283104 A Theoretical Study of Mechanisms for Chain Transfer to Monomer Reactions in Alkyl Acrylates

Tuesday, October 30, 2012: 2:00 PM
415 (Convention Center )
Nazanin Moghadam1, Masoud Soroush1, Andrew M. Rappe2, Shi Liu2, Sriraj Srinivasan3 and Michael C. Grady4, (1)Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA, (2)Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA, (3)Arkema, King of Prussia, PA, (4)DuPont Experimental Station, Wilmington, DE

Alkyl acrylates are widely used as primary binders in coatings formulations for the automobile industry [1-3]. The basic nature of acrylic resins and the plants producing the resins have changed considerably over the past decades, as a result of environmental limits on allowable volatile organic contents (VOCs) of resins [4-6].  High temperature (>100ºC) polymerization of alkyl acrylates allows for the production of high-solids low-molecular-weight resins, but it also permits secondary reactions to occur at higher rates, including spontaneous initiation (in the absence of known added initiators), chain transfer to monomer (CTM) and polymer, back-biting, and β-scission [1, 2, 7-11]. The polydispersity index, which is a measure of the breadth of the polymer chain length distribution, was found to be 1.5 to 2.2 in high-temperature homo-polymerization of alkyl acrylates [10, 12]. This indicates that various types of chain transfer reactions occur abundantly at high temperatures.

Propagation rate constants of alkyl acrylates and methacrylates were determined using different density functional theory (DFT)-based hybrid functionals [13]. In our previous work, we used B3LYP/6-31G* to study several mechanisms of CTM reaction in methyl acrylate (MA) [14]. Hydrogen abstraction from methyl substituent group in the monomer by the live chain radical was identified as the most probable mechanism for CTM in MA.  Before this work, there was no computational/theoretical comparison of CTM mechanisms in high-temperature free-radical polymerization of alkyl acrylates.

This paper presents newer results from a detailed computational investigation of four possible mechanisms of CTM in self-initiated homo-polymerization of methyl, ethyl and n-butyl acrylate. We have used B3LYP, X3LYP and M06-2X, and WB97X-D functionals, and 6-31G(d), 6-31G(d,p), 6-311G(d), and 6-311G(d,p) basis sets.  Energy barriers and rate constants of the reactions involved in the four mechanisms have been estimated. The effects of live polymer chain length, the type of mono-radical that initiated the live polymer chain, and the type of live polymer chain radical (tertiary vs. secondary) on the kinetics of the reactions have been studied. The rigid rotor harmonic oscillator (RRHO) approximation has been applied to investigate the thermodynamics and kinetics of the reactions. Hydrogen abstraction from methylene substituent group in the monomer by the live chain radical has been found to be the most probable mechanism for CTM in ethyl acrylate  and n-butyl acrylate. EA and n-BA live chains, initiated by two different types of monoradicals obtained via self initiation, have showed nearly similar rate constants of hydrogen abstraction from monomer. We have determined that increasing the chain length of the live polymer has negligible effect on the barriers and rate constants of chain transfer to monomer. The transition state geometries and activation barriers have been validated using different levels of theory.

References:

[1] 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, 149-168.

[2] Rantow, F. S.; Soroush, M.; Grady, M.; Kalfas, G.; Spontaneous Polymerization and Chain Microstructure Evolution in High Temperature Solution Polymerization of n-Butyl Acrylate, Polymer2006, 47, 1423-1435.

[3] Barth, J.; Buback, M.; Russell, G. T.; Smolne, S.; Chain-Length-Dependent Termination in Radical Polymerization of Acrylates, Molecular. Chem. Phys.2011, 212, 1366-1378.

[4] Superintendent of Documents, Title 1, US Government Printing Office, Washington, DC, 1990, P. 1.

[5] Superintendent of Documents, Clean Air Act Amendments of 1990, Title 111, US Government Printing Office, Washington, DC, 1990, P. 236.

[6] R. S. Reisch; Paints & Coatings, Chem. Eng. News,1993, 71 (42), 34.

[7] 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. A2010, 114, 7975-7983.

[8] 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. A2009, 113, 10787-10794.

[9] Nikitin, A. N.; Hutchinson, R. A.; Wang, W.; Kalfas, G. A.; Richards, J. R.; Bruni, C.; Effect of Intramolecular Transfer to Polymer on Stationary Free-Radical Polymerization of Alkyl Acrylates, 5 – Consideration of Solution Polymerization up to High Temperatures, Macromol. React. Eng.2010, 4, 691-706.

[10] Quan, C.; Soroush, M.; Grady, M. C.; Hansen, J. E.; Simonsick, W. J.; High-Temperature Homopolymerization of Ethyl Acrylate and n-Butyl Acrylate: Polymer Characterization, Macromolecules 2005, 38, 7619-7628.

[11] Peck, A. N. F.; Hutchinson, R. A.; Secondary Reactions in the High Temperature Free Radical Polymerization of Butyl Acrylate, Macromolecules2004, 37, 5944-5951.

[12] Srinivasan, S.; Kalfas, G.; Petkovska, V. I.; Bruni, C.; Grady, M. C.; Soroush, M.; Experimental Study of Spontaneous Thermal Homopolymerization of Methyl and n-Butyl Acrylate, Journal of Applied Polymer Science2010, 118, 1898-1909.

[13] Yu, X.; Pfaendtner, J.; Broadbelt, L. J.; Ab Initio Study of Acrylate Polymerization Reactions: Methyl Methacrylate and Methyl Acrylate Propagation, J. Phys. Chem. A2008, 112, 6772-6782. [14] Moghadam, N.; Soroush, M.; Srinivasan, S.; Rappe, A. M.; Grady, M. C.; Computational Study of Chain Transfer to Monomer Reactions in Thermal Polymerization of Methyl Acrylate,  Paper 749c, AIChE Annual Meeting 2011.  

[14] Moghadam, N.; Soroush, M.; Srinivasan, S.; Rappe, A. M.; Grady, M. C.; Computational Study of Chain Transfer to Monomer Reactions in Thermal Polymerization of Methyl Acrylate,  Paper 749c, AIChE Annual Meeting 2011.


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