(300d) Computational Study of the Cyclohexanone-Based Initiation Mechanism In Thermal Polymerization of Methyl Acrylate and Methyl Methacrylate

The free radical polymerization of acrylates and methacrylates has been the subject of great interest to both fundamental and industrial research due to their various applications such as automobile coatings and adhesives[1-5]. The spontaneous thermal polymerization of acrylates and methacrylates is a promising approach to produce novel resins economically[6-10]. The high-temperature prompted secondary reactions such as back-biting reactions reduce the molecular weight of produced resins. Resins with low average molecular weights require less solvent, typically volatile organic compound, in order to be brushable and sparable. The absence of an external initiator also saves the cost for expensive initiator and after-reaction purification process as no-additional moiety is introduced[10].  Junker et al.[9] also reported that highly uniform macromonomers with unsaturated end-groups were obtained in the high temperature auto-initiated polymerization of acrylates. These highly functional macromonomers can further participate in polymerization leading to the synthesis of polymers with complex architectures. However, the absence of traditional controlling agent in spontaneous thermal polymerizations to some extent limits our ability to control the reactions. Here, we are suggesting to take advantage of solvent effect to achieve the controlled spontaneous thermal polymerization as the reaction medium can influence the reaction via various ways to different extents.

Cyclohexanone (CH), a widely used solvent, has been reported to be capable of initiating the polymerization of methyl methacrylate (MMA) at 75°C[11,12]. A recent experimental study also found that spontaneous thermal polymerization of methyl acrylate (MA) at 120°C has the highest reaction rate and final conversion in cyclohexanone compared to polymerization conducted in dimethyl sulfoxide and xylene[2]. All these experimental studies point out that cyclohexanone might provide additional initiation mechanisms generating free radicals.

To the best of our knowledge, no experimental or computational study has convincingly characterized the CH-mediated initiation mechanism in details. In the present work, four different types of mechanisms, the Kaim mechanism[11], the Flory mechanism[13], the α-position hydrogen addition reaction and the Mayo mechanism[14] are studied with density functional theory(DFT) and second-order Møller–Plesset (MP2) calculations to

elucidate the true nature of cyclohexanone-based initiation reaction in MA and MMA. Transition state for each mechanism is located with B3LYP/6-31G* and assessed with MP2/6-31G*. Activation energies and rate constants are calculated and compared to determine which mechanism is more likely to be responsible for the initiation. Our results show that Diels-Alder adduct formed from a cyclohexanone and a monomer can transfer a hydrogen atom to a second monomer, generating free radicals for initiation. We propose that cyclohexanone via the Mayo mechanism, together with the monomer self-initiation reactions[3,5], is likely to explain the cyclohexanone-facilitated polymerization in thermal polymerization of MA and MMA.

References

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[3]         Srinivasan, S., Lee, M. W., Grady, M. C., Soroush, M., Rappe, A. M., Computational Study of the Self-Initiation Mechanism  in Thermal Polymerization of Methyl Acrylate, J. Phys. Chem. A, 2009, 113, p. 10787.

[4]         Srinivasan, S., Lee, M. W., Grady, M. C., Soroush, M., Rappe, A. M., Self-Initiation Mechanism in Spontaneous Thermal Polymerization of Ethyl and n-Butyl Acrylate: A Theoretical Study, J. Phys. Chem. A, 2010, 114, p. 7975.

[5]         Srinivasan, S., Lee, M. W.; Grady, M. C., Soroush, M., Rappe, A. M., Computational Evidence for Self-Initiation in Spontaneous High-Temperature Polymerization of Methyl Methacrylate, J. Phys. Chem. A, 2011, 115, p. 1125.

[6]         Walling, C., Briggs, E. R., Mayo, F. R., The kinetics of the thermal polymerization of styrene, J. Am. Chem. Soc., 1946. 68(7), p.1145.

[7]         Grady, M.C.,  Quan, C., Soroush, M. Thermally initiated polymerization process. US Patent Application Number 60/484,393, filed on July 2, 2003

[8]         Quan, C., Soroush, M., Grady, M.C., Hansen, J.E., Simonsick, W.J. Characterization of thermally polymerized n-butyl acrylate and ethyl acrylate, Macromolecules, 2005, 38(18), p. 7619.

[9]         Zorn, A. M.; Junkers, T. , Barner-Kowollik, C. , Synthesis of a Macromonomer Library from High-Temperature Acrylate Polymerization, Macromol. Rapid Commun., 2009, 30, p. 2028.

[10]       Grady, M.C., Simonsick, W.J., Hutchinson, R.A., Studies of higher temperature polymerization of n-butyl methacrylate and n- butyl acrylate, Macromol. Symp., 2002, 182, p. 149.

[11]       Kaim, A., Polymerization of Vinyl Monomers Initiated with Cyclohexanone, Journal of Polymer Science Part C-Polymer Letters, 1984, 22, p. 203.

[12]       Kaim, A., Kinetics of Polymerization of Methyl Methacrylate Initiated with Cyclohexanone. Part I, J. Polym. Sci. Pol. Chem., 1984, 22, p.1891.

[13]       Flory, P.J., The mechanism of vinyl polymerizations, J. Am. Chem. Soc., 1937, 59,  p.241.

[14]        Mayo, F.R., The dimerization of styrene, J. Am. Chem. Soc., 1968, 90(5), p. 1289.