Photocopying Living Chains. 2. Time-Dependent Measurements

In the preceding article, we reported a novel experimental technique, the “photocopy” method, that allowed the first ever measurements of living chain molecular weight distributions (MWDs) in free radical polymerization (FRP) at steady state. The method entails flooding the FRP at some instant with “photoinhibitor” radicals created photochemically from an appropriate precursor using a short laser pulse. These radicals are extremely slow in initiating new living chains, yet they couple with existing ones (and one another) at near diffusion-controlled rates and carry a fluorescent label. The effect is to freeze the growth of and simultaneously label the living chains that existed just before the laser pulse. In this article we first address the issue of photoinhibitor radical addition to monomer (at rate <EQUATION><EQNDATA>%@mt;sys@%τ%@sx@%add%@be@%-1%@sxx@%%@mx@%</EQNDATA> </EQUATION>), which creates new unwanted labeled living chains that distort the labeled living MWD measurements. Being unable to measure <EQUATION><EQNDATA>%@mt;sys@%τ%@sx@%add%@be@%-1%@sxx@%%@mx@%</EQNDATA> </EQUATION> by standard methods due to its extremely small value, we have studied the dependence of the total detected amount of labeled chains on the concentration of photoinhibitor radicals produced per pulse. These experiments suggest, albeit indirectly, that pollution should have a small effect when the photoinhibitor molecule di(1-naphthyl, phenyl methyl) sulfone (DNPMS) is used. Second, we present measurements of living chains whose steady state is perturbed by a series of photocopying pulses, applied with a period shorter than the time required to reestablish the steady state. These pulses have a dual role:  (i) with each pulse all living chains are removed from the FRP instantly, creating a “vacuum” at t = 0, and (ii) the MWD of the living chains recovering toward steady state is “copied” into a labeled dead MWD. In this “reverse post-effect” experiment, monitoring the concentration ψ<INF>l</INF>(t) and the mean length &Nmacr;(t) of living chains as a function of laser period t<INF>o</INF>, allows us to estimate the propagation velocity v<INF>p</INF> (number of monomers added to a living chain per second) and the mean steady-state living chain lifetime <EQUATION><EQNDATA>%@mt;sys@%τ%@sx@%living%@be@%o%@sxx@%%@mx@%</EQNDATA> </EQUATION>.