrations used. nation reactions (c and d) take place during the The blank experiment without the RAFT agent RAFT polymerization process as well. The occur- became turbid after a conversion of a few percent. 3602 DE BROUWER ET AL.

Scheme 3. Proposed reaction scheme. Under the inﬂuence of UV light, the polymer dissociates and forms a dithiobenzoate radical and a block copolymer radical. The polymer radicals can recombine to form triblock copolymers or react with intact polymeric RAFT species to form an intermediate radical that can be terminated by a second block copolymer radical.

The heterogeneity was caused by precipitation theoretical value of 4.4 ⅐ 103 Da obtained from eq due to the poor solvent properties of butyl acetate 1. The polydispersity of the final product is 1.06. for high molar mass STY/MAh copolymer. The The application of the macromolecular RAFT molar mass of the resulting polymer exceeded the agent (4; experiment 5) allowed the preparation exclusion limit of the applied columns (M Ͼ 2 ⅐ 106 of low-polydispersity poly[(ethylene-co-butylene)- Da). block-(styrene-co-maleic anhydride)] polymers. The experiment with the RAFT agent (1; ex- Although the reaction mixture is heterogeneous periment 4) remained homogeneous during the at room temperature, it forms a clear, single- entire polymerization, and a GPC analysis of phase solution at the reaction temperature of 60 samples that were periodically drawn from the °C. The first few samples at low conversion phase- reaction mixture revealed a controlled growth separate when cooled to room temperature into a (Fig. 6). Because of the nonvolatile character of red PEB-rich phase and a colorless monomer-rich the MAh monomer, gravimetric conversion mea- phase. As conversion increases, the STY/MAh surements are rather inaccurate, but the molar block grows and solubilizes the PEB block to form mass of the final sample (number-average molec- a homogeneous solution at room temperature. ular weight ϭ 4.1 ⅐ 103 Da) is close to the expected Low-conversion samples exhibited a bimodal molar mass distribution (Fig. 7). The first and

Figure 6. Normalized logarithmic molar mass distri- Figure 7. Normalized logarithmic molar mass distributions for samples taken during experiment 4, the butions for samples taken during experiment 5, the controlled copolymerization of STY and MAh. growth of an STY/MAh block onto the PEB chain. CONTROLLED RADICAL COPOLYMERIZATION BY RAFT 3603 large peak is the starting polyolefin RAFT agent, The authors thank the Foundation for Emulsion Poly- and the second is the block copolymer, which is off merization (SEP) for funding and Shell Chemicals for a very high molecular weight. The usual explana- providing the Kraton L-1203. They are very grateful to tion for this type of behavior is a low transfer Mr. Meijerink and Mrs. Frijns-Bruls from DSM Re- constant to the RAFT agent. It seems unlikely in search, Geleen, The Netherlands for their GPC analyses of the STY/MAh copolymers. this case, as this behavior was not observed in experiment 4 with the RAFT agent (1), because for both RAFT agents the electronic structure close to the reactive dithioester moiety are simi- REFERENCES AND NOTES lar. A second reason to discount this explanation is the gradual growth of the remaining PEB some- what later in the polymerization. This would be 1. Solomon, D. H.; Rizzardo, E.; Cacioli, P. Eur. Pat. Appl. 135 280 A2, 1985. highly unlikely if a RAFT agent with a low trans- 2. Rizzardo, E. Chem Aust 1987, 54, 32. fer constant was used. It is, therefore, assumed 3. Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; that local inhomogeneities in the reaction mix- Hamer, G. K. Macromolecules 1993, 26, 2987. ture, the aggregation of PEB molecules, cause 4. Otsu, T. J Polym Sci Part A: Polym Chem 2000, 38, propagating radicals to grow in a microenviron- 2121. ment that has a considerably lower concentration 5. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashi- of dithioester groups than expected on the basis of mura, T. Macromolecules 1995, 28, 1721. macroscopic calculations. As conversion in- 6. Wang, J. S.; Matyjaszewski, K. Macromolecules creases, the production of more block copolymer 1995, 28, 7901. makes the reaction mixture more homogeneous, 7. Trivedi, B. C.; Culbertson, B. M. Maleic Anhydride; so the local environment becomes similar to the Plenum: New York, 1982; Chapter 3, p 364. 8. Lin, C.-W.; Lee, W.-L. J Appl Polym Sci 1998, 70, global environment. The low molar mass found at 383. a higher conversion is presumably the starting 9. Stehling, U. M.; Malmstro¨m, E. E.; Waymouth, polyolefin RAFT agent, suggesting that some of R. M.; Hawker, C. J. Macromolecules 1998, 31, the transfer agent is not consumed. 4396. The final product, a pink powder, has a molar 10. Schellekens, M. A. J.; Klumperman, B. J Macromol mass close to the predicted value of 1.1 ⅐ 104 Da Sci Rev Macromol Chem Phys 2000, 40, 167. and a polydispersity of 1.12. Its material proper- 11. Jancova, K.; Kops, J.; Chen, X.; Batsberg, W. Mac- ties and applicability as an adhesion-promoting romol Rapid Commun 1999, 20, 219. additive are currently under investigation and 12. Waterson, C.; Haddleton, D. M. Polym Prepr 1999, form the basis of a future publication. 218, 1045. 13. Ashford, E. J.; Naldi, V.; O’Dell, R.; Billingham, N. C.; Armes, S. P. Chem Commun 1999, 1285. 14. Benoit, D.; Hawker, C. J.; Huang, E. E.; Lin, Z.; CONCLUSIONS Russell, T. P. Macromolecules 2000, 33, 1505. 15. Chen, G.-Q.; Wu, Z.-Q.; Wu, J.-R.; Li, Z.-C.; Li, It has been demonstrated that well-defined and F.-M. Macromolecules 2000, 33, 232. low-polydispersity polyolefin block copolymers can 16. Le, T.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT be prepared with a macromolecular RAFT agent. In Int. Appl. WO 9801478 A1 980115, 1998; Chem addition, the copolymerization of STY and MAh can Abstr 1998, 128, 115390. be performed under living conditions, something 17. Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; that has proven to be very difficult until now. The Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, combination of both achievements allowed the G. F.; Moad, C. L.; Moad, G.; Thang, S. H. Macro- preparation of poly[(ethylene-co-butylene)-block- molecules 1998, 31, 5559. (styrene-co-maleic anhydride)], a polymer that is 18. Chong, Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 2071. expected to prove useful in coating applications. 19. Thang, S. H.; Chong, Y. K.; Mayadunne, R. T. A.; Furthermore, the highly colored and labile dithio- Moad, G.; Rizzardo, E. Tetrahedron Lett 1999, 40, benzoate group could be removed from the polymer 2435. chain by UV irradiation facilitating a greater range 20. Bouhadir, G.; Legrand, N.; Quiclet-Sire, B.; Zard, of polymer architectures and perhaps future practi- S. Z. Tetrahedron Lett 1999, 40, 277. cal applications such as postgrafting or crosslink- 21. Sanayei, R. A.; O’ Driscoll, K. F.; Klumperman, B. ing. Macromolecules 1994, 27, 5577.