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Understanding Kinetics of Spontaneous Thermal Polymerization of Alkyl Acrylates: An Experimental Study

Sriraj Srinivasan1, Christopher Bruni2, George A. Kalfas2, Michael C. Grady2, and Masoud Soroush1. (1) Department of Chemical and Biological Engineering, Drexel University, 32nd & Chestnut Streets, Philadelphia, PA 19104, (2) DuPont Marshall Research Laboratory, 3401 Grays Ferry Avenue, Philadelphia, PA 19146

Some low molecular weight, high solids, acrylic resins, now widely used in paints and coatings, are produced at elevated temperatures (ca. above 150oC) using azo or peroxy thermal initiators [1-3]. Typically, initiators are more expensive than other reactants in these polymerization processes. Furthermore, the presence of unreacted thermal initiators in coatings lowers their resistance to UV radiation. Grady et al. [4] demonstrated sustained, reproducible, spontaneous polymerization of alkyl acrylates (e.g., methyl, ethyl and n-butyl acrylates), without the addition of thermal initiators, at higher temperatures (ca. above 120oC).

Kinetics of free-radical polymerization at high temperatures reveal significant differences from classical “low-temperature” polymerization kinetics. Quan et al. [5] studied chain transfer reactions such as backbiting, beta-scission, and inter/intra molecular hydrogen abstraction reactions. Rantow et al. [6] conducted mechanistic modeling of high temperature polymerization of n-butyl acrylate to determine the mechanism of spontaneous initiation. These studies did not lead to conclusive evidence on the nature of the initiation mechanism or the type of initiating species.

This paper presents results from our experimental study of (a) the influence of oxygen and the type of solvent on spontaneous initiation in alkyl acrylates, and (b) the possibility of self initiation via a diradical mechanism. It has been speculated previously [6] that spontaneous initiation can occur via the formation of peroxy compounds by the reaction of monomer with oxygen, which further decompose to generate free radicals. To verify the hypotheses, we have conducted polymerization of alkyl acrylates in the presence and absence of oxygen. The conversion and molecular weight distribution of the polymer formed have been compared. To understand the effect of solvent polarity on the nature of initiating species, we have carried out experiments using solvents with different polarity (dielectric constant). The conversion profiles have been compared. Spontaneous initiation in thermal polymerization of methyl methacrylate has been postulated to be via a diradical mechanism [7, 8], which has been verified indirectly by measuring the quantities of four membered cyclic compounds formed in the final polymer mixture [9]. In order to determine the validity of the same mechanism in the alkyl acrylate spontaneous polymerizations, we have conducted experiments and used GC measurements to quantify the concentration of cyclic compounds formed.

Methyl and n-butyl acrylate monomers (BASF, 99.5%) are passed through an inhibitor removal column before use. Solvents, xylene (ExxonMobil Chemical Co., an 80/20 mix of xylene isomers and ethyl benzene with boiling point range 137-143 oC), chlorobenzene, cyclohexanone and 4 methoxy phenol as sample diluent (99 % ACROS) are used as received. Experiments are carried out in a 1 liter RC1 calorimeter (Mettler-Toledo, GmBH, Schwerzenbach, Switzerland). In each of the experiments, monomer and solvent mass fractions are about 0.4 and 0.6, respectively. Polymer concentration is determined by gravimetric and chromatographic analyses, and residual monomer and cyclic compound concentrations by gas chromatographic (GC) analyses. GC analyses are carried out using Hewlett–Packard (HP) 6890 GC-FID system with a 30 m DB-5 separation column. Molecular weight distributions are determined using an HP 1090 high performance liquid chromatography (HPLC) system, equipped with an HP 1047A refractive index (RI) detector and a four-column set configuration.


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