Pyrolysis of 5-methylenebicyclo[2.2.1]hept-2-en-7-one produces 5-methylene-1, 3-cyclohexadiene (MCH). We have studied MCH as a model for the Diels-Alder dimer of styrene (AH). The styrene dimer is postulated to be involved in radical production, by a molecule-assisted homolysis (MAH) process, in the spontaneous polymerization of styrene. Surprisingly, however, no increase in the rate of polymerization of styrene is observed upon addition of MCH, even though MCH reacts rapidly with styrene to give ene products. The rate of radical production from the reaction of styrene with MCH must be at least 20 times slower than that attributed to AH and styrene. A detailed analysis is presented of the fractions of AH and MCH that undergo the various reactions open to these reactive species: initiation, chain transfer, and ene reaction. Surprisingly, <2% of the AH undergoes the MAH reaction; however, this process can be detected since it leads to the formation of long-chain polymer molecules. The fraction of AH that undergoes chain transfer also is very small, despite the large transfer constant. Most of the AH reacts with styrene to give trimeric products, probably both by an ene process and by cage radical recombination. The situation is similar for MCH, except that the fraction undergoing the MAH initiation is even smaller or is zero. Data are also presented on the time for AH to reach its steady state level in styrene at 60 °C, judged from the UV absorbance at 320 nm. It is argued that an induction period should be observed in the rate of thermal polymerization if the Diels- Alder MAH initiation mechanism is correct. However, no induction period has been reported at 60 °C. A rationalization is presented that attempts to explain the different rates of radical production from ene reactions and/or MAH processes from MCH and AH. The apparent faster rate of MAH initiation by AH relative to MCH can be rationalized by postulating a more open transition state in an ene-like reaction caused by steric effects for AH or to other differences between the MCH- and the AH-styrene reactions. Alternatively, however, it is argued that our results may require rejection or modification of the proposed MAH reaction of AH as the initiation step in the polymerization of styrene. The transfer constant of MCH is the largest ever reported for a hydrocarbon, and indeed one of the largest on record, about 9 at 60 °C in styrene. © 1979, American Chemical Society. All rights reserved.
