Article pubs.acs.org/Macromolecules
Intermolecular Transfer to Polymer in the Radical Polymerization of n‑Butyl Acrylate Nicholas Ballard, Shaghayegh Hamzehlou, and José M. Asua* POLYMAT and Kimika Aplikatua Saila, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Etorbidea 72, 20018, Donostia/San Sebastián, Spain S Supporting Information *
ABSTRACT: Transfer to polymer in the radical polymerization of acrylic monomers results in chain branching and has significant implications for both reaction kinetics and the microstructure of the resulting polymer. While intramolecular transfer to polymer is more prevalent than the intermolecular pathway, intermolecular transfer to polymer is of particular importance for understanding polymer microstructures. Despite this, the magnitude of the rate coefficient is uncertain, and therefore, predicting the effects of intermolecular transfer remains a challenge. Herein, we seek to provide an estimate of the rate of intermolecular transfer of butyl acrylate by conducting reversible addition−fragmentation chain transfer (RAFT) polymerizations in the presence of a low molecular weight dead polymer. In these experiments, intermolecular transfer to polymer yields a characteristic low molecular shoulder in the polymer detected by UV in the SEC. We obtain a value significantly higher than those previously estimated and discuss the implications this has on branch formation, molecular weight and gel formation in radical polymerization of acrylic monomers as well as the formation of dead chains in controlled radical polymerization.
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INTRODUCTION
The second pathway for formation of midchain radicals and branch points is via intermolecular transfer, whereby a hydrogen atom is abstracted from another polymer chain in the mixture, forming a dead chain and a midchain radical which upon propagation yields a long chain branch (see Scheme 1). It has been shown by numerous groups that the rate of intramolecular transfer to polymer is far greater than that of intermolecular transfer and the overwhelming majority of branch points are formed via the intramolecular pathway.6,7,18−21 Although intermolecular transfer is less prevalent than intramolecular transfer, and as such has little influence on the reaction kinetics, it has a strong effect on the microstructure of the polymer produced. For example, intermolecular transfer to polymer followed by termination by combination is known to be the cause of gel formation in the bulk polymerization and emulsion polymerization of acrylic monomers.7,18,22 The size and extent of the gel networks that are formed are critical factors in the rheological properties of films cast from polymer solutions/dispersions,23−26 and therefore, intermolecular transfer to polymer exerts a significant influence over the commercial applications of acrylic polymers. Despite the importance of intermolecular transfer to polymer in the radical polymerization of acrylic monomers, relatively little effort has been made to quantitate this effect. Plessis and
Transfer to polymer during free radical polymerization leads to formation of a radical in the polymer backbone and, upon monomer propagation from this midchain radical, a branch point in the chain.1−3 In acrylic polymers, branching has a significant influence on the rheological properties of the polymers and impacts on the physical characteristics of polymer films, making it an area of great interest for many commercial applications.4,5 Branch points in polymerization of acrylic monomers form by one of two pathways. In the first case, a hydrogen atom is abstracted from within the polymer chain in an intramolecular transfer reaction. This typically occurs via a 6-membered ring transition state to yield a midchain radical (MCR), which generates a short chain branch upon propagation. The slow rate of propagation from the tertiary center results in a reduction in the rate of polymerization and a buildup of midchain radicals, such that the fraction of midchain radicals in polymerizations of acrylic monomers is typically greater than 0.7.6−11 At low monomer concentrations and high temperatures the midchain radical is also susceptible to scission resulting in a macromonomer species and a secondary radical.12−14 In addition, the midchain radical can undergo a series of n:n + 4 hydrogen abstractions which results in migration of the radical along the polymer backbone and can lead to a distribution in the chain length of short chain branched upon propagation,15,16 although in the presence of monomer this mechanism is less likely.17 © XXXX American Chemical Society
Received: June 4, 2016 Revised: July 13, 2016
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DOI: 10.1021/acs.macromol.6b01195 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
The aim of this work is to quantitate the effects of intermolecular transfer in the radical polymerization of butyl acrylate. To this end, RAFT polymerizations of nBA were conducted in the presence of a high quantity of preformed poly(nBA). RAFT polymerization of nBA in the presence of trithiocarbonates is known to lead to monodisperse polymer chains of well-defined molecular weight.30−32 The ratio RAFT/ nBA was chosen in such a way that the molecular weight of the polymer produced during the RAFT polymerization was different from the preformed poly(nBA). Therefore, the two populations were distinguishable in the SEC chromatograms, and due to the high UV absorbance of the RAFT end group, the location of the RAFT agent in the populations can be easily detected. The presence of the RAFT end group in the preformed polymer requires the occurrence of intermolecular transfer to polymer and therefore, this process can be detected and quantified. By fitting of the molecular weight distribution (MWD) of the living fraction of chains across a wide temperature range with a mathematical model, the Arrhenius parameters of the intermolecular transfer to polymer reaction were obtained. Finally, the implications of the new estimate of the rate of intermolecular transfer on branching fraction, molecular weight and gel formation in free radical polymerization and the formation of dead chains in controlled radical polymerization of acrylic monomers are explored.
Scheme 1. Pathways to Branch Formation via Intermolecular and Intramolecular Transfer to Polymer in Radical Polymerization of Acrylic Monomers
co-workers gave a value of the rate coefficient for intermolecular transfer (ktr,p) of 0.178 M−1 s−1 at 75 °C based on the fitting of a mathematical model to experimental measurements of gel content in the seeded semibatch emulsion polymerization of n-butyl acrylate (nBA).18 In their model, equal reactivities of the secondary and tertiary radicals toward intermolecular transfer to polymer were assumed, which leads to an unrealistic situation in which intermolecular transfer of the tertiary radicals becomes highly probable. In the RAFT polymerization of nBA for the preparation of star polymers, Boschmann and Vana observed a star coupling which was ascribed to intermolecular transfer to polymer events. On the basis of a mathematical model they estimated a value of 0.33 M−1 s−1 at 60 °C.27 This model, however, did not account for the occurrence of intramolecular transfer and thus neglected the effects of midchain radicals on the polymerization. On the basis of the measurements of branching content of poly(n-butyl acrylate) (poly(nBA)) formed using various concentrations of monomer, a much higher value of 200 M−1 s−1 at 80 °C has also been quoted in the literature.28 Thus, the estimated values for the rate coefficient of intermolecular transfer to polymer vary approximately by 3 orders of magnitude. Moghadam et al. performed a theoretical study on the pathway of intermolecular transfer to polymer in polymerization of acrylates.29 Their study highlighted that the most likely reaction pathway is indeed that shown in Scheme 1, and although the level of theory used was not high enough to accurately predict the kinetics of the reaction, analysis of the transition states yielded an activation energy of around 55 kJ/ mol.
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EXPERIMENTAL SECTION
Materials. n-Butyl acrylate (nBA, Quimidroga, technical grade) was purified by distillation and was kept at −20 °C until use. 2[(butylsulfanyl) carbonothioyl sulfanyl] propanoic acid (TTCA-4) was prepared according to Ferguson et al.,33 2,2′-Azobis(2-methylpropionitrile) (AIBN, > 98%, Aldrich), Toluene (>99.5%, Aldrich), 1octanethiol (>98.5%, Aldrich), potassium hydroxide (>85%, Aldrich), 1,1′-Azobis(cyclohexanecarbonitrile) (ACHN, 98%, Aldrich), and tertbutyl peroxybenzoate (Luperox P, 98%, Aldrich) were used as received. All other solvents were purchased from Scharlab, were of technical grade, and were used without purification. Synthesis of Low Molecular Weight Poly(butyl acrylate). AIBN (300 mg) was dissolved in a mixture of nBA (200 g), 1octanethiol (20 g) and toluene (200 g) in a 1 L double walled glass reactor equipped with anchor type stirrer, nitrogen inlet, condenser, and thermocouple. The mixture was purged with nitrogen for 30 min then heated to 60 °C for 6 h. The conversion of the monomer was X = 0.88 as determined by gravimetry. The solution was washed four times with an aqueous solution of potassium hydroxide (500 mL, 0.1 M) and then washed three times with a mixture of methanol−water with a weight ratio of 75:25. The viscous liquid obtained was dried under vacuum at 60 °C for 48 h. Analysis by headspace GC showed a residual thiol content of
