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It is well recognized that the photoinitiator represents the key component of any light curable formulation through radical process. Indeed, the photoinitiator is responsible for the absorption of the incident light radiation producing the primary radical species that initiate the conversion of multifunctional monomers, prepolymers and oligomers to the final crosslinked network. Many efforts have been therefore dedicated to the optimization of the relevant performances of the initiator for industrial applications. In particular, tailor made photoinitiators, characterized by high extinction coefficient in the spectral region matching the lamp emission, high efficiency, related to both quantum yield for radical generation and reactivity with monomer(s), as well as easy solubility in the formulation, have been selected for different application fields. However, one of the main drawbacks of common photoinitiators is due to the presence of residual unreacted molecules and photolysis byproducts which may migrate onto the surface of the cured material with undesired consequences, such as the emission of toxic and/or bad smelling substances, change of colour, etc. In this respect, polymeric photoinitiators, i.e. polymers bearing covalently linked side-chain photoreactive moieties, have recently gained interest, as the anchorage to a polymer backbone may suppress, or strongly reduce, the migration within the crosslinked material of the above mentioned low molecular weight molecules, thus substantially overcoming the problem. In addition, it has been observed in many cases that polymeric photoinitiators may also afford an improvement of photoinitiation activity, as the result of energy migration between excited- and ground- state photosensitive moieties along the polymer chain, or of intramolecular reactions, giving rise to more active radical species, favoured by a higher local concentration of reactive moieties in the macromolecular micro environment. A reduced possibility of radical-radicalrecombination due to the protection of the active radical species offered by the steric hindrance ofthe macromolecular initiator may also occur.
In this context, with the aim to produce photoinitiating systems characterized by improved performances in terms of higher efficiency, higher monomer conversion and reduced presence of migratable toxic residues in the formulation, it appeared of interest to investigate the photoinitiating behaviour of novel methacrylic polymeric photoinitiators bearing in the side chain the photosensitive moiety of camphorquinone (CQ) combined with a tertiary amine possessing abstractable a hydrogen atoms as coinitiator, a system which is specifically employed in dentistry in the photocuring by visible light of methacrylic formulations for restorative dental resins. The anchorage of camphorquinone to a polymeric backbone could in fact produce enhanced activity with respect to the low molecular weight system presently employed; moreover, the release of toxic substances, such as the amine coinitiator, from the final crosslinked resin was also expected to be reduced in consequence of the macromolecular nature of the photoreactive moieties.
