Novel techniques in chromatography mass spectrometry for the investigation of free radical polymerization mechanisms

Abstract

A novel procedure allowing for the first time the determination of absolute molecular weight distributions and species concentrations in mixtures of functional homopolymers is reported. The method makes use of the accurate concentration determined by refractive index (RI) detection of polymers separated by size exclusion chromatography (SEC). Online electrospray ionization mass spectrometry (ESI-MS) is used to establish the molecular weight (MW) and relative concentrations of the individual functional polymers eluting from the chromatographic column. Influences of the instrumental mass bias are avoided via the application of a sophisticated algorithm that optimally unites the data of both individual detectors. The developed method was shown to enable the determination of the MW irrespective of the chemical identity of the monomer and has an upper MW limit of around 15 kg•mol-1 for the employed quadrupole ion-trap mass analyzer. For poly(methyl methacrylate) the MW determined by online SEC/ESI-MS was shown to be lower than those obtained by the classical techniques of light-scattering and viscosimetry by 5 to 14%. The obtained values agreed with MW determined by 1H-NMR to within less than 2%. For binary and ternary mixtures of functional synthetic polymers, the MW of the pristine monofunctional polymers and the restored values showed less than 8% deviation. The developed method was first applied to the pulsed-laser polymerization of a series of methacrylates. Accurate MW determination by SEC/ESI-MS significantly improved the precision of the propagation rate coefficient, kp for these monomers. The mass accuracy of online ESI-MS is furthermore shown to be transferable to SEC systems with RI-detection by providing pairs of Mark-Houwink parameters in relationship to poly(methyl methacrylate), a typical standard in SEC. MS was furthermore used to investigate the oxidative degradation reactions of styrene and acrylate polymers obtained from reversible addition fragmentation polymerization (RAFT). A radical degradation pathway was identified, that lead to the formation of hydroperoxide-functional polymer as the major degradation species in the presence of (peroxide-containing) cyclic ethers. The degradation reaction was subsequently exploited in a one-pot reaction, providing for the first time a facile means of converting the dithioester or trithiocarbonate functionality of RAFT polymers, into a versatile hydroxide endgroup.