Photocopying living chains. 1. Steady-state

We present the first ever measurements of living chain molecular weight dis tributions (MWDs), psi degrees (N), in free radical polymerization (FRP), u sing a new technique, the "photocopy method". Though living chains are the fundamental objects in FRP, their MWDs have eluded measurement until now, p rincipally due to their very short lifetimes (less than or similar to 1 s). In the photocopy method, the living population is converted, essentially i nstantaneously, to a labeled inert one by "photoinhibitor" molecules activa ted by a short laser pulse. This floods the FRP with photoinhibitor radical s, which ideally W are extremely slow to initiate new living chains yet (ii ) couple with existing living chains (and each other) at near diffusion-con trolled 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 conversi on methyl methacrylate FRP. Our measured MWDs are exponential as predicted by the classical Flory-Schulz theory (which ignores the chain length depend ence of the termination rate constant, kt), but only for chains longer than the mean living chain length (N) over bar. For N < (N) over bar (0) our da ta are consistent with a stretched exponential as predicted by modern FRP t heories accounting for N dependence of kt. However, the small N data may al so be accounted for by nonideal effects, initiation of new living chains by photoinhibitors, which lead to power law behavior. Another complication is that thermal initiation persists during the photocopying process in its pr esent form. Thus, post-laser-pulse initiated living chains react with photo inhibitor radicals, distorting the measured MWDs from that of the steady-st ate living chains. From the measured living and dead MWDs, we infer living and dead chain concentrations and mean lengths and the fraction of living c hains terminating via coupling. Finally, using reported values of propagati on rate constants, we estimate mean living chain lifetime, polymerization r ate, and the average termination rate constant.