Evan H. Williams

The radical polymerization of dimethacrylate monomers—the use of NMR spectrometry to follow the development of network structure

Abstract 13 C NMR spectrometry has been used to identify and assay residual unsaturation in the cure of ethylene glycol dimethacrylate, and distinguish between monomer molecules and pendant double-bonds. Direct evidence of heterogeneity is provided by the distinction between constrained and unconstrained monomer. The predictions of a simulation using a kinetic gelation model coincide with the experimental estimations. Cure estimated by differential scanning calorimetry does not correlate linearly with residual unsaturation.

Changes of proton-enhanced, magic-angle-spinning 13C-NMR dynamic parameters during cure of tetra(ethylene glycol)dimethacrylate

Abstract Dynamic NMR and mechanical properties TSL, T1p(C) at 60 kHz, Tg at ≈1 Hz and its half width, log decrement (Δmax) and compression modulus (E) were measured for poly[tetra(ethylene glycol)dimethacrylate] (PTEGDMA) at different extents of cure. The β, γ and water-induced transitions were also noted. The spin-lock, cross-polarization time-constant, TSL, declined to its limiting value before the vitrification point. T1p(C), relaxation time in the rotating 13C field, increased sharply at or just after vitrification. The change is greatest at quaternary C, decreasing through CH2, CH2O and CH3, being barely significant in the latter. The observed changes affirm that T1p(C) is sufficiently influenced by spin-lattice relaxations to provide a monitor of the damping of mid kHz components during cure. The small change observed at CH3 shows that this group is not the origin of changes in relaxation rates at other groups. Correlations with E and Tg show the importance of mid kHz components of group motion to the cooperative motions determining the response to macroscopic strain at lower frequencies. MAS 13C-NMR showed that PTEGDMA contains residual unsaturation at limiting cure and that two types, constrained and unconstrained, can be distinguished.

Fluorine-19 nuclear magnetic resonance study of the inclusion of fluoro- and difluoro-trans-cinnamates by α-cyclodextrin

19 F n.m.r. studies of the inclusion of o-, m-, p- and α-fluoro-trans-cinnamates and o, p- and α,p-difluoro-trans-cinnamates by α-cyclodextrin (αCD) have shown that two inclusion equilibria [graphic omitted] are established in D2O solution at pD = 8.5 ± 0.1. Typically at 294 K, K1= 111 ± 13 dm3 mol–1 and K2= 23 ± 2 dm3 mol–1 for α,p-difluoro-trans-cinnamate. Chemical-shift and other data indicate that the predominant S·αCD complex is that in which the carboxylate group of the fluorocinnamate enters the wide end of the αCD cavity, delineated by twelve secondary hydroxy groups, first. The S·(αCD)2 complex probably has a structure in which the fluoro-trans-cinnamate is encapsulated by two αCD with the wide ends of their cavities in close proximity.

The points of contact between poly(methyl methacrylate) and absorbed water as indicated by PE/MAS 13C NMR

SummaryThe development of the PE/MAS 13C NMR spectrum of poly(d8MMA) cross-polarized by the protons of water is used to follow the distribution of water as it is absorbed by the polymer. In the early stages, water is in contact with the backbone CD2 and quaternary C. It makes contact with side-chain carbons later. A non-random distribution of close contacts between water and polymer molecules was also observed when P(d8 MMA) was precipitated from d8-acetone by water.

Some Problems Concerning the Mechanism of the Isotactic Polymerization of Methyl Methacrylate Initiated by Organomagnesium Compounds in Toluene Solution—Application of Modified Dilatometer and NMR Methods

Abstract A dilatometer capable of being rapidly filled with monomer and initiator solutions mixed under carefully controlled conditions and a modification of an NMR T1 program, adapted to store and display on recall the results of frequent sweeps at 30-s intervals of relevant proton resonances of a polymerizing mixture, are reported. These techniques have been applied to the identification of the optimum conditions for efficient, initiation of isotactic polymerization and investigation of the mechanism of propagation. It is shown that heat treatment does not remove the coordinated THF (1 molecule per RMg group) when toluene-soluble “tBuMgBr” or “PhMgBr” initiators are prepared and that the optimum THF concentration is slightly in excess of this. The nature of the solute in the toluene-soluble initiator solutions is discussed. The kinetics of the polymerization change from internal zero order to internal order of one with respect to monomer as the temperature increases from 225–275 K. This is consistent wi...

A proton magnetic resonance study of ligand exchange on pentakis(N,N-dimethylformamide) dioxouranium(VI)ion

Abstract Proton magnetic resonance studies show that UO 2 (DMF) 2+ 5 , pentakis(N,N-dimethylformamide)dioxouranium(VI) ion, is the greatly predominant dioxouranium(VI) species in mixed solutions of DMF and d 2 -methylene chloride. The virtual independence of the rate of intermolecular exchange of DMF of free DMF concentration over a 20.3 fold concentration range is interpreted in terms of a dissociative exchange mechanism. A typical set of kinetic data for this process is k ex (220K) = 199 ± 11 s −1 , ΔH ♯ = 31.9 ± 0.3 kJ mol −1 and ΔS ♯ = −53.4 ± 1.5 J K −1 mol −1 , where the observed exchange rate = 5 k ex [UO 2 (DMF) 2+ 5 ], and the concentrations of UO 2 (DMF) 2+ 5 , DMF and d 2 -methylenechloride are respectively 0.008648, 0.04323 and 15.05 mol dm −3 . The observed magnetic equivalence of the methyl groups in bound DMF over the experimental temperature range 170–300 K is shown to arise as a consequence of the cis and trans DMF methyl groups experiencing significantly different magnetic shielding modifications upon coordination to the dioxouranium(VI) ion probably in part as a consequence of the magnetic anisotropy of that ion.

13C-NMR parameters of some poly[oligo(ethylene glycol) dimethacrylates]

Les deplacements chimiques obtenus sont compares a ceux des monomeres et du polymethacrylate de methyle

The exchange of trimethylphoshpate on Magnesium(II). A phosphorus-31 nuclear magnetic resonance study

Abstract The exchange of trimethylphosphate (TMP) on Mg(TMP) 2+ 6 has been reinvestigated in d 2 -methylenechloride and d 6 -acetone diluents using 31 P NMR spectroscopic methods. The rate of exchange = 6k ex [Mg(TMP) 2+ 6 , and a typical set of kinetic data is: k ex (225 K) = 683 ± 47 s −1 Δ # = 51.3 ± 0.5 kJ mol −1 and ΔS # = 39.5 ± 2.0 J K − mol −1 for a solution in which [Mg(TMP) 2+ 6 [TMP] and [d 2 methylenechloride] = 0.116, 0.604, and 12.7 mol dm −3 respectively. The small variation in k ex (225 K) observed as [TMP] is varied, in conjunction with other evidence, suggests that a D exchange mechanism probably operates. The species Mg(TMP) 2+ 5 is observed in d 2 -methylenechloride, and acetone is found to compete with TMP for sites in the first co-ordination sphere of magnesium(II).

NMR of tracer molecules in networks

Abstract Liquid NMR techniques were used to elucidate the molecular organization and dynamics of poly[tetra(ethylene glycol)dimethacrylate] and poly[tetra(ethylene glycol)diacrylate] as a function of cure. The change in line width of the monomer was used to determine the extent of cure and indicate the mobility in the polymer matrix. Line widths of different parts of the monomer peaks broadened at different extents of cure. Non-reacting tracer molecules (dimethylsulphoxide, methyl acetate, decalin, cyclohexane and dodecane) were also introduced into the curing system at low concentrations and their line widths observed. It was found that they broadened at differing rates and persisted well into the later stages of the cure when the monomer peaks had broadened into the baseline. In typical cures the monomer peaks broadened simply but, in atypical cases, the line shapes showed that there were two monomer domains one constrained and the other unconstrained. All tracers became constrained with increasing cure, except dodecane. The line shape of pure monomer was also followed as a function of temperature and found to broaden in similar fashion to the monomer during a typical cure.

The kinetics of the polymerization of methyl methacrylate initiated by t-butylmagnesium bromide in THF, toluene or mixed solvent

Abstract The kinetics of the solution polymerization of methyl methacrylate in THF, toluene and their mixtures were studied between 200 and 300 K using dilatometry (in the systems where it was valid), gravimetric determination and monitoring monomer and polymer concentrations by NMR spectrometry. The reaction followed zero order kinetics at 200 K, first order kinetics at 275 K and mixed order in between. At both the limits and intermediate range, the reaction followed an integrated rate equation consistent with terminationless propagation proceeding through a complex between monomer and the propagating species. Above 275 K, termination and side reactions were evident and the yields of high mol. wt polymer were small. Density-temperature calibrations for monomer in THF, toluene and mixtures were constructed for the range 190–283 K. However for polymerizations in toluene-rich mixtures, where very high mol. wt polymer forms, the contraction did not correlate linearly with conversion.