Photocopying Living Chains. 1. Steady-State

ABSTRACT: We present the first ever measurements of living chain molecular weight distributions (MWDs), ªo(N), in free radical polymerization (FRP), using a new technique, the “photocopy method”. Though living chains are the fundamental objects in FRP, their MWDs have eluded measurement until now, principally due to their very short lifetimes (j1 s). In the photocopy method, the living population is converted, essentially instantaneously, to a labeled inert one by “photoinhibitor ” molecules activated by a short laser pulse. This floods the FRP with photoinhibitor radicals, which ideally (i) are extremely slow to initiate new living chains yet (ii) couple with existing living chains (and each other) at near diffusion-controlled rates and (iii) carry a fluorescent label. Thus, the living chains are “frozen ” and labeled. They are subsequently detected selectively using GPC equipped with a fluorescence detector (a second detector simultaneously detects unlabeled chains). We applied the photocopy method to low conversion methyl methacrylate FRP. Our measured MWDs are exponential as predicted by the classical Flory-Schulz theory (which ignores the chain length dependence of the termination rate constant, kt), but only for chains longer than the mean living chain length Nh o. For N < Nh o, our data are consistent with a stretched exponential as predicted by modern FRP theories accounting for N dependence of kt. However, the small N data may also be accounted for by nonideal effects, initiation of new living chains by photoinhibitors