G. Gordon Cameron

Polymerization of poly(dimethylsiloxane) macromers: 1. Copolymerization with styrene

Abstract The synthesis and characterization of methacrylate-ended macromers ( M n 500 to 10 000) and their copolymerization with styrene (M2) is described. The experimental errors in the values of the reactivity ratios r1 render them meaningless. Values of r2 can be determined with more precision and increase from 1.06 to 1.55 as the molecular weight of the macromer increases. This behaviour is due to steric effects, not diffusion-controlled propagation. It is shown that the assumptions that 1 > r 1 [ M 1 ] [ M 2 ] and r 2 > [ M 1 ] [ M 2 ] are only valid for macromers of M n > ca. 10 000 .

Thermal degradation of polystyrene-II the role of abnormalities

Abstract The degradation characteristics of thermally synthesized polystyrenes, decomposing at sub-volatilization temperatures in the presence of an inhibitor (1,4-diaminoanthraquinone), support the theory that the weak spots in these polymers are distributed at random within the polymer backbone, rather than at chain ends. Transfer reactions do not appear to be significant at these low temperatures. The evidence indicates that these “weak links” are not head-to-head bonds, branch points or unsaturated structures. However, such thermolabile abnormalities (particularly the last-mentioned) reduce the pyrolytic stability of thermal polystyrenes relative to “living” anionic polymers by lowering the overall activation energy for bond rupture in the phase of decomposition immediately after “weak link” scission. It is possible that the labile structures which decompose in the initial stage of degradation are oxygenated groups, probably peroxide links, but their identity has not been unequivocally proved.

Polymerization of methyl methacrylate in solution

Abstract The rate enhancing effects of aromatic solvents (1-fluoro-, 1-chloro-, 1-bromonaphthalene and toluene) on the photo- and thermal sensitized polymerization of methyl methacrylate by azoinitiators at 25° and 60° respectively were investigated by measuring k p , k r and R l by means of rate, molecular weight and sector experiments. The rate of polymerization is affected by the nature and concentration of the solvent partly from the viscosity dependence of termination and partly from the solvent dependence of the propagation coefficient. A minor solvent effect on the rate of initiation was also noted.

Segment size in synthetic polymers by the spin-probe method

Abstract Following the approach of Kusumoto an equation is derived which relates the correlation time for the tumbling of a nitroxide spin probe in a polymer matrix with the parameter f, the ratio of the volumes of the probe and the polymer segment undergoing motion at T>Tg. For poly(vinyl acetate) probed with a series of nitroxides the correlation between f and the molecular volumes of the probes is poor, possibly because of wide variations in probe flexibility and polarity. An approximate version of the equation also permits evaluation of f from the parameter T50G. Values of f calculated in this manner for the probe 2,2,6,6-tetramethyl-4-hydroxypiperidin-1-oxyl-benzoate in nine different polymers suggest that polymers with high glass transition temperatures have relatively bulky segments.

Solvent effects in free radical copolymerization of styrene and methacrylonitrile

Abstract The reactivity ratios for the free radical copolymerization at 60°C of methacrylonitrile (M1) and styrene have been determined in benzene, benzonitrile, acetonitrile and benzyl alcohol solutions. In the first three systems there is a minor solvent effect on r1 which may be attributable to changes in the dielectric constant of the solution. In benzyl alcohol there is a marked solvent effect, which has not been satisfactorily explained, on both r1 and r2.

The thermal degradation of poly(ethylene oxide) and its complex with NaCNS

Abstract Poly(ethylene oxide) (PEO) of molecular weight (Mw) ca 5 × 106 and its complex with NaCNS have been thermally degraded at temperatures in the range 320–330°. Examination of the polymeric residues shows that in both cases there is a precipitous decrease in Mw during degradation consistent with a random scission reaction, the complex decomposing much faster than the pure PEO. The products of complete degradation up to 500° can also be accounted for in terms of a radical reaction initiated by random scission of backbone bonds. The salt appears to participate in the degradation as evidenced by the general decrease in stability and by the appearance of HCN and relatively large amounts of ethylene and of CO2 in the pyrolysis products from the PEO-NaCNS complex. A complex of PEO and LiClO4 ([EO]/[Li] = 8) was much less stable thermally than either the pure PEO or the thiocyanate complex.

E.s.r. and saturation transfer e.s.r. spectroscopy of spin-probed poly(vinyl acetate): A new spectral transition correlating with glass transition temperature

Abstract From the temperature dependence of the rotational frequency of a nitroxide spin probe dispersed in poly(vinyl acetate) the size of the polymer segment involved in the glass to rubber relaxation can be estimated. The calculation is based on a free volume model which relates f , the ratio of the volumes of the spin probe and the polymer segment, to the rotational frequency of the probe. An increase in f with increasing size of the probe is demonstrated. Saturation transfer electron spin resonance spectra of two nitroxide probes are shown to undergo a rapid change with temperature at a temperature T R which is characteristic of both the probe and its environment. T R corresponds to a pseudo-isofrequency point and is analogous to, though at a lower characteristic frequency than, the widely-used parameter T 50G . For the systems examined, T R correlates with the glass to rubber relaxation.

Electron spin resonance studies of spin-labelled polymers—VIII: Relaxation processes in low density polyethylene

Abstract Low density polyethylene was spin-labelled with an oxazolidine- N -oxyl group. Correlation times for the motion of the label were calculated both in the slow-motion region and in the motionally-narrowed region. In the latter situation, the central line shape was successfully simulated. A correlation map revealed the α- and β-relaxation processes in solid polyethylene, with activation energies of 48 and 28 kJ mol −1 , respectively. The segmental reorientation processes of polyethylene in the melt and in solution had activation energies of 15·2 and 22·9 kJ mol −1 , respectively. Taking into account the effect of viscous drag of the solvent, the two activation energies were almost the same suggesting that the same relaxation process occurs in the bulk molten polymer and in the isolated molecule. The model compound 2,2′,5,5′-tetramethyl-3-oxazolidinyloxy was synthesized and used as a spin-probe in the polymer. The spin-probe experiments gave a transition temperature attributed to the γ -relaxation.

Electron spin resonance studies of spin-labelled polymers—I segmental relaxation of polystyrene in solution

From an analysis of the widths of the hyperfine lines in the ESR spectrum of a nitroxidelabelled polystyrene in toluene solution, the correlation times for rotational diffusion at three temperatures have been measured. The values agree well with published data from NMR studies of polystyrene in solution. The value of the activation energy for the relaxation process, 4·3 kcal. mole−1, is close to published values for dielectric relaxation of para-substituted polystyrenes. These comparisons provide strong evidence for associating the ESR correlation times with some form of segmental motion of the polymer backbone.

Effects of poor solvents on radical-radical termination of polymerization

Abstract It is shown that in poor solvents for the polymer the rate coefficient for radical-radical termination in the polymerization of styrene and methyl methacrylate is diminished compared with that in bulk monomer. It is suggested that this effect is due to the formation of tightly coiled macroradicals which hinder the radical-radical reaction process.