1332 Macromolecules 1999, 32, 1332-1340 Radical Ring-Opening Copolymerization of 2-Methylene 1,3-Dioxepane and Methyl Methacrylate: Experiments Originally Designed To Probe the Origin of the Penultimate Unit Effect G. Evan Roberts, Michelle L. Coote, Johan P. A. Heuts, Leesa M. Morris, and Thomas P. Davis* School of Chemical Engineering & Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received August 27, 1998; Revised Manuscript Received December 14, 1998 ABSTRACT: Pulsed-laser polymerization experiments were conducted on the copolymerization of 2-methylene 1,3-dioxepane (MDO) and methyl methacrylate (MMA) at 40 °C. Reactivity ratios, based on the terminal model, were determined from copolymer compositional analyses as rMDO ) 0.057 and rMMA ) 34.12. The average propagation rate coefficients, 〈kp〉, were determined for a number of feed compositions ranging from an MDO mole fraction of 0 to 0.916. It was not possible to obtain a 〈kp〉 value for the homopolymerization of MDO as the PLP experiment was compromised by high chain transfer rates to polymer and monomer. The data were fitted to a number of copolymerization models, viz. the terminal and the explicit and implicit penultimate unit models and the bootstrap solvent dependence model. It was found that the terminal and penultimate unit models for copolymerization could not adequately describe the experimental results. A fit to a bootstrap model was successful, but the values obtained indicated a very large, unprecedented, partitioning effect. Introduction sounds trivial, such detailed studies have only been The pulsed-laser polymerization (PLP) technique1 has performed in a few specific cases, and the bulk of now become firmly established as a powerful tool to experimental data reported in the literature implicitly measure propagation rate coefficients in free radical assume the validity of the IPUE model without testing polymerization.2,3 Several groups have extended PLP this assumption. studies into copolymerization and terpolymerization The main problem in deciding among the various - 〈 〉 reactions.4 10 In nearly all cases observed so far, the copolymerization models is that the composition and kp terminal (or Mayo-Lewis) model fails to describe the values experimentally measured are not sensitive enough to allow for unequivocal discrimination. This was shown experimental 〈kp〉 and copolymer composition data si- multaneously. Most workers now believe that the main quite clearly by extensive studies on styrene (STY)- shortcomings of the terminal model can be assigned to methyl methacrylate (MMA) copolymerization where a strong penultimate unit effect on many copolymeri- even a large, careful experimental design did not permit 8,16 zation reactions, although opinions on the origin of the model discrimination. The challenge is therefore to penultimate unit effect may be different. The theoretical design new experiments that will permit either a direct framework that seems to have supplanted the terminal measure of the penultimate unit effect (negating the model is the implicit penultimate unit model (IPUE) need for model fitting) or experiments that simplify the developed by Fukuda and co-workers.11 This model model fitting procedure by minimizing the number of assumes that there is a substantial contribution from fit parameters. In this study, we choose the latter the γ-substituent (penultimate unit) which alters the approach, and the system we selected to investigate is reactivity but not the selectivity of the radical. As a the copolymerization of MMA with 2-methylene 1,3- consequence, the general penultimate model, as origi- dioxepane (MDO), a monomer that undergoes ring- nally proposed by Merz et al.,12 reduces to a set of much opening in free-radical polymerization. simpler mathematical equations. However, the general Radical ring-opening polymerization has been studied validity of this assumption, as justified in the stabiliza- extensively by Bailey17 and Endo.18,19 Most previous tion energy model proposed by Fukuda et al.,11 has been studies have concentrated on the synthetic aspects of questioned in recent theoretical studies13,14 which con- monomer and polymer syntheses, and there is very little clude that implicit penultimate unit effects can only kinetic and mechanistic data available in the literature. occur under specific circumstances. Therefore, we be- The radical ring-opening polymerization of MDO to yield polycaprolactone has been subject of a number of lieve that the general penultimate unit model (called - the explicit penultimate unit model by Fukuda et al.)15 studies17,18,20 22 which show that 100% ring-opening provides in general a physically more realistic, though occurs at 25 °C and the mechanism shown in Figure 1 mathematically more complex, description of the chemi- is generally assumed to be valid. The rate-determining cal reaction kinetics. The task at hand is thus to provide step has been shown to be the bimolecular propagation experimental evidence for these penultimate unit effects step. The only previous work on copolymerization of and to determine situations in which the simplified MDO monomer has been restricted to monomer reactiv- model, i.e., the IPUE model, is applicable. Although this ity ratio measurements with styrene.22 The work described in this paper is the first attempt * To whom correspondence should be addressed: E-mail: t.davis@ (to our knowledge) to measure propagation rate coef- unsw.edu.au. ficients for the ring-opening polymerization of a cyclic 10.1021/ma9813587 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/19/1999 Macromolecules, Vol. 32, No. 5, 1999 Ring-Opening Copolymerization 1333 °C, and MDO was distilled into a collection flask containing potassium carbonate and potassium hydroxide pellets (yield, 28.5%). The product MDO obtained was found to contain some BMDO contaminant which we found difficult to remove. Therefore in all the PLP experiments detailed in this paper we corrected the MDO concentration to allow for a 6% contamination by BMDO (as found by NMR). For the CLD experiments we double-distilled the MDO and obtained a purity of 99% (by NMR). This matter is discussed further in the paper. Pulsed-Laser Polymerization. The PLP procedure has been described in a number of our earlier publications8,16,23 and will not be elaborated on here. All of the PLP data were obtained at 40 °C. Figure 1. Scheme depicting the generally accepted mecha- nism for the radical ring-opening of MDO. The facile ring- Chain Transfer to Monomer Experiments. The chain opening reaction occurs exclusively at room temperature transfer to monomer constant was determined using the chain 24,25 leading to the formation of polycaprolactone. length distribution method (CLD). A stock solution of azobisisobutyonitrile (AIBN) (0.0107 g) in MDO (0.976 g) was prepared. This stock solution was successively diluted with MDO to make a number of solutions with decreasing initiator concentrations. Each solution was degassed via freeze-thaw cycles and then placed in a thermostated oven at 40 °C. The resultant polymer was precipitated in methanol, redissolved in acetone, and reprecipitated in methanol prior to drying at 20 °C in vacuo. SEC Analyses. Molecular weights were determined by SEC using tetrahydrofuran (THF) eluent at 1 mL min-1. Four 5.0 µm bead columns (Polymer laboratories) were used in series (103,104,105, and 106 Å). Two detectors were used: a PL differential refractive index detector and a Viscotek model 250 differential viscometer. NMR Analyses. 13C and 1H NMR spectra were recorded on a 300 MHz (Bruker ACF300) spectrometer using CDCl3 (Aldrich) as solvent. These were used to determine both the composition of the copolymers and the extent of ring-opening Figure 2. Chain end structures of the three radical types in the MDO homopolymers. In all of the experiments con- present in the system (assuming a negligible kinetic contribu- ducted with MDO we found 100% ring-opening. tion from antepenultimate units). The MMA radical end group may be preceded by either another MMA unit (A) or a MDO Results and Discussion (ethylene) unit (B). The MDO radical end group is always preceded by another ethylene-type unit (C). Determination of Mark-Houwink-Kuhn- Sakurada (MHKS) Parameters for PMDO in THF. ketene acetal. The specific experimental design was Attempts were made to measure the MHKS parameters originally envisaged as an attempt to simplify copolym- for PMDO using the polymers synthesized for chain erization kinetics, so that the penultimate unit effect transfer to monomer studies. The approach used has on propagation could be restricted to an influence on been described in earlier papers and utilizes SEC with just one terminal-radical type in a copolymerization online viscometric detection.23,26,27 Ten different samples system. This is illustrated in Figure 2, which shows that were subjected to MHKS analyses, and we observed the MMA terminal radical can have either a MMA or considerable scatter in the MHKS plots. This scatter can an ethylene penultimate unit. In contrast, the MDO be ascribed to the low molecular weights of the poly- terminal unit (defining a unit as consisting of two mers, caused by the high chain transfer constant to substituted carbon atoms in the polymer backbone, monomer (vide infra) and branching originating from which for MDO is effectively an ethylene unit) always backbiting reactions.20,28 Four of the 10 samples gave has the same preceding unit, viz. another ethylene unit. reasonably linear MHS plots and showed fair agree- - Therefore any penultimate unit effect which is observed ment, yielding MHKS parameters, K ) 23 × 105 dL‚g 1 may be attributed solely to the one on the MMA radical. and R)0.71. To investigate the possible influence of branching on these polymers we used 1H NMR in Experimental Section conjunction with assignments for branching reactions obtained for PMDO polymers.20,28 Branching was found Synthesis of MDO Monomer. MDO was synthesized in a two-step process via 2-bromomethyl-1,3-dioxepane (BMDO).