			Home About us Contact

Mid-chain Radicals (mid-chain + radical)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Pulsed Laser Polymerization of Alkyl Acrylates: Potential Effects of the Oxygen Presence and High Laser Power,

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 1 2006
Patrice Castignolles
Abstract Summary: Unexpected difficulties are encountered in the determination of propagation rate coefficients (kp) in free radical polymerization of alkyl acrylates by pulsed laser polymerization (PLP), mainly due to intramolecular transfer to polymer.1 This article is focused on the role played by the high laser power in these difficulties and the possible reactions of mid-chain radical with residual oxygen. Removing the oxygen by simple bubbling of nitrogen is sufficient to avoid alteration of the polymerization kinetics of acrylates by residual oxygen under PLP conditions. Moreover, no degradation of polymer (or solvent) has been detected after irradiation with the high laser power typically used in PLP experiments. However, it has been shown that this high laser power completely prevents from having a temporally and spatially homogeneous radical concentration in the PLP cell. A model is proposed here to simulate the pulsed laser polymerization taking initiator consumption and laser energy absorption into account. According to our simulation results, this non-negligible initiator consumption and laser power absorption can indeed have a positive influence, i.e., it favors the obtainment of a bimodal molar mass distribution fulfilling the IUPAC consistency criteria. This observation may contradict the idea that PLP-SEC is not suitable to determine accurate kp values for acrylates above 20,30,°C. Instantaneous MMDs formed after N pulses. Simulation taking initiator consumption and laser absorption into account. [source]

EPR Analysis of n -Butyl Acrylate Radical Polymerization

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 23 2009
Johannes Barth
Abstract Via electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n -butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at ,50,°C and by the 7-line spectrum of MCRs at +70,°C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs allows for estimating the rate coefficient k for monomer addition to a mid-chain radical. For 70,°C, k is obtained to be 65.5 L,·,mol,1,·,s,1. [source]

Effect of Intramolecular Transfer to Polymer on Stationary Free Radical Polymerization of Alkyl Acrylates, 2,

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 2 2006
Improved Consideration of Termination
Abstract Summary: Procedures are developed to estimate kinetic rate coefficients from available rate data for the free radical solution polymerization of butyl acrylate at 50,°C. The analysis is based upon a complete mechanistic set that includes the formation of mid-chain radicals through backbiting and their subsequent reaction, and contains no assumptions on how the rate coefficient for cross-termination of mid-chain and end-chain radicals is related to the two homo-termination rate coefficients. After a thorough statistical analysis, the results of the fitting are combined with other recent literature data to provide a complete set of individual rate coefficients for the butyl acrylate system. Monomer addition to a mid-chain radical is estimated to be slower than addition to a chain-end radical by a factor of more than 400. The termination of two mid-chain radicals is estimated to be two orders of magnitude slower than termination of two end-chain radicals, with the cross-termination rate coefficient close to the geometric mean. Formation of a mid-chain radical by intramolecular chain transfer to polymer by a chain-end radical. [source]

The role of mid-chain radicals in acrylate free radical polymerization: Branching and scission

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2008
Thomas Junkers
Abstract The past 5 years have seen a significant increase in the understanding of the fate of so-called mid-chain radicals (MCR), which are formed during the free radical polymerization of monomers that form highly reactive propagating radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most prominent. Typically, a secondary propagating acrylate-type macroradical (SPR) can easily transfer its radical functionality via a six-membered transition state to a position within the polymer chain (in a so-called backbiting reaction), creating a tertiary MCR. Alternatively, the radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up-to-date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high-temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7585,7605, 2008 [source]