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SynopsisThe diffusion coefficient of oxygen in poly(2-hydroxyethyl methacrylate) has been explicitly measured using an optical technique based on fluorescence quenching. This measurement represents the first explicit determination of Do, in PHEMA. A diffusion coefficient of oxygen in PHEMA of 1.36 X 10W7 cm2/s at 20°C was obtained from this measurement. This value is shown to agree well with permeability data for PHEMA, the free volume theory of diffusion, and with values of Do, that have been explicitly measured in other methacrylate hydrogels.

. (2000) Direct evidence for the interaction of the mechanisms of thermally initiated and atom transfer radical polymerization. Macromolecules, 33 (25) Introduction. Autopolymerization of styrene 1-5 remains relatively unimportant in standard free radical polymerization, but Matyjaszewski, Fukuda, and Hawker have noted that it is of great importance at the elevated temperatures used in living free radical polymerizations (LFRP) mediated by nitroxyls such as TEMPO. [6][7][8] Autopolymerization of styrene in ATRP systems has been observed by Angot et al. 9 and Matyjaszewski et al. 10 during syntheses of star polymers of styrene at high monomer concentrations and at temperatures of 100°C or higher. In both cases, sides peaks at lower molecular weights were observed in the size exclusion chromatograms of the products that were attributed to linear polystyrene arising from the thermal initiation of the monomer.In a recent paper we published the synthesis of a poly-(methyphenylsilane)-graft-polystyrene copolymer. 11 This was achieved by the bromomethylation of poly(methylphenylsilane) (PMPS), 12 with the resulting bromomethylated poly(methylphenylsilane) (BrMePMPS) 13 being used as a macromolecular initiator for the bulk ATRP of styrene at 110°C using Cu(I)Br and bipyridine (bipy) as a catalyst system. 14,15 Although thermal initiated polymerization has been noted in both LFRP and ATRP reactions, its interaction with these systems has only been shown for LFRP. 16 In this paper we show how a detailed study of this grafting reaction has yielded valuable information concerning the interaction of the mechanisms of ATRP and the underlying autopolymerization.Results and Discussion. PMPS 12 and BrMePMPS 13 were synthesized according to literature procedures. The polystyrene grafts were grown by ATRP. 11,17 The course of the reaction was followed by 1 H NMR spectroscopy and size exclusion chromatography. The reaction time until gelation for the ATRP reaction was found to vary greatly with graft chain length, from 7 h for a calculated degree of polymerization (calculated DP) of ∼70 to 72 h for a calculated DP of ∼200. It should also be noted that the reaction time required for a grafting reaction to high molecular weight is significantly longer than for the equivalent homopolymerization using a Cu(I)Br (bipy) catalyst system. 14 The plot of M n versus conversion for the low molecular weight grafting reaction (calculated DP ∼ 70) is linear, indicating that the polymerization has living character (Figure 1). The near perfect overlay of the size exclusion chromatograms of a typical low molecular weight graft copolymer recorded at 334 nm (characteristic of the PMPS σ-conjugated silicon backbone) and at 254 nm using a UV detector demonstrates that all of the material present contains

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Atom transfer radical polymerisation (ATRP) was used in an attempt to prepare a graft polymer based on poly(methylphenylsilane) (PMPS). A partially bromomethylated PMPS was subjected to ATRP conditions in order to grow poly(methyl methacrylate) (PMMA) grafts onto that backbone. Following the grafting reaction, a substantial increase in molecular weight was observed by size exclusion chromatography, analysis using refractive index detection. However, a bimodal product molecular weight distribution was observed using a UV detector set at wavelengths at which only the PMPS backbone can be detected. The lower molecular weight fraction was identified as homopolymeric PMPS although it appears to include some material with short grafts, while the higher molecular weight fraction corresponds to PMPS with long grafts. These observations are interpreted as the formation of a substantial amount of PMMA that is not grafted onto PMPS through a radical process initiated independently of the ATRP. However, since MMA is barely susceptible to thermal polymerisation, this is not easily rationalised. © 2002 Society of Chemical Industry