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 precu rsor using a short laser pulse. These radicals are extremely slow in initia ting new living chains, yet they couple with existing ones (and one another ) at near diffusion-controlled rates and carry a fluorescent label. The eff ect is to freeze the growth of and simultaneously label the living chains t hat existed just before the laser pulse. In this article we first address t he issue of photoinhibitor radical addition to monomer (at rate tau (-1)(ad d)), which creates new unwanted labeled living chains that distort the labe led living MWD measurements. Being unable to measure tau (-1)(add) by stand ard methods due to its extremely small value, we have studied the dependenc e of the total detected amount of labeled chains on the concentration of ph otoinhibitor radicals produced per pulse. These experiments suggest, albeit indirectly, that pollution should have a small effect when the photoinhibi tor molecule di(l-naphthyl, phenyl methyl) sulfone (DNPMS) is used. Second, we present measurements of living chains whose steady state is perturbed b y a series of photocopying pulses, applied with a period shorter than the t ime 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 XIV D of the living chains reco vering toward steady state is "copied" into a labeled dead MWD. In this "re verse post-effect" experiment, monitoring the concentration (phi nu)(t) and the mean length (N) over bar (t) of living chains as a function of laser p eriod t, allows us to estimate the propagation velocity vp (number of monom ers added to a living chain per second) and the mean steady-state living ch ain lifetime tau (o)(living).