in chain-growth polymerization,polymer molecules generally grow to full size in a time-scale, wich is much smaller than the time required for high conversion of monomer to polymer. the lifetime of a growing polymer molecule may be less than a few seconds for a free radical polymerization, which is a typical example of chain-growth polymerization, while a typical polymerization time to obtain high monomer conversion may be several minutes or hours such as that for the step-growth polymerization. chain-growth polymerizations require an active center, which may be a free radical, cation, or anion. once an active center is created, the polymer chain grows extremely rapidly, and when the growing chain is deactivated by a termination reaction, the polymer chain is ''dead" and no longer takes part as a reactant. with free radical polymerization, however, the so-called dead polymer chain is not always truly dead, because under certain circumstances it may itself react with radicals. the active center may initiate the growth of several polymer chains.