Dominik Voll

Quantifying photoinitiation efficiencies in a multiphotoinitiated free-radical polymerization.

Online size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl-substituted acetophenone-type photoinitiators being employed in a single cocktail to initiate the free-radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl-type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments.

Detailed investigation of the propagation rate of urethane acrylates

Temperature dependent propagation rate coefficients, kp, are determined for four acrylate monomers containing a carbamate moiety via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) technique. Therefore, the Mark–Houwink–Kuhn–Sakurada coefficients K and a of the respective polymers were additionally determined via triple-detection SEC. The monomers under investigation were synthesized from hydroxyethyl acrylate, hydroxyl(iso)propyl acrylate as well as phenyl isocyanate and hexyl isocyanate, respectively, in all four possible combinations. For 2-(phenylcarbamoyloxy)ethyl acrylate (PhCEA) an activation energy of 14.3 kJ mol−1 and a frequency factor of A = 1.2 × 107 L·mol−1 s−1 are obtained for kp. The MHKS parameters for poly(PhCEA) are K = 8.3 × 10−5 dL g−1 and a = 0.677. For 2-(phenylcarbamoyloxy)isopropyl acrylate (PhCPA) an activation energy of 14.2 kJ mol−1 and a frequency factor of A = 4.9 × 106 L mol−1 s−1 are found for kp and the MHKS parameters for poly(PhCPA) read K = 10.3 × 10−5 dL g−1 and a = 0.657. The activation parameters of kp of 2-(hexylcarbamoyloxy)ethyl acrylate (HCEA) are EA = 13.3 kJ mol−1 and A = 6.6 × 106 L mol−1 s−1 with K = 36.0 × 10−5 dL g−1 and a = 0.552 for poly(HCEA). For 2-(hexylcarbamoyloxy)isopropyl acrylate (HCPA) EA is 14.1 kJ mol−1 and A = 6.6 × 106 L mol−1 s−1 with K = 26.0 × 10−5 dL g−1 and a = 0.587 for poly(HCPA). All rate measurements were performed in 1 M solutions in butyl acetate. The fast propagating nature and reduced activation energy of the monomers may be understood on the basis of the increased nucleophilicity that is induced by the carbamate functionality present in all monomers. Rate-increasing effects from solvent polarity and/or from H-bonding can, however, not be excluded and might also contribute to the observed high propagation rates.

Determination of vinyl acetate propagation rate coefficients via high frequency pulsed laser polymerization

Abstract New data on the propagation rate coefficient, kp, of vinyl acetate (VAc) are obtained via the IUPAC recommended method of pulsed laser polymerization - size exclusion chromatography (PLP-SEC) operated at 500 Hz laser repetition rate. An apparent dependency of the experiment’s outcome on the laser pulsing rate is identified with kp being about 25 % larger at 500 Hz compared to 100 Hz. The temperature dependence of kp was determined to fit ln kp = 17.12 - 2621 K/T; data is in generally good agreement with literature values when the previous underestimation of kp by 100 Hz laser pulsing experiments is taken into account.