Laboratoriumsapparate

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Crossover in dynamics of polymeric liquids: Back to Tll?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polystyrene (PS), were analyzed in the broad frequency range at temperatures above the glass transition (T_g ). At high temperatures, the spectra followed the qualitative scenario suggested by mode‐coupling theory (MCT) of the glass transition. The crossover temperature (T_c ) was defined to be approximately 1.35 T_g in PIB and approximately 1.15 T_g in PS. At lower temperatures (T < T_c ), the light scattering spectra deviated strongly from the idealized MCT scenario. Different signs of the dynamic transition around T_c are discussed. The difference between the suggested interpretation and an old idea of the liquid–liquid transition in polymeric liquids is stressed: we describe the transition as purely dynamic in nature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2785–2790, 2000     

Conditions for deuterium exchange mediated by iridium complexes formed in situ

A series of iridium-based complexes formed in situ, containing pyridine, phosphines, triphenylarsine, triphenylstibine, and triphenylamine as ligands, has been screened for ability to mediate ortho-exchange of hydrogen in a series of model substrates. Improved incorporation into a number of substrate classes has been achieved. The electronic properties and number of ligands at the metal centre are instrumental in determining which catalysts are best suited to exchange in any given substrate.     

Kalk

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How well can renewable resources mimic commodity monomers and polymers?

This highlight discusses the recent progress aimed at maximizing the potential of biomass for commodity monomers and polymers. These efforts are no longer solely academic issues. In recent years, a variety of alkene, diene, aromatic, and condensation type monomers have utilized renewable resources, such as cellulose, lignin, plant oils, starches, and monoterpenes in commercial polymers. Generally, these multifaceted efforts involve pretreatment of biomass with thermal, chemical, or physical methods followed by a catalyst sequence that entails a combination of acid‐catalysis, bio‐catalysis, or metal‐based catalysis. In this regard, synthesis strategies for ethylene, propylene, α‐olefins, methylmethacrylate, 1,3‐butadiene, 1,3‐cyclohexadiene, isoprene, 1,3‐propanediol, 1,4‐butanediol, and terephthalic acid are discussed as well as opportunities for other renewable‐based monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Controlling crosslinking in thermosets via chain transfer with monoterpenes

Renewable resources have been utilized to control the degree of crosslinking during ring‐opening metathesis polymerizations (ROMP). In this regard, the addition of monoterpenes to thermosets created a sustainable method for adjusting the physical properties and thermal stability. A number of monoterpenes with alkenes were examined as monofunctional reactants. The presence of these monoterpenes, such as β‐pinene, during the ROMP of dicyclopentadiene altered the degree of crosslinking and plasticized the thermoset. The resulting physical properties were tailored to obtain desired modulus and glass transition (T_g) temperatures. The influence of time, temperature, weight percent β‐pinene, and different metathesis catalysts was examined by dynamic mechanical analysis, thermogravimetric analysis, and FTIR spectroscopy. A comparison of β‐pinene with other monoterpenes, such as carvone, limonene oxide, myrcene, and d‐limonene, resulted in a reactivity profile for this general method. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011