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Mechanism of Spontaneous Initiation in High-Temperature Polymerization of N-Butyl Acrylate: a Theoretical Study

Sriraj Srinivasan1, Myung Won Lee2, Michael C. Grady3, Masoud Soroush1, and Andrew M. Rappe4. (1) Department of Chemical and Biological Engineering, Drexel University, 32nd & Chestnut Streets, Philadelphia, PA 19104, (2) Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104, (3) DuPont Marshall Lab, 3401 Grays Ferry Ave., Philadelphia, PA 19146, (4) Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104-6323

The growing requirement for environmental friendly polymeric resins has rendered a major change in the manufacturing of paints and coatings. Low-solids, high-molecular weight acrylic resins have been replaced with highly functional, high-solids, low-molecular-weight resins [1, 2, 3]. Such desired resins have been produced mainly using high temperature (ca. above 100 oC) polymerization. The dynamics of high-temperature polymerization are very different from its low-temperature counterpart, as at high temperatures different reaction mechanisms govern the process [4, 5]. For this reason, low-temperature polymerization models have been found to be inherently incapable of predicting the dynamics of high-temperature polymerization. Efforts have been made to understand better the high-temperature polymerization dynamics. Grady et al. [6] discovered sustained, reproducible, spontaneously-initiated polymerization of n-butyl acrylate at high temperatures in the absence of any known extraneously added initiators. Quan et al. [7] characterized the polymers using electronspray ionization/Fourier transform mass spectrometry and nuclear magnetic resonance spectroscopy, to understand chain branching reactions. Felix et al. [8] performed kinetic studies using mechanistic macroscopic modeling to evaluate contribution of different reactions to overall rate of polymerization. So far, these laboratory and marcroscopic-modeling studies have been inconclusive in identifying the initiation mechanism and the initiating species in spontaneous thermal polymerization of acryl acrylates.

Spontaneously initiated thermal polymerization has been known for styrene and methyl methacrylate [9]. Mayo's mechanism of self initiation is widely recognized as the mechanism of initiation in styrene and other cyclic molecules [10,11]. Diradical mechanism of self-initiation proposed by Flory [12] is speculated to initiate high temperature polymerization of methyl methacrylate (MMA), although no experimental evidence is available as of yet.

In this study, we present results from density functional theory calculations [13] that we have conducted to identify the initiating species and the mechanism of initiation in high-temperature thermal polymerization of n-butyl acrylate. The study involves calculation of reactant, transition state, intermediate, and product geometries using B3LYP/6-31G(d). The molecular geometries of n-butyl acrylate monomer and diradical have been calculated on the singlet and triplet potential energy surfaces [14]. The rate constants and activation energy of the diradical formation have been calculated. The diradical has been found to be a stable intermediate on the triplet energy surface, which is in agreement with previously known hypotheses [9]. The presence of a stable Diels-Alder (DA) intermediate, a dimer, has been found on the singlet energy surface, which had never been reported before. The calculated rate constant for diradical formation was found to be few orders of magnitude lower than the previously reported experimental values [8]. This difference can be due to errors arising from experimental measurements that had been made in time-scales (in minutes) higher than those known for formation of diradicals (10-610-9 s). Monoradicals are widely recognized to initiate polymerization reactions, not diradicals. In view of this, we have studied the formation of monoradicals via hydrogen abstraction by the diradical and the DA dimer from a third monomer. Activation energy and rate constant for the monoradical formation have been calculated, and the resulting values have been compared to the literature values reported in [8].


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