[4 + 2]-Cycloadditionen mit inversem Elektronenbedarf,XVII. Oxepin und 2,7-Dimethyloxepin als Dienophile bei Diels-Alder-Cycloadditionen mit inversem Elektronenbedarf

[4 + 2] Cycloadditions with Inverse Electron Demand, XVII. Oxepin and 2,7-Dimethyloxepin as Dienophiles in Diels-Alder Cycloadditions with Inverse Electron Demand The cycloaddition reactions of oxepin ( 1 ) with the tetrazine 3 and the triazine 12 yield the dihydrooxepino[4,5-d]pyridazine 5 and the oxepino[4,5-c]pyridine 13 , resp., via [2 + 4] cycloaddition as well as the phthalazine 9 and the isoquinoline 14 , resp., most probably via [6 + 4] cycloaddition. On the contrary, 2,7-dimethyloxepin ( 15 ) only gives the products 16 and 20 , which result from [2 + 4] cycloaddition. Oxidation of 16 by silver(I) oxide leads to the oxepino[4,5-d]pyridazine 17 , which as well as 20 in hydrochloric acid rearranges to the pseudoazulenes 19 and 21 , resp.     

Enthalpy relaxation of rigid poly(vinyl chloride)

The changes which take place on annealing rigid PVC in the vicinity of the glass transition have been followed by differential scanning calorimetry. The changes appear as an increase in the glass-transition temperature and a decrease in the enthalpy with time of annealing. For annealing at 75°C, the enthalpy after 50–100 hr approaches the value characteristic of the equilibrium liquid state. The results obtained for annealing at 65°C and 75°C are in accord with those expected for the relaxation of an amorphous material, and are at variance with those expected on the basis of crystallization taking place on annealing. The enthalpy relaxation process is characterized by a distribution of activation energies centered about 18.8 kcal mole^?1, and seems to reflect a multiplicity of molecular processes.     

Crystallization of polyethylene at high pressure: A kinetic view

A model for the crystallization kinetics of polymers is outlined and is used to interpret observations of the crystallization of polyethylene at high pressures. This model introduces a distinction between σ_e the lamellar surface energy which controls the lamellar thickness, and σ_e′, the surface nucleus surface energy which controls the growth rate. Differential scanning calorimetry and electron microscopy data for several polyethylenes crystallized at pressures of up to 8 kb are presented. From the dependence of lamellar thickness on the crystallization undercooling at 5 kb, it is found that σ_e increases markedly with pressure leading to the formation of very thick crystals at high pressures. The magnitude of the increase in σ_e is in agreement with σ_e values calculated from the dependence of melting temperatures on pressure. The nucleus surface energy σ_e′ is not expected to vary significantly with pressure, and estimates of growth rates of 5 kb which indicate that the growth rate does not vary significantly with pressure at constant under-cooling confirm this. Fractionation effects and the differences in behavior between different polyethylenes are also discussed.     

Gene Synthesis [New Synthetic Methods (77)]

Oligonucleotide synthesis, until a few years ago the rather exotic preserve of a few experts, has become an integral part of the arsenal of molecular-biological techniques. The last decade, in particular, has seen unbelievably rapid development in this area. DNA synthesis has been automated and can now produce genes greater than 1000 base pairs in length. Tailor-made synthetic genes also permit the synthesis of altered or even novel proteins (de novo protein design) by gene-technological methods. Together with modern methods of gene isolation, sequencing, and expression, gene synthesis has played a major part in the enormous advances achieved in gene technology.