Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

Abstract

Abstract The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile‐1‐methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ≪ pyrene < phenanthrene, fluorene ≈ 2‐methoxynaphthalene ≈ biphenyl < anthracene < 2‐methylnaphthalene < 1‐methoxynaphthalene < 2,3,6‐trimethylnaphthalene < 2,3‐dimethylnaphthalene ≈ naphthalene < 1‐methylnaphthalene < 2,6‐dimethylnaphthalene < p ‐terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k q for the formation of exciplex as well as the self‐quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k q were calculated from the Stern‐Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2‐methylnaphthalene ≈ 1‐methylnaphthalene ≈ 2,3‐dimethylnaphthalene < 2,6‐dimethylnaphthalene < acenaphthene < p ‐terphenyl < 1‐methoxynaphthalene. The study of the concentration effect reflecting the self‐quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self‐quenching efficiency of aromatic hydrocarbons: 2‐methoxynaphthalene ≈ pyrene > anthracene > 1‐methoxynaphthalene > fluorene > 2,6‐dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6‐trimethylnaphthalene > 2,3‐dimethylnaphthalene > 1‐methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above‐mentioned sequences.