Photoinitiators-bulk effects

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

As a photoinitiator for the polymerization of bulk methyl methacrylate, benzil was found to be considerably less efficient than benzoin or benzoin methyl ether (20), e,g. at photoinitiator concentrations of 10 M, the polymerization rate observed using benzoin was eight times that observed using benzil, despite the fact that the latter was found to absorb three times as much of the incident radiation (Fig. 1). The photo-initiating efficiency of benzil was improved by a factor of three on addition to the methyl methacrylate of 10% v/v tetrahydrofuran, whereas the same additive had no appreciable effect on rates of benzoin- and benzoin methyl ether-photoinitiated polymerizations direct evidence that photoinitiation by benzil proceeds by a hydrogen abstraction mechanism rather than by fragmentation. 

In addition to (i) nascent product V,R,T resolution, (ii) comparisons with results obtained under bulk conditions, (iii) yield spectra, (iv) steric effects, and (v) branching ratios, several experiments with subpicosecond time resolution have been reported by Zewail, Bernstein, and co-workers for CO2-H1 complexes [42,43]. These measurements show that OH product builds up on a time scale of a few picoseconds following pulsed photoinitiation. This is compatible with the anticipated unimolecular decomposition nature of the reaction in which OH accrues via intermediates such as HOCO , where denotes substantial vibrational excitation. 

Bulk semiconductors and powders have been used as initiators for radical polymerization reactions [140-144], Recently the study has been extended to semiconductor nanoclusters [145-147]. It was found that polymerization of methyl methacrylate occurs readily using ZnO nanoclusters. Under the same experimental conditions, no polymerization occurred with bulk ZnO particles as photoinitiators [145], In a survey study, several semiconductor nanoclusters such as CdS and Ti02, in addition to ZnO, were found to be effective photoinitiators for a wide variety of polymers [146], In all cases nanoclusters are more effective than bulk semiconductor particles. A comparison of the quantum yields for polymerization of methyl methacrylate for different nanoclusters revealed that Ti02 < ZnO < CdS [146]. This trend is parallel with the reduction potential of the conduction band electron. The mechanism of polymerization is believed to be via anionic initiation, followed by a free-radical propagation step. 

Piletsky et al. [81] had found that using a coated hydrogen abstraction photoinitiator (see Scheme 4e) very thin MIP films, which were covalently anchored and covered the entire surface of the base material, could be synthesized by a photo-initiated cross-linking graft copolymerization. This approach had been first explored with benzophenone as photo-initiator and a membrane from polypropylene as support. MIP synthesis and recognition were possible in/from water, and significantly less cross-linker than with bulk preparations was necessary to obtain the highest template specificity. Both effects were explained by a contribution of the soUd polymer support to the stabilization of the imprinted sites. The approach is very flexible because no premodification is necessary. 

In conclusion, reviewing the literature and experimental material obtained from stationary photoinitiated radical polymerization of bifunctional (meth)acrylates in bulk allowed us to obtain the kinetic equation describing the process at all depths of conversion. A mechanism of 3-D polymerization combining the kinetic schemes of homophaseous and heterophaseous processes has been proposed on the basis of the microheterogeneous model and also effective rate constants of these processes have been estimated. 

The interface between two immiscible electrolyte solutions is well suited to studying artilicial photosynthesis. Indeed, the products of a photoinitiated electron-transfer reaction can be separated on either side of the interface, thereby breaking the cage effect that is dominant in bulk solutions. 

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