CHAIN TRANSFER TO SOLVENT IN 2-STATE ANIONIC-POLYMERIZATION .2. SLOW EXCHANGE BETWEEN PROPAGATING SPECIES

The effect of dow exchange between growing chains on the average degre e of polymerization (DP) and the molecular weight distribution (MWD) o f polymers formed in a two-state polymerization accompanied by chain t ransfer to solvent is considered. For the case when centres of only on e type are capable of chain transfer it is shown that the exchange can be regarded as fast or slow, depending on the ratio of characteristic times of exchange (tau(ex)) and chain transfer (tau(tr)) for these ce ntres. The conditions for fast exchange, when the MWD of the polymer b ecomes independent of the rate of exchange, are derived. Under otherwi se the same conditions, polymer of the narrowest MWD is formed if the exchange is fast, the number-average DP being independent of the excha nge rate. With increasing ratio tau(ex)/tau(tr), the weight-average DP of the polymer increases and the MWD changes from unimodal to bimodal . For the case when ''non-transferring'' centres are dormant, it was a lso found that at slow exchange the number-average DP is greater and t he MWD is less broadened than at fast exchange.