Theoretical Study of the Photochemical Initiation in Nitroxide-Mediated Photopolymerization

WOS:000338184600010International audienceNitroxide-mediated photopolymerization (NMP2) is a promising novel route to initiate radical polymerization. In NMP2, alkoxyamines bounded to a monomer are attached to a chromophore. Upon light absorption, the excitation energy is transferred from the chromophore to the alkoxyamine moiety, inducing the cleavage of the oxygen-carbon bond and thus initiating the polymerization. The NMP2 mechanism depends strongly on several factors like the type of chromophore, the monomer, the connectivity pattern, etc. This complexity makes it difficult to design new NMP2 initiators with increased polymerization efficiency and selectivity. In the present article, we characterize by means of quantum mechanical calculations the main steps of the NMP2 initiation for alkoxyamines attached to aromatic ketones. We show how the excitation energy can be transferred from the chromophore to the alkoxyamine moiety, and present two easily computed parameters which can account for the selectivity of the O-C bond photocleaveage. Finally, using results obtained for a series of isomers, we give some rules that may help the design of more efficient NMP2 initiators

Cooling and Freezing of Fruits and Fruit Products

The use of low temperatures is an important factor for maintaining postharvest quality of fruits and greatly influences the rate of deterioration. The objective of cooling is to prolong the commercial life of perishable products, by decreasing metabolic activity without causing chilling or freezing injuries, reduction of microbial growth and water loss of the product, between others. On the other side, freezing results in improved effects with respect to shelf life of fruits and availability throughout the year, however various undesirable changes occur due to ice formation.In the present chapter pre-cooling treatments and refrigerated storage of fruits are analyzed. The use of controlled and modified atmosphere storage, novel technologies (thermal treatments, UV-C irradiation, etc.), minimally processed fruits and the use of edible coatings are analyzed.With reference to fruit freezing the formation of ice in tissues (nucleation and crystal growth, intra and extracellular ice), initial freezing point, state diagrams, glass transition temperature, freezing rates and the different equipment used for freezing are described.The effect of freezing rate on fruit quality, physical and chemical modifications, nutritional properties and microbial stability of frozen fruit are discussed. Preparatory operations for freezing such as pre-treatments of fruits using sugar syrups, and dehydrofreezing are explained. Recommended packaging and industrial freezing methods, shelf-life of frozen fruits and new trends in freezing technology are also analyzedFil: Chaves, Alicia Raquel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos; ArgentinaFil: Zaritzky, Noemi Elisabet. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos; Argentin