Photocopying Living Chains. 1. Steady-State

Citations Over Time

Abstract

We present the first ever measurements of living chain molecular weight distributions (MWDs), ψo(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 (≲1 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 N̄o. For N < N̄o, 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, which lead to power law behavior. Another complication is that thermal initiation persists during the photocopying process in its present form. Thus, post-laser-pulse initiated living chains react with photoinhibitor radicals, distorting the measured MWDs from that of the steady-state living chains. From the measured living and dead MWDs, we infer living and dead chain concentrations and mean lengths and the fraction of living chains terminating via coupling. Finally, using reported values of propagation rate constants, we estimate mean living chain lifetime, polymerization rate, and the average termination rate constant.

Related Papers

• A GEOMETRIC MEAN IN THE FURUTA INEQUALITY(2002)
• Susquehanna Chorale Spring Concert "Roots and Wings"(2017)
• Коммуникaтивно- прaгмaтический aнaлиз дипломaтических бумaг (нa основе вербaльных нот)(2018)
• → ФОРМИРОВAНИЕ ГОТОВНОСТИ БУДУЩИХ ПЕДAГОГОВ К ОРГAНИЗAЦИИ РAБОТЫ ПО РAЗВИТИЮ ВAЛЕОЛОГИЧЕСКОЙ КУЛЬТУРЫ ШКОЛЬНИКОВ(2023)