Measurement of propagation rate coefficients using pulsed-laser polymerization and matrix-assisted laser desorption/ionization mass spectrometry

Abstract

The IUPAC Working Party on modeling free-radical kinetics' noted that pulsed-laser polymerization (PLP) (e.g., ref 2) provides ameans of measuring the propagation rate coefficient (k,) that is virtually assumption-free. PLP involves pulsed-laser illumination of monomer and photoinitiator, under conditions such that a significant amount of macroradicals is terminated by a very short radical species formed in the pulse immediately subsequent to that in which the longer macroradical was formed. If one can identify the degree of polymerization up of polymer terminated as above, then k, is given by up/ [MI tp, tf being the time between pulses and [MI the monomer concentration. This paper shows that matrix-assisted laser desorptionlionization (MALDI) mass spectrometry can be used to measured up and provides an alternative to the only present means of carrying out this measurement, gel permeation chromatography (GPC).Modeling polymerization under PLP conditions-shows that (i) up can be identified as the point of inflection occurring at the lowest molecular weight in the number molecular weight distribution (MWD) of the formed polymer (this point of inflection is within a few percent of vp for all models for polymerization kinetics tested thus far) and (ii) up also corresponds closely to the lowmolecular-weight point of inflection on the weight MWD, and hence on a GPC trace.Despite its advantages, PLP suffers from some major limitations,interaZia from the need to use GPC to identify v , . These include (i) the need for molecular-weight calibration, through either a reliable monodisperse standard or "universal" calibration and (ii) the necessity of accumulating sufficient polymer. Optimization with regard to these constraints is often opposed by having to ensure that (i) laser illumination of the sample is uniform (including negligible attenuation of the laser beam over the length of the sample cell); (ii) [MI changes negligibly over the course of the experiment5i6 (both of which restrict the total amount of polymer that can be formed); (iii) a large proportion of polymer is formed by termination involving one chain that started in a given pulse and a second chain of a very low degree of polymerization that formed in the next pulse (as distinct from chain stoppage by termination between two chains that both have a large degree of polymerization, etc.); and (iv) the GPC is calibrated to provide accurate MWDs.MALDI offers the potential for obtaining the number MWD directly and is applicable to polymers of moderate molecular weight (up to lo5 under favorable condition^).'^ derivative c 10 10 molecular weight weight distribution calculated from MALO1 (b) MALDI I I : , , : ' I t MALDI signal = number distribution) derivative I 10 I O molecular weight Figure 1. (a) GPC trace (full line), which is the weight MWD, number MWD (broken line) and derivative of weight MWD (points), for product from PLP of MMA at -8 "C, as described in the text. (b) Corresponding quantities for the same sample from MALDI, deduced from the data of Figur...

A Perfect Couple: PLP/SEC/ESI‐MS for the Accurate Determination of Propagation Rate Coefficients in Free Radical Polymerization

“…In the current study, methyl methacrylate (MMA), ethyl methacrylate (EMA) and butyl methacrylate (BMA) were used and PLP experiments were performed over a range of temperatures to allow determination of Arrhenius parameters. This is a significant step forward from the simple analysis of model monomers MMA and styrene at constant temperature which has been attempted so far using PLP/MALDI-MS [47][48][49][50] with the exception of an elaborate study by Willemse et al on the chain length dependency of k p . [31] Benzoin was employed as the radical photoinitiator as it dissociates into two radical species of roughly the same molecular weight.…”

“…[45,46] MALDI-MS has been investigated as a tool to measure MWD in conjunction with PLP-experiments using methyl methacrylate and styrene as model monomers as early as 1993. [31,[47][48][49][50] Although the general applicability was demonstrated, PLP/MALDI-MS has since not made the step towards an established technique for the determination of k p . One likely reason for this has recently been summarized by Schnöll-Bitai et al, who showed that quantitative determination of MWD requires a number of data transformations to be made prior to analysis.…”

Employing MALDI-MS on Poly(alkylthiophenes): Analysis of Molecular Weights, Molecular Weight Distributions, End-Group Structures, and End-Group Modifications

“…6,7 More recently, it has also been used to characterize many synthetic polymers. [8][9][10][11][12] It has been shown to be a viable tool for end-group analysis as well as product confirmation for synthetic polymers. Another advantage of MALDI-TOF MS for synthetic polymer analysis is that the absolute molecular masses can be determined as opposed to relative molecular weights by GPC, and this determination is independent of polymer structures.…”

Determination of the Propagation Rate Coefficient (kp) and Termination Mode in the Free-Radical Polymerization of Methyl Methacrylate, Employing Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry for Molecular Weight Distribution Analysis

“…It is obvious that the PLP method would benefit greatly if there was an alternative to GPC-MWD analysis. Danis et al 9 suggested the use of matrixassisted laser desorption ionization time-of-flight (MAL-DI-TOF) mass spectrometry (MS) for MWD analysis. Their work yielded a propagation rate coefficient significantly at odds with that obtained from GPC data.…”