Biomacromolecules 2005, 6, 2091-2095
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Catalytic Polymerization of a Cyclic Ester Derived from a “Cool” Natural Precursor Donghui Zhang, Marc A. Hillmyer,* and William B. Tolman* Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431 Received February 1, 2005; Revised Manuscript Received March 10, 2005
(-)-Menthide, a seven-membered lactone derived from the natural product (-)-menthol, was polymerized using a structurally defined zinc-alkoxide catalyst to form an aliphatic polyester. The polymer was fully characterized by NMR spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. The polymerization reaction occurred in a controlled fashion and polymer samples with Mn values up to 90 kg mol-1 were obtained by varying the catalyst loading. Monitoring of the rate of polymerization by in situ FT-IR spectroscopy (ReactIR) revealed a first order dependence on (-)menthide. The temperature dependence of the observed rate constant between 30 and 90 °C was well described by the Arrhenius equation and gave Ea ) 38.4 ( 0.9 kJ mol-1. Thermodynamic parameters (∆Hp° ) -16.8 ( 1.6 kJ mol-1, ∆Sp° ) -27.4 ( 4.6 J mol-1 K-1) for the polymerization of (-)-menthide were also determined by measuring the equilibrium monomer concentration at different temperatures ranging from 40 to 100 °C. The equilibrium monomer concentrations at 25 and 100 °C were calculated to be 0.031 ( 0.018 and 0.120 ( 0.063 M, respectively. Introduction The necessity to replace petroleum-based plastics with materials based on sustainable technologies has stimulated fundamental research aimed at the preparation of a newgeneration of polymers derived from bio-renewable sources.1 One such resource is (-)-menthol, a natural product known for its “cooling” characteristics when applied to the skin or inhaled. Extracted from the plant Mentha ArVensis, thousands of tons of (-)-menthol are harvested each year and used in the pharmaceutical, confectionery, and flavor and fragrance industries.2 Many biosynthetic derivatives of (-)-menthol are available, and chemical conversions of (-)-menthol and its derivatives are widespread.2 In one specific example, the readily available ketone derivative of (-)-menthol, (-)menthone,3 can be converted to the seven-membered lactone (-)-menthide (1) through a simple Baeyer-Villiger reaction (Figure 1).4 Drawing an analogy to the well-established chemistry of the related monomers -caprolactone,5 lactide,6 glycolide,6,7 and propiolactone,8 ring-opening polymerization of 1 may be envisioned to give a potentially useful aliphatic polyester (Figure 1). Indeed, this polymerization using Na to yield a “thick white hygr. gel” after 2 h at 170 °C was reported in 1958,9 but since that very limited description, we have not found any other report. In this paper, we present a detailed study of the polymerization of 1 to give high molecular weight polymers under mild conditions using a discrete Zn-based catalyst recently discovered in our laboratories.10 Fundamentally important thermodynamic and kinetic parameters associated with the polymerization are presented. * To whom correspondence should be addressed. E-mail: hillmyer@ chem.umn.edu (M.A.H.);
[email protected] (W.B.T.).
Figure 1. Synthesis of (-)-menthide (1) from (-)-menthone and polymerization of 1 using a discrete Zn alkoxide catalyst (Zn). “BV” ) Baeyer-Villiger reaction.
Results and Discussion The synthesis of 1 from (-)-menthone using a mixture of sodium persulfate and concentrated sulfuric acid was first reported in 1899 by Baeyer and Villiger.11 While “green” Baeyer-Villiger reactions are being actively pursued,4,12,13 for convenience we chose to use meta-chloroperbenzoic acid14 to oxidize (-)-menthone to 1. Analytically pure 1 was readily obtained on multigram scale in >70% yield by recrystallization and sublimation of the crude product. NMR (1H and 13C{1H}), HR-MS, MS, IR, melting point, and optical rotation characterization data for 1 were all consistent with data reported in the literature.13,15 For all of the polymerization studies reported here, we chose to use one of the most active, well-defined zinc alkoxide catalysts reported (Zn, Figure 1).10 A typical polymerization entailed dissolution of 1 in toluene ([1]0 ) 1.83 M) at room temperature followed by addition
10.1021/bm050076t CCC: $30.25 © 2005 American Chemical Society Published on Web 04/23/2005
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Figure 2. 1H and 13C{1H} NMR spectra of poly-1 in CDCl3 [both scales are in ppm].
of Zn ([1]0/[Zn]0 ) 200) under an inert atmosphere. After approximately 8.5 h of stirring followed by termination by exposure to air, 96% of 1 was converted to a polymeric product with a molecular weight of 63 kg mol-1 and a PDI of 1.3 by SEC using polystyrene standards. 1H and 13C{1H} NMR data (Figure 2) are consistent with the repeating units resulting from the expected ring-opening process with no epimerization of the chiral centers in 1. The MALDI mass spectrum of a low molecular weight sample of poly-1 (Figure 3) was consistent with the expected C10H18O2 repeat unit structure. Furthermore, the molecular weights of individual polymer chains were consistent with poly-1 containing one ethoxy and one hydroxy end group. Insertion of the ethoxy group from Zn and termination by atmospheric water accounts for this observation. In addition, the MALDI data show evidence for cyclic oligomers of poly-1 (* peaks, Figure 3), a common side product in many cyclic ester polymerizations that stems from intramolecular transesterification.16,17 Collectively, these data confirm that the polymerization of 1 to poly-1 occurs to give a polymer with the expected polyester repeating unit and end group structure. The rate of polymerization of 1 by Zn as a function of temperature was determined by monitoring the decay of 1 by in situ FT-IR spectroscopy (ReactIR; [1]0 ) 1.0 M and [Zn]0 ) 0.005 M). The decay of [1] over time was found to be exponential, indicative of a rate law that exhibits a first order dependence on [1] (Figure S1). Assuming a first order dependence on [Zn] (i.e., - d[1]/dt ) kp[Zn][1]) by analogy to what was found in more extensive studies of the kinetic behavior of this catalyst in the polymerization of lactide,10 we calculate a second-order rate constant kp of (1.4 ( 0.5) × 10-2 M-1 s-1 at 30 °C. This rate constant is several orders of magnitude slower than for the related polymerization of lactide (kp ) 2.2 M-1 s-1 at 25 °C in CH2Cl2),10 which is
Zhang et al.
Figure 3. MALDI-MS spectrum of low molecular weight poly-1 (Mn ) 2.0 kg‚mol-1, PDI ) 1.2). #: the mass peaks correspond to the sum of an integral number of repeating units (170.25), ethanol (46.07), and sodium ion (22.99) from the matrix, consistent with poly-1 terminated with hydroxy and ethoxy ester end-groups; *: the mass peaks correspond to the sum of an integral number of repeating units (170.25) and sodium ion (22.99) from the matrix, consistent with macrocyclic esters formed through intramolecular transesterification.
Figure 4. Plot of ln([1]eq/[1]ss) versus T-1. Conditions: toluene, [1]0 ) 0.20 M, [1]0:[Zn]0 ) 100:1. (2, 1: Two sets of independent experiments were performed under the same conditions).
not surprising given the significant steric congestion around the ester functionality in 1. The temperature dependence of the pseudo first-order rate constant between 30 and 90 °C is well described by the Arrhenius expression and gave Ea ) 38.4 ( 0.9 kJ mol-1 (Figure S2). At 90 °C using 0.5 mol % Zn, the t1/2 for the polymerization of 1 is approximately 14 min. Thermodynamic information on the polymerization reaction was obtained by measuring the amount of 1 remaining at equilibrium ([1]eq) at temperatures between 40 and 100 °C using [1]0 ) 0.20 M and [Zn] ) 0.002 M in toluene. Thermodynamic parameters were then calculated using eq 1 with a standard state concentration of 1 ([1]ss) of 1.0 M. ln
( ) [1]eq [1]ss
)
∆Hp° ∆Sp° RT R
(1)
A plot of ln([1]eq/[1]ss) versus 1/T is linear (Figure 4) providing a standard state enthalpy of polymerization
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Catalytic Polymerization of a Cyclic Ester
Figure 5. Dependence of molecular weight (Mn, b) and polydispersity (PDI, 2) on conversion for the polymerization of 1. Conditions: toluene, room temperature, [1]0 ) 1.84 M, [1]0:[Zn]0 ) 200:1. Table 1. Data for Ring-Opening Polymerization of 1a [1]0:[Zn]0
conversion (%)
Mn (theory)
Mn (SEC)
PDI (SEC)
yield (%)
10 50 100 200 400
96.3 96.5 96.2 95.3 86.4
1.6 8.5 16.4 32.4 58.8
3.3 14.3 41.6 63.1 91.0
1.6 1.2 1.4 1.3 1.1
80 85 83 85
a
Figure 6. Dependence of polydispersity (PDI, b) and molecular weight (Mn, 2) of poly-1 on the reaction time after full conversion was reached. Condition: toluene, room temperature, [1] ) 1.84 M, [1]0: [Zn]0 ) 50:1.
Conditions: [1]0 ) 1.84 M, toluene, RT.
∆Hp° ) -16.8 ( 1.6 kJ mol-1 and a standard state entropy of polymerization ∆Sp° ) -27.4 ( 4.6 J mol-1 K-1. These thermodynamic data give values of [1]eq of 0.031 ( 0.018 M at 25 °C and 0.120 ( 0.063 M at 100 °C. For the unsubstituted seven-membered ring, -caprolactone, the corresponding thermodynamic parameters (measured in a CH2Cl2/toluene solvent mixture) are ∆Hp° ) -14.0 kJ mol-1 and ∆Sp° ) -6.0 J mol-1 K-1.5 Thus, considering the experimental error associated with ∆Hp° and the different solvent systems used, the exothermicity of the polymerization of 1 (a reflection of its ring strain) is roughly comparable to that of -caprolactone. The ∆Sp° for polymerization of 1 is significantly more negative than that for -caprolactone, as expected from previously presented arguments that invoke Thorpe-Ingold effects.18 With detailed knowledge of the rate of polymerization and thermodynamic parameters in hand, we explored molecular weight control of poly-1 by varying the catalyst/initiator (Zn) loading. These data are summarized in Table 1. The conversions were measured by integrating 1H NMR resonances due to 1 and poly-1 in the reaction mixture, and the yields were obtained gravimetrically after precipitation of the high molecular weight products. The molecular weight (by SEC versus polystyrene standards) of poly-1 could generally be controlled as evinced by the linear increase in the molecular weight of poly-1 using initial Zn concentrations ([Zn]0) ranging from 4.60 to 184 mM and [1]0 ) 1.84 M (Figure S3). The polydispersity indices for poly-1 in these experiments ranged between 1.1 and 1.6. To further explore the controlled nature of this polymerization, we followed the molecular weights and PDI values for poly-1 as a function of monomer conversion at a single [Zn]0 value (Figure 5). A linear increase in molecular weight with conversion was observed, and all of the PDI values were less than 1.1 with the exception of the sample at the highest conversion. At higher conversions, the initially unimodal
Figure 7. Corresponding SEC traces of poly-1 at different reaction times after full conversion was reached. Condition: toluene, room temperature, [1] ) 1.84 M, [1]0:[Zn]0 ) 50:1.
peak in the SEC data develops a high molecular weight shoulder, and thus the PDI values are larger. We explored this phenomenon in more detail in a separate polymerization experiment by following the molecular weight and PDI as a function of time after full conversion of 1 (Figure 6). Although the number average molecular weight of poly-1 was essentially constant over the course of the experiment, the PDI values for the samples increased steadily. The SEC traces for samples taken from this experiment are revealing (Figure 7). The high molecular weight shoulder grows in over time and eventually merges with the low molecular weight peak to recover a unimodal trace with an increased PDI. This behavior is consistent with a slow intermolecular transesterification process occurring after complete conversion of 1.17,19 Although polymerization/depolymerization equilibria can also lead to broadening of the molecular weight distribution, this process normally does not result in the bimodal broadening of PDI.20 In the case of the polymerization of 1 by Zn, the data are consistent with active polymer chain ends participating in intermolecular transesterifcation reactions, thus resulting in randomization of the molecular weight distribution of the sample in a predictable manner after the equilibrium conversion of 1 has been reached. Conclusions We have performed a detailed study of the ring-opening polymerization of 1, a (-)-menthol derivative, and demonstrated that 1 can be readily polymerized in the presence of a recently reported zinc-alkoxide catalyst (Zn). The reactions occur in a controlled fashion and poly-1 samples with
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molecular weights up to 91 kg mol-1 were readily prepared by varying the catalyst loadings or controlling the monomer conversion. Thermodynamic parameters were obtained for the polymerization of 1, with enthalpy data that suggest that the ring strain for 1 is roughly comparable to that for -caprolactone. Analysis of the polymerization rate showed that the reaction follows kinetics that are first order in [1] with a rate significantly slower than the polymerization of lactide under similar conditions. Most importantly, we have demonstrated the simple conversion of a naturally occurring terpene derivative to a monomer that is readily polymerized to yield a potentially useful high molecular weight material. Experimental Section General Considerations. All air- or moisture-sensitive compounds were handled under a nitrogen atmosphere, either using standard Schlenk-line techniques or in a glovebox. Toluene and dichloromethane used for polymerizations were purified by passing through activated alumina-based columns (Glass Contour, Laguna Beach, CA), and all other solvents were used as received from a commercial source without further purification. (-)-Menthone (90%) and 3-chloroperoxybenzoic acid (mCPBA, 77%) were used as received from Aldrich without further purification. The zinc catalyst was synthesized as previously reported.10 NMR spectra were collected on a Varian INOVA-300 or Varian INOVA-500 spectrometer. Chemical shifts were reported in the units of ppm and referenced to the protio impurities in the deuterated solvents for 1H and corresponding 13 C resonances. Molecular weights (Mn and Mw) and polydispersity indices (Mw/Mn) were determined by size exclusion chromatography (SEC) using polystyrene standards. Samples were analyzed at 40 °C using a Hewlett-Packard highpressure liquid chromatograph equipped with three Jordi poly(divinylbenzene) columns of 104, 103, and 500 Å pore sizes and a HP1047A differential refractometer. The progress of selected polymerizations was monitored using a Mettler Toledo ReactIR 4000 equipped with a diamond probe. Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) was performed on a Bruker Reflex III MALDI-TOF mass spectrometer. Samples were deposited in a dithranol/sodium trifluoroacetate matrix. Internal calibration of the instrument was performed using PEO standards. Optical rotation measurements were performed using a JASCO DIP-370 system. FT-IR spectra were collected on a Thermo Nicolet Avatar 370 FT-IR spectrometer. Melting points were measured using a Mel-temp 3.0 melting point machine. GC/MS experiments were performed on a HewletPackard HP G1800A GCD system. Thin-layer chromatography (TLC) was performed on prepared silica gel TLC plates (0.2 mm silica gel 50 F-254, plastic plates). Column chromatography was performed using silica gel 60 (70-230 mesh) with the optimal solvent system determined by TLC. Synthesis of (-)-Menthide (1). 3-Chloroperoxybenzoic acid (14.4 g, 0.0642 mol) was added in batches to a solution of (-)-menthone (9.20 mL, 0.0532 mol) in anhydrous dichloromethane (150 mL) under a flow of nitrogen at 0 °C with stirring. The mixture was allowed to warm to room
Zhang et al.
temperature and was stirred overnight. Some white precipitate formed and was removed by filtration. The filtrate was then washed with saturated aqueous solutions of NaS2O4 (1 × 200 mL), Na2CO3 (1 × 200 mL), distilled water (2 × 200 mL), and brine (1 × 200 mL). The organic layer was separated, dried over anhydrous MgSO4 and filtered. Evaporation of solvent from the filtrate afforded a yellow oil, which was purified by passing through a silica column using a mixture of pentane and ethyl acetate (3:1) as eluent. A colorless oil was obtained and dissolved in minimal pentane (30 mL). By allowing the pentane solution to stand at -20 °C overnight, the product was obtained as white needles (6.7 g, 74%). These were further purified by sublimation (45-50 °C, 0.4 mmHg). [R]25D ) -10.5 (c, 4.0 g/100 mL, CHCl3); mp 46.3-48.0 °C; FT-IR (neat, cm-1) 2962, 2937, 2874, 1715, 1275, 1232, 1164, 1019, 1004; GC/MS (m/z): 55, 69, 81, 99, 127; HRMS-CI (m/z): [M + Na]+ Calcd. for C10H18O2Na, 193.1204; found 193.1211; 1H and 13C{1H} NMR spectra are shown in Figure S4; these and the optical rotation data are consistent with those reported in the literature.13,15 Synthesis of Poly[(-)-menthide] (poly-1). Polymerizations were performed either at room temperature or elevated temperatures depending on the catalyst loading. The glassware used was oven dried at 150 °C, treated with a solution of Me2SiCl2 (1.0 M in CH2Cl2), and oven dried at 200 °C for a minimum of 3 h before use. For the purpose of comparison of data among different samples, the initial monomer concentration was kept the same unless otherwise stated. A stock solution of catalyst Zn was prepared such that [Zn] ) 0.050 M in toluene and was stored under nitrogen at -30 °C. The amount of catalyst used varied depending on the desired molecular weight of the resulting polymers. A representative procedure is given as follows. In the glovebox, a vial was charged with 1 (50.0 mg, 0.294 mmol) and toluene (101 µL). A volume of the catalyst stock solution (59.0 µL, 2.94 µmol) was injected into the monomer solution with stirring. The reaction mixture was stirred at room temperature for an appropriate time (overnight in this case). The reaction was then quenched by exposure to the air outside the glovebox. The solvent was then evaporated, and the remaining polymer (38 mg, 80%) was washed with cold hexane and dried at 50 °C in a vacuum oven. 1H NMR (CDCl3) δ 4.72 (m, 1H), 2.30 (dd, JAB) 14.7 Hz, J ) 5.6 Hz, 1H), 2.07 (dd, JAB) 14.7 Hz, J ) 8.6 Hz, 1H), 1.94 (m, 1H), 1.81 (m, 1H), 1.43-1.63 (m, 2H), 1.31 (m, 1H), 1.17 (m, 1H), 0.94 (d, J ) 6.6 Hz, 3H), 0.88 (d, J ) 6.6 Hz, 6H); 13C{1H} NMR (CDCl3) δ 173.1, 78.5, 42.1, 32.8, 31.3, 30.5, 28.6, 19.9, 18.8, 17.7. Kinetic Experiments. The progress of a polymerization reaction was monitored in situ by a ReactIR 4000 spectrometer. The reaction vessel containing a solution of monomer and catalyst and the ReactIR probe were pre-assembled in the glovebox before being quickly attached to the IR spectrometer outside the glovebox. The decay of a peak at 1273.8 cm-1 due to 1 was monitored to more than 4 halflives. A nonlinear fit of the decay curve to At ) (A0 - A∞) exp(-kobst) + A∞ gave the observed rate constant (kobs) (Figure S1). By assuming a first order dependence on [Zn],
Catalytic Polymerization of a Cyclic Ester
the second-order rate constant (kp) was calculated by using kobs ) kp[Zn]. Measurement of Thermodynamic Parameters. Polymerization reactions were performed at different temperatures and quenched by exposure to the air after a certain period of time. The reaction time was chosen to be long enough to ensure attainment of equilibrium between monomer and polymer and was determined by reference to the kinetic results. A representative procedure is given as follows. In the glovebox, a vial was charged with 1 (50.0 mg, 0.294 mmol) and toluene (1.411 mL). A volume of the catalyst stock solution (59.0 µL, 2.94 µmol) was injected into the monomer solution with stirring. The reaction mixture was stirred at 40 °C for 15 h. The reaction was then quenched by exposure to the air outside the glovebox. Analysis by 1H NMR spectroscopy indicated 80.4% conversion of the monomer, corresponding to [1]eq ) 0.039 M. Acknowledgment. We thank the Initiative for Renewable Energy and Environment (IREE) at the University of Minnesota and the National Science Foundation (CHE0236662) for funding this research. Supporting Information Available. 1H and 13C{1H} NMR spectra of 1 in CDCl3, FT-IR spectrum of 1, typical ReactIR data for a polymerization of 1 by Zn in toluene, the Arrhenius plot of ln(kobs) versus T-1 and dependence of molecular weight (Mn) on ([1]0 - [1]t)/[Zn]0 for the polymerization of 1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 1078. (b) Wedin, R. Chemistry 2004, Spring, 30. (2) Hopp, R. Rec. AdV. Tobacco Sci. 1993, 19, 3. (3) Brown, H. C.; Garg, C. P. J. Am. Chem. Soc. 1961, 83, 2952. (4) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Chem. ReV. 2004, 104, 4105. (5) Save, M.; Schappacher, M.; Soum, A. Macromol. Chem. Phys. 2002, 203, 889.
Biomacromolecules, Vol. 6, No. 4, 2005 2095 (6) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. ReV. 2004, 104, 6147. (7) Chujo, K.; Kobayashi, H.; Suzuki, J.; Tokuhara, S.; Tanabe, M. Makromol. Chem. 1967, 100, 262. (8) (a) Gresham, T. L.; Jansen, J. E.; Shaver, F. W. J. Am. Chem. Soc. 1948, 70, 998. (b) Asahara, T.; Katayama, S. Kogyo Kagaku Zasshi 1966, 69, 725. (c) Fukui, K.; Yuasa, S.; Kagitani, T.; Shimizu, T.; Sano, T. JP 38,026,596, 1963. (d) Marans, N. S. U.S. Patent 3,111,469, 1963. (e) Matsumura, S.; Beppu, H.; Toshima, K. ACS Symp. Ser. (Enzymes Polym. Synth.) 1998, 684, 74. (f) Noltes, J. G.; Verbeek, F.; Overmars, H. G. J.; Boersma, J. J. Organomet. Chem. 1970, 24, 257. (g) Ouhadi, T.; Heuschen, J. M. J. Macromol. Sci., Chem. 1975, A9, 1183. (h) Yamashita, M.; Takemoto, Y.; Ihara, E.; Yasuda, H. Macromolecules 1996, 29, 1798. (i) Jedlinski, Z.; Kurcok, P.; Kowalczuk, M. Polym. Int. 1995, 37, 187. (9) Hall, H. K., Jr.; Schneider, A. K. J. Am. Chem. Soc. 1958, 80, 6409. (10) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W. W.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350. (11) Baeyer, A.; Villiger, V. Ber. Dtsch. Chem. Ges. 1899, 32, 3625. (12) Inokuchi, T.; Kanazaki, M.; Sugimoto, T.; Torii, S. Synlett. 1994, 1037. (13) Kaneda, K.; Ueno, S.; Imanaka, T.; Shimotsuma, E.; Nishiyama, Y.; Ishii, Y. J. Org. Chem. 1994, 59, 2915. (14) (a) Canan Koch, S. S.; Chamberlin, A. R. Synth. Commun. 1989, 19, 829. (b) Still, W. C.; Darst, K. P. J. Am. Chem. Soc. 1980, 102, 7385. (15) (a) Asakawa, Y.; Matsuda, R.; Tori, M.; Hashimoto, T. Phytochemistry 1988, 27, 3861. (b) Jakovac, I. J.; Jones, J. B. J. Org. Chem. 1979, 44, 2165. (c) Shono, T.; Matsumura, Y.; Hibino, K.; Miyawaki, S. Tetrahedron Lett. 1974, 1295. (d) Odinokov, V. N.; Ishmuratov, G. Yu.; Yakovleva, M. P.; Muslukhov, R. R.; Safiullin, R. L.; Volgarev, A. N.; Komissarov, V. D.; Tolstikov, G. A. Dokl. Chem. 1992, 326, 842. (16) Libiszowski, J.; Kowalski, A.; Szymanski, R.; Duda, A.; Raquez, J.-M.; Dege´e, P.; Dubois, P. Macromolecules 2004, 37, 52. (17) Duda, A.; Penczek, S. Biopolymers; Wiley-VCH: Weinheim, Germany, 2001; Vol. 3b, pp 371-429. (18) For example see: Forbes, M. D. E.; Patton, J. T.; Myers, T. L.; Maynard, H. D.; Smith, D. W., Jr.; Schulz, G. R.; Wagener, K. B. J. Am. Chem. Soc. 1992, 114, 10978. (19) (a) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Marcromol. Rapid Commun. 1997, 18, 325. (b) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Macromol. Symp. 1997, 123, 93. (c) Penczek, S.; Duda, A.; Szymanski, R. Macromol. Symp. 1998, 132, 441. (20) See, for example: Wang, Y.; Hillmyer, M. A. Macromolecules 2000, 33, 7395.
BM050076T
