Effect of alkyl groups on the rate constants of propagation and termination in the radical polymerization of dialkyl itaconates

Polymerization of dialkyl itaconates with dimethyl azoisobutyrate ( 5 ) was studied in benzene at 50°C by means of electron spin resonance (ESR). The monomers used are dimethyl ( 1 ), diethyl ( 2 ), dibutyl ( 3 ) and di‐2‐ethylhexyl ( 4 ) itaconates. All the polymerization systems involve ESR‐observable propagating polymer radicals under the actual polymerization conditions. The polymerization rate ( R p ) and degree of polymerization of the resulting polymer increase in going from shorter to longer alkyl groups. The ESR‐determined rate constants of propagation ( k p ) and termination ( k t ) decrease as the alkyl chain becomes longer. k p of 1 is 3,3 times higher than that of 4 , while k t of 1 is 590 times higher than that of 4 . Thus, the steric effect due to the alkyl group suppresses much more termination than propagation, leading to the fact that R p increases as the alkyl group becomes larger.