Pulsing chain-transfer agent

Grubbs et al. [151] reported a pulsed-addition protocol involving the use of a chain transfer agent, for example, a symmetrical internal cis-olefin, for terminating the ROMP and regenerating the initiator for further homo or block copolymer preparation runs. 

Fig. 6 shows the GPC trace from a pulsed-laser photoinitiated experiment with MMA monomer at 58 C (with benzoin as photo-initiator) containing 2 mole % triethylamine as chain-transfer agent. Fig. 7 shows the same data plotted as nP(M). The straight-line region at higher M is most apparent, as predicted by eq 5. The behaviour at lower M can be ascribed to termination. The slope of the straight-line region can be obtained from a series of such data for different concentrations of chain-transfer agent, and the results plotted as a function of [A]/[M], as in Fig. 8. From this and eq 5, A tr,A is found to be 88 9 dm mohl s-1, in reasonable accord with the literature value [40] of 110 dm mol l s Since the latter was obtained by the Mayo method, it is felt that the present result is in fact more accurate. The present technique also has the advantage that Ap can be measured in the same system, as can A t,m (as apparent from Fig. 8).

Pulsed Laser Polymerization-Size Exclusion Chromatography. The careful determination of the chain length distribution of the polymer produced via a pseudostationary pulsed laser experiment allows to obtain accurate values for the propagation rate coefficient p. The polymerizable system—containing monomer and photoinitiator, occasionally solvent and transfer agents— is irradiated by a pulsed laser beam and the chain length distribution formed is subsequently analyzed by SEC. The determination of the additional peaks and its points of inflection on the low molecnlar side, respectively, gives a value for Lo,n which can easily be inserted into eqnation 123. 

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