INITIATION OF POLYMERIZATION WITH SUBSTITUTED ETHANES .13. FREE-RADICAL POLYMERIZATION OF METHYL-METHACRYLATE AND STYRENE WITH SUBSTITUTED SUCCINONITRILES

Tetraarylsuccinonitriles 2 were synthesized via oxidative dimerization of diarylacetonitriles 1 in basic media. The thermal decomposition of 2 results in two identical diarylcyanomethyl semiradicals 3. The diss ociation energy (E(a)) measured using the thiophenol radical scavengin g technique was found to amount to 94,3 +/- 9,2 kJ/mol in the case of tetraphenylsuccinonitrile 2a. Tetraarylsuccinonitriles 2 show an atypi cal ''stepwise'' initiation mechanism in free-radical polymerization, particularly in the case of methyl methacrylate (MMA) as monomer. In t he initial phase of the polymerization reaction a very high concentrat ion of diarylcyanomethyl primary radicals 3 leads to the formation of short-chain telechelics with both end groups originating from the init iator. In the further course of the polymerization these MMA telecheli cs are able to re-form radicals by the scission of thermolabile carbon -carbon bonds and by the release of initiator end groups, and so the M MA telechelics 're-initiate' the free-radical polymerization (''resusc itable free-radical polymerization''). In styrene polymerization, tetr aarylsuccinonitriles 2 indeed cause a pronounced primary radical termi nation effect, but the styrene telechelics formed in contrast to MMA t elechelics are thermostable, and therefore no ''re-initiation'' effect occurs. The X-ray structure determination of the styrene telechelic 2 ,2,3,5,5-pentaphenyladiponitrile 4a exhibits a bond length of the ther molabile carbon-carbon bond of 158,7 pm as compared to 162,8 pm of the corresponding MMA telechelic tetraphenyl-3-methoxycarbonyl-3-methylad iponitrile and emphasizes the higher thermal stability of 4.