Chain-length-dependent termination in radical polymerization: Subtle revolution in tackling a long-standing challenge

Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods

http://web.tecnico.ulisboa.pt/jgmartinho/PDF/2011/Progress%20in%20Polymer%20Science%2036_568-602_2011.pdf

Dye-labelled polymer chains at specific sites: Synthesis by living/controlled polymerization

The knowledge of accurate rate coefficients for individual steps of free-radical polymerization (FRP) is of scientific interest and of application-oriented importance. For a wide variety of homopolymerizations and for many copolymerizations, reliable propagation rate coefficients, k p, are accessible via the IUPAC-recommended method of PLP-SEC (pulsed laser polymerization - size-exclusion chromatography). For termination rate coefficients, kt, the situation is less favorable. Even for very common monomers, no kt benchmark data sets are available. Moreover, instead of having one recommended technique for measuring kt, there are a plethora of such methods. Seventeen of the most prominent approaches for measuring kt are here reviewed, including innovative ones that have emerged over the last decade. The methods have been subdivided into two categories: (i) 'Kinetic methods', in which analysis of the time dependence of concentrations is essential, and (ii) 'MWD methods', in which the analysis of the molecular weight distribution plays the dominant role. The methods are evaluated with respect to their potential for providing routine access to measuring kt as a function of monomer conversion and of free-radical chain length. Moreover, it has been considered whether expensive instrumentation or highly demanding analysis is required for a particular method and whether a method is applicable to many types of monomers. A table summarizes all these evaluations in a readily accessible form. The use of kinetic methods appears to be generally preferable over MWD-based methods. The largest potential is currently seen for methods in which polymerization is induced by a single laser pulse and where the subsequent time evolution of either monomer concentration or free-radical concentration is measured. © 2005 Elsevier Ltd. All rights reserved.

Fluorescence Sensing of Glucose Using Glucose Oxidase Modified by PVA-Pyrene Prepared via “Click” Chemistry

The kinetics of intermolecular excimer formation are reported for a polystyrene sample containing one pyrene end group (PS-Py). This reaction serves as a model for the termination process in free-radical polymerization. Fluorescence decay profiles were measured for both monomer and excimer emissions, and from these, the transient contribution to the diffusion-controlled reaction was evaluated. In this way, the mutual diffusion coefficients characteristic of the reaction could be determined. These values (Do = 1.4 X 104 cm2 a-1, toluene 20 "C; 1.2 X lP cm2 s-l, cyclohexane, 34.5 "C) for the polymer of M, = 10 400 (M, f M, = 1.05) are remarkably close to values reported for polystyrene ihelf (M, = 10 700) as determined by quasielastic light scattering (Do = 1.57 x lo+, toluene 20 OC; 1.23 X lP, cyclohexane, 34.5 "C). This close correspondence between the two seta of values implies that, in this concentration region, excimer formation (and the termination reaction for polymers of this length) is controlled by center-of-mass diffusion of the polymer.

Marko Strukelj

Papers

Intermolecular excimer formation for pyrene-end-capped polystyrene : a model for the termination process in free-radical polymerization