As the energy as well as the kinetic barrier to break a C-H bond or a C-C bond in a polymer is rather high, only very few polymer radicals are formed that could react with oxygen (in the auto-oxidation chain) and ultimately form an excited carbonyl (termination step) that could relax by emitting a CL photon.
Among several possible termination steps in which free radicals can react with each other with a certain probability after a certain number of cycles in the propagation step, the depicted termination steps render an electronically excited carbonyl group RO *.
The overall specific rate constant for the termination step may be written as :
Combining both termination mechanisms leads to the overall specific rate constant for the termination step given by Eq.
The reversible termination step
is an equilibrium process that strongly favors the dormant or inactive state so that most of the growing chains are dormant at any time.
As inferred from the results of dynamic DSC runs, a termination step accounting for the consumption of active species must be also present in the reaction mechanism.
The presence of termination steps leading to inactive species is still a matter of debate.
[k.sub.t] rate constant for the termination step
The chain entanglement effects in the termination step are also accounted for.
The increase in molecular weight with flow rate is primarily due to the higher monomer concentrations for the higher flow rates (lower conversions) which result in acceleration of the propagation reaction relative to the termination step. Because of high recycle ratios, the variation in flow rates causes only small changes in the heat transfer coefficient and as a result, the temperature profile in reactor 1 is not sensitive to flow rates.
He detailed the chemistries and cure mechanisms involving initiation, propagation, and termination steps
during the curing process.