SELF-CONDENSING VINYL POLYMERIZATION: THEORETICAL ASPECTS AND APPLICATION TO GROUP TRANSFER POLYMERIZATION OF METHACRYLATES.

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1 SELF-CNDENSING VINYL PLYMERIZATIN: THERETICAL ASPECTS AND APPLICATIN T GRUP TRANSFER PLYMERIZATIN F METHACRYLATES. Peter. F. W. Simon, Wolfgang Radke, Deyue Yan, a) Axel H. E. Müller Institut für Physikalische Chemie, Universität Mainz, D Mainz, Germany, and Krzysztof Matyjaszewski Department of Chemistry, Carnegie-Mellon University, Pittsburgh, PA Recently, Fréchet et al. 1,2 reported a new way of forming hyperbranched polymers, i.e. the so called self-condensing vinyl polymerization (SCVP). This reaction involves a monomer of the general structure A=B-C*, in which the C* is a group capable of initiating the polymerization of vinyl groups A=B. In contrast to Flory s approach of polycondensation of a monomer with AB 2 functionality, 3 the chain initiation is the addition of an initiating C* group to the double bond of another monomer, leading to a dimer with two active sites B* and C* and one double bound A=B. Both the initiating C* group and the and the newly created B* group can react with the vinyl group of another molecule in the same way. SCVP has been applied to various kinds of living polymerization, i.e. cationic 1,2, radical 4, and atom transfer radical polymerization. 5 We recently reported the application to group transfer polymerization. 6 We have theoretically studied the kinetics, the molecular weight distribution (MWD), and the degree of branching, DB, using a new definition of this parameter. 7,8 The calculated MWD is extremely broad, the polydispersity index being equal to the number-average degree of polymerization: P w / P n = P n. It is even broader than that for the polycondensation of AB 2 type monomers. The kinetics of the polymerization process follow first order with respect to the monomer and both the number-average degree of polymerization, with time. P n, and the polydispersity index increase exponentially a) Permanent Address: Shanghai Jiao Tong University, Department of Applied Chemistry, 1954 Hua Shan Road, Shanghai , The People's Republic of China

2 P n, P w, P w /P n 10 P w P n ' P' w /P' n P w ' P n ; P w /P n τ = k M 0 t Fig. 1: Dependence of MWD averages and polydispersity index on reduced time, τ = km 0 t, for SCVP including ( ) and excluding (----) residual monomer. Comparison of the theoretical results with published experimental data 1 indicates that the rate of addition of an active center to a vinyl group decreases with increasing degree of polymerization. w n Since there are two different active centers in SCVP, namely C* and newly formed B*, the effect of non-equal reactivities on the kinetics and on P n is also discussed. It is shown that the kinetics and the MWD of the process are strongly affected. A numerical analysis reveals that the polydispersity index increases from P / P = 2 for k B = 0 (corresponding to a polycondensation of linear molecules) to P w /P n > P n for k C > k B, and to Pw '/ Pn ' 0.63 P n ' for k C >> k B when the large fraction of residual monomer molecules is not taken into account. 2

3 0,5 0,4 DB, FB 0,3 0,2 DB (AB*) DB (AB 2 ) 0,1 FB (AB*) FB (AB 2 ) 0,0 0,0 0,2 0,4 0,6 0,8 1,0 conversion, x Fig. 2: Dependence of the degree of branching, DB, and the fraction of branch points, FB, on conversion of vinyl groups, x, for SCVP (A=B-C*) and for polycondensation of AB 2 monomers. Using a new definition of this parameter, the degree of branching is calculated as a function of conversion (Fig. 2). At full conversion, DB is somewhat smaller for SCVP (DB = 0.465) than for AB 2 systems (DB = 0.5). The effect of non-equal reactivities on DB is also discussed. It is shown that at a ratio of rate constants of the two kinds of active centers, r = k B = 2.6, a maximum value of DB = 0.5 is reached (Fig. 3). For the limiting case r << 1, a linear polymer resembling a polycondensate A=B-C-[-A-B*-C-] n -A-B*-C* is formed whereas for r >> 1 a slightly branched vinyl polymer with the limiting linear structure should be obtained. A=B-C-[-A-B(C*)-] n -A-B*-C*. 3

4 1,00 C* = 1 - B* 0,75 fractions, DB 0,50 DB 0,25 r = 2.6 0,00 1E-3 0,01 0, r = k B Fig. 3: Fractions of active centers and degree of branching as a function of r = k B for full conversion Hyperbranched polymethacrylates were obtained by introducing selfcondensing group transfer polymerization (SCGTP) of 2-(2-methyl-1-triethylsiloxy-1-propenyloxy)ethyl methacrylate (MTSHEMA) in THF catalyzed by tetrabutylammonium bibenzoate at -50 C. 6 A B SiEt 3 C* MTSHEMA Absolute calibration curves for PMTSHEMA and the linear vinyl analogue, poly[2-(isobutyryl)ethyl methacrylate], PIBHEMA, were constructed using SEC-viscometry and SEC-light scattering, respectively. The calibration curve for PIBHEMA shows for lower elution volumes a slight difference to the PMMA calibration curve. In comparison to PIBHEMA, the calibration curve for PMTSHEMA is strongly shifted to higher molecular weights, due to the lower hydrodynamic volume of the branched structure. From the measured intrinsic viscosities and the molecular weights obtained from universal calibration, Mark-Houwink plots were established for both polymers. The low value of the Mark-Houwink exponent found for PMTSHEMA (α = 0.40) compared to PIBHEMA (α = 0.79) shows undoubtedly a densely packed structure resulting from the hyperbranched topology. Acknowledgment: This work was supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 262. References 1) Fréchet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.; 4

5 Grubbs, R. B. Science 1995, 269, ) Aoshima, S.; Fréchet, J. M. J.; Grubbs, R. B.; Henmi, M.; Leduc, L. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1995, 36, 531 3) Flory, P. J. J. Am. Chem. Soc. 1953, 74, ) Hawker, C. J.; Fréchet, J. M. J.; Grubbs, R. B.; Dao, J. J. Am. Chem. Soc. 1995, 117, ) Gaynor, S.G.; Edelman, S.; Matyjaszewski, K. Macromolecules 1996, 29, ) Simon, P.F.W.; Radke, W.; Müller, A.H.E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 38(1), in press 7) Müller, A. H. E.; Yan, D.; Wulkow, M. Macromolecules, submitted 8) Yan, D.; Müller, A. H. E.; Matyjaszewski, K. Macromolecules, submitted 5