Article pubs.acs.org/Macromolecules
The Role of Chain Transfer Agent in Reducing Branching Content in Radical Polymerization of Acrylates Nicholas Ballard, José C. de la Cal, and José M. Asua* POLYMAT and Grupo de Ingeniería Química, Dpto. de Química Aplicada, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Etorbidea 72, 20018 Donostia/San Sebastián, Spain S Supporting Information *
ABSTRACT: Intramolecular transfer to polymer, and the reactions of the resulting midchain radical, are important processes in determining both the reaction kinetics and microstructure of acrylic polymers produced by radical polymerization. Herein, the causes of the reduction in branching content when adding chain transfer agent (CTA) to the polymerization mixture are unveiled. Quantitative NMR spectroscopy is used to determine the distribution of product species from the reaction of n-butyl acrylate in the presence of CBr4. It is shown that the reduction of branching is the result of the combined effect of a lower backbiting rate and the removal of the resulting midchain radical via transfer to CTA. The first effect is due to the minimum chain length required for backbiting to occur, and the second one is operative at high CTA/monomer ratios but becomes negligible at high concentrations of monomer.
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INTRODUCTION
radical, and hence reaction kinetics can also be affected by transfer to polymer processes.2,22,23 An analysis of the kinetics of the intramolecular transfer reaction reveals that the instantaneous degree of branching is a simple function of monomer concentration and the rate coefficients of backbiting and propagation. The branching fraction is given by the ratio of propagation from a tertiary radical center and the sum of propagation from the tertiary radical and a secondary radical such that
Radical polymerization of acrylic monomers results in polymeric materials that can contain both long and short chain branches that are the result of intermolecular and intramolecular transfer to polymer, respectively.1−11 It has been demonstrated that under typical reaction conditions intramolecular transfer dominates2,12,13and that the backbiting reaction of the propagating radical species proceeds via a sixmembered transition state to give a midchain radical.14,15 This midchain radical produced by intramolecular transfer can undergo several processes including β-scission to produce macromonomers and an active secondary radical,16 translocation of the midchain radical via a hopping mechanism,14,17,18 termination to produce multiarm chains, and propagation to give a branch point (see Scheme 1). At moderate temperatures ( 3) there is virtually no chain length dependence on the propagation rate coefficient; thus, kp/kbb is practically constant for all chain lengths. If a stronger chain length dependence on the rate coefficient for propagation is assumed, then one would expect a change in the extent of backbiting but also in the polymerization rate, which is not observed experimentally (see Figure 4). In order to check the second alternative (that backbiting requires a minimum chain length of 3), the process was simulated by means of a Monte Carlo approach (see Supporting Information). The Monte Carlo method is useful because the constraint of a minimum chain length for backbiting can be considered not only for the chain length but also, as each chain can be tracked individually, for the three propagation steps needed after backbiting before backbiting can happen again, which is difficult in deterministic models. The parameters of the model (kbb/kp, ktr/kp, and Rterm/kp[M], where kbb, kp, and ktr are the rate coefficients for backbiting, propagation, and chain transfer to CTA, respectively, and Rterm is the rate of all termination events other than chain transfer to CTA) were estimated by fitting the experimental 990
DOI: 10.1021/ma502575j Macromolecules 2015, 48, 987−993
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simulations (Supporting Information). In this case, following the model described in ref 28, linear−exponential probability distribution functions were used instead of the classical exponential ones used in the simulations leading to Figure 5. It was found that even at high concentrations of CTA, chain transfer to CTA is not fast enough to affect the competition between backbiting and propagation, namely that the effect attributed to the reduction of branching in controlled radical polymerization is not operative in the present case. The data of the distribution of species resulting from the midchain reaction (Cq, CMCBr, and CMM) given in Table 1 can be used to estimated ktr*/kp* (the ratio of rate coefficients for transfer to CBr4 and propagation of the midchain radicals) using the approach summarized in the Supporting Information. The estimated value was ktr*/kp* = 10.4. This means that for a bulk polymerization ([M]0 = 6.8 M) carried out at 90 °C using a concentration of 0.055 M of CBr4 the expected contribution of patching to the decrease of branching would be only 8%. The data in Table 1 also allow a more uncertain estimation of kβ/kp* (with kβ being the rate of β-scission of the midchain radicals) using the method explained in the Supporting Information. The estimated value was kβ/kp* = 0.14 mol L−1, which, using literature values of the rate coefficients for propagation from the tertiary center at 90 °C (91 mol L−1 s−1),11 gives kβ = 12.7 s−1, which is higher has been previously estimated at this temperature (kβ = 0.95 s−1 calculated by solution polymerization at high temperatures,11 kβ = 4.42 s−1 estimated from methyl acrylate trimer polymerization35).
Figure 4. Experimental time evolution of monomer conversion for solution polymerization of n-butyl acrylate ([M] = 0.715 mol L−1) at 90 °C with varying concentrations of CBr4 ([CBr4] = 0 (squares), 0.0136 (circles), 0.0343 (upward pointing triangle), and 0.055 mol L−1 (downward pointing triangle)). For the sake of comparison of the rates, the time has been corrected to remove the initial short inhibition period that was observed in all reactions.
values of number-average molecular weight and the relative amount of backbiting events for runs 1−4. It should be pointed out that Rterm/kp[M] is the ratio of the averages rates of termination and propagation along the polymerization, which is expected to be similar in runs 1−4 as they were carried out at the same temperature, in the same solvent, with the same concentrations of initiator and monomer, and reaching similar final conversions. The satisfactory fit between the model and experimental data in Figure 5 shows that this effect suitably accounts for the reduction in the number of backbiting events, which will consequently lead to a lower branching fraction. It has to be pointed out that both the value for the average rate of backbiting and the value of the transfer constant are in reasonable agreement with theoretical values.14,34 Run 5 could not be included in the estimation because, due to the higher monomer concentration, the kinetics and the final conversion differ significantly from runs 1−4, and thus Rterm/kp[M] should be different. Nevertheless, as the backbiting/propagation ratio was not affected by Rterm/kp[M], the estimated parameters were used calculate this ratio for run 5 obtaining a value of 2.42, which is in reasonable agreement with the experimental value. The effect of a fast third process on the competition between backbiting and propagation was also checked by Monte Carlo
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CONCLUSIONS
In conclusion, we have performed a comprehensive analysis on the reason for reduction in branching content when a high concentration of chain transfer agent is employed in radical polymerization of acrylic monomers. By using quantitative 13C and 1H NMR, it has been demonstrated that upon increasing the amount of chain transfer agent used, branching decreases because the rate of backbiting decreases and because at high CTA/monomer ratios, patching of the midchain radical through chain transfer to the CTA is significant. This latter effect becomes negligible at high concentrations of monomer. On the other hand, it is shown that over a range of monomer concentrations branching decreases because backbiting decreases with increasing [CTA]. The reason for this is that as the kinetic chain length becomes shorter when [CTA] increases,
Figure 5. Comparison between model (circles) and experimental data (squares) for ratio of backbiting to propagation for solution polymerization of n-butyl acrylate (10.5 wt %) at 90 °C with varying concentrations of CBr4. Estimated values of the parameters kbb/kp = 1.77 × 10−2 and ktr/kp = 0.92, Rterm/kp[M] = 1.50 × 10−2. 991
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the fraction of monomer units in the polymer chain that can suffer backbiting (those longer than 3) becomes smaller. Simulations show that even at high concentrations of CTA, chain transfer to CTA is not fast enough to affect the competition backbiting/propagation, namely that the effect attributed to reduction of branching in controlled radical polymerization is not operative in the present case.
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ASSOCIATED CONTENT
S Supporting Information *
Complete 1H and 13C NMR spectra, details of deterministic and Monte Carlo simulations, and methods used to estimate rate coefficients. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (J.M.A.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Diputación Foral de Gipuzkoa, University of Basque Country (UFI 11/56), Basque Government (GVIT373-10 and Etortek Nanoiker IE11-304), and Ministerio de Economiá y Competitividad (CTQ2011-25572) are gratefully acknowledged for their financial support.
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