(259a) Self-Initiation in High-Temperature Polymerization of Alkyl Acrylates - a Theoretical Study of the Mechanism

An attractive method of producing low molecular-weight solvent-borne polymer resins is to carry out solution free-radical polymerization at high temperatures (above 120 C). At these high temperatures, secondary reactions, that are insignificant at low temperatures, have very strong effects on the polymerization rate and molecular structure formation. For example, alkyl acrylates undergo significant, sustained, reproducible, spontaneous (with no added known thermal initiator), thermal homo- and co-polymerization at temperatures above 140 C, as reported in 2002 [1, 2] ? methacrylates and styrene have been known for several decades to undergo reproducible spontaneous polymerization at high temperatures.The species that initiate the chain reactions in spontaneous thermal polymerization of alkyl acrylates and the initiation mechanism are still unknown. By identifying the initiating species and the initiation mechanism, the chemical structure of the free-radicals that are generated can be obtained. This kinetic insight is essential to achieve a better control of the polymer microstructural properties [3]. Hence, a quantitative understanding of the secondary reactions allows one to design safer, environmentally-friendlier and more efficient solvent-borne resin processes.

The observation of the spontaneous thermal polymerization led to several speculations on the initiating species including: a) residual oxygen in trace quantities reacting with the monomer to form peroxide, which at high temperature dissociates to release free radicals that initiate the polymerization [4], and b) the mechanism proposed by Flory [5], where two acrylate monomers react to form an intermediate diradical, which further combines with a third monomer to abstract a hydrogen atom to form a stable monoradical. Efforts to use spectroscopic analyses on captured free radicals with spin trap agents to determine their structures, which eventually can assist in identifying the initiating species and the initiation mechanism, have not been largely successful [6].

In this work, we apply computational quantum chemistry using GAMESS [7] to determine the possibility of self-initiation through estimating the reaction barrier and rate constants of the reaction. The estimated kinetic parameters will be used to describe the polymerization dynamics quantitatively. The model predictions will then be compared with measurements of micro-structural polymer properties to validate the postulated mechanism for the self initiation. Computational quantum chemistry has been used to calculate the molecular geometry, and vibrational frequencies of various monomers such as ethylene [8], acrylonitrile [9], styrene [10], methyl methacrylate [11], and methyl acrylate [12]. The energy barrier and rate constants have been calculated via transition state theory and verified against experimental data for initiation and/or propagation reactions in free-radical polymerization of these monomers. The major advantage of using this technique is to obtain the structures of the intermediates and transition states that elude spectroscopic measurements. It not only provides a better understanding of the reaction mechanisms of several elementary reactions, but also improves the kinetic models to describe free-radical polymerization. Assessment studies to identify suitable procedures that offer a reasonable compromise between cost and accuracy have been done [13]. It has been shown that equilibrium geometry and vibration frequencies, energy barriers and rate constants of these monomers can be calculated with density functional theory [14] [B3LYP/6-31G(d)] at reasonable accuracy.

In this study, unrestricted B3LYP method with 6-31G(d) basis set has been used to estimate geometry of the reacting species, and energy barriers. The tetramethylene diradical formation from two ethylene molecules has been studied as a model system prior to alkyl acrylates; because little information on self initiation of ethylene has been reported, and also we see it to serve as a precursor to understand the initiation reaction with alkyl acrylates. Singlet and triplet states have been considered in the calculation of the minimum energy reaction path. An energy barrier of 43.9 kcal/mole has been calculated for this reaction. The ring opening mechanism of cyclobutane to form the tetramethylene diradical has also been studied, and an energy barrier of 68.4 kcal/mole has been obtained. The reaction of methyl acrylate self-initiation has been studied to predict and estimate intermediate diradical structure on which there is limited information. The self-initiated diradical and monoradical formation rate constants, frequency factor, and activation energy for the reaction at various temperatures have been calculated. Similar studies are also being conducted for other important alkyl acrylate monomers such as ethyl and n-butyl acrylate.

References

[1] Quan, C., Grady, M.C., Soroush M., Characterization and Kinetics of Hightemperature Polymerization of n-Butyl Acrylate. Preprints of AIChE Annual Meeting, Indianapolis, IN, November, 2002.

[2] Quan, C., Soroush, M., Grady, M.C., Hansen, J.E., Simonsick, W.J., Characterization of Thermally Polymerized n-Butyl Acrylate and Ethyl Acrylate, Macromolecules, 38(18), p. 7619, 2005.

[3] Callias, P.A., Moskal, M.G., Higher Solids, Higher Quality Acrylic Coating Resins Produced with Organic Peroxide Intiators. Arkema Inc., King of Prussia, PA, USA. www.arkema-inc.com.

[4] Conant, J.B., Peterson, W.R., Polymerization Reactions Under High Pressure. II. The Mechanism of the Reaction. J. Am. Chem. Soc., 54, p. 628, 1932.

[5] Flory, P.J., The Mechanism of Vinyl Polymerization, J. Am. Chem. Soc., 59, p.241, 1937.

[6] Ah Toy, A., Chaffey-Millar, H., Davis, T. P., Stenzel, M. H., Izgorodina, E. I., Coote, M. L., Barner-Kowollik, C. Thioketone Spin Traps as Mediating Agents for Free Radical Polymerization Processes. Chem. Comm. , p. 835, 2006.

[7] Schmidt, M.W., Baldridge, K.K., Boatz, J.A., Elbert, S.T., Gordon, M.S., Jensen, J.H., Koseki, S., Matsunaga, N., Nguyen, K.A., Su, S., Windus, T.L., Dupuis, M., Montgomery, J.A., J. Comput Chem.,14, p.1363, 1993.

[8] Heuts, J.P.A., Gilbert, R.G., Radom, L., A Priori Prediction of Propagation Rate Coefficients in Free-Radical Polymerizations: Propagation of Ethylene. Macromolecules, 28, p. 8771, 1995.

[9] Huang, D.M., Monteiro, M.J., Gilbert, R.G., A Theoretical Study of Propagation Rate Coefficients for Methacrylonitrile and Acrylonitrile. Macromolecules, 31, p. 5175, 1998.

[10] Khuong, K.S., The Mechanism of Self-Initiated Thermal Polymerization of Styrene. Theoretical Solution of a Classic Problem, J. Am. Chem. Soc., 127(4), p. 1265-1277, 2005.

[11] Gunaydin, H., Salman, S., Tuzun, S.N., Avci, D., Aviyente, V., Modeling the Free Radical Polymerization of Acrylates. Intl. J. Quant. Chem., 103, p. 176, 2005.

[12] Bebe, S., Yu, X., Hutchinson, R.A., Broadbelt, L.J., Estimation of Free Radical Polymerization Rate Coefficients Using Computational Chemistry, Macromol. Symp., 243, p.179, 2006.

[13] Wong, M.W., Radom, L., Radical Addition to Alkenes: An Assessment of Theoretical Procedures. J. Phys Chem, 99,p. 8582, 1995.

[14] Hohenberg, P., Kohn, W., Inhomogeneous Electron Gas. Phys. Rev., 136, p.864, 1964.