Catalytic reactions, photoinitiators

The catalytic reaction involving light absorption by a substrate and catalyst species is called photocatalysis [29], At the end of the reaction cycle, the photocatalyst is regenerated to its original state. Sometimes, however, the agent that initiates certain chemical transformations under the action of light is itself consumed in the reaction or process then it is called a photoinitiator [127-129]. 

In all three of these designs, chemical amplification is the result of photoinitiated chain or catalytic reactions where irradiation is used only to initiate a chain reaction or to generate a catalyst within localised areas of a resist film. 

In a reply on this note Mirbach [13] emphasized that the quantum yield of product formation only is not a sufficient criterion to identify whether a reaction belongs to the first category, here named as photoinitiated catalytic reactions. When the quantum yield of the initial photoreaction, forming the catalyst is very low, the overall quantum yield may be less than unity, although the catalyst can enter many cycles of thermal substrate conversion. Furtheron, Mirbach classified such photoreactions which need a photon to start a new cycle into three fundamentally different processes Photoassisted catalytic reactions, catalyzed photoreactions, and sensitized photoreactions. 

Fig. 2 Simplified representation of photocatalytic reaction pathways ) photo induced catalytic reaction, N denotes a nominal catalyst which yields photochemically the real catalyst C (2) photoinduced chain reaction, here I means any photoinitiator (3) photoinduced chain reaction initiated by a sacrificial agent I (4) photoassisted reaction with N as nominal catalyst (photo-assistor) (5) photoassisted reaction, the catalyst N is a short-lived intermediate in ground state (6) sensitized photoreaction, N represents a sensitizer (7) catalyzed photo-reaction, C is to be considered as a real catalyst.

In order to probe the photoinitiation that occurs in these catalytic reactions, flash photolysis has been carried out with fran -RhCl(CO)(PMe3)2, trans-RhCl(CO)(P(p-tolyl)3)2, and rran -RhCl(CO)(PPh3)2. Photolysis of trans-RhCl(CO)(PPh3)2 leads to CO dissociation, followed by subsequent competing... 

The primary aim of most studies on Lewis acid controlled copolymerization has been the elucidation of mechanism and only low conversion polymerizations are reported. Sherrington et al.m studied the high conversion synthesis of alternating MMA-S copolymers in the presence of Lewis acids on a preparative scale. Many Lewis acids were found lo give poor control (i.e. deviation from 50 50 composition) and were further complicated by side reactions including cross-linking. They found that the use of catalytic BCI- as the Lewis acid and photoinitiation gave best results. 

Hydroperoxides are potent photoinitiators and their decomposition into inactive products is an important method of stabilization in polyolefins for it prevents generation of radicals by reactions such as (4), (5), (14), and (15). Decomposition of hydroperoxides can be achieved by a reaction with phosphite esters or nickel chelates or by a catalytic action by a range of compounds including dithiocarbamates and mercaptoben-zothiazoles. Decomposition of hydroperoxides formed during processing is important for the stabilization ofPVC. 

Cho and Hong (2005) used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated cationic photopolymerization of 1,4-cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4-diethylthioxanthone (DETX) in the presence of a diaryliodonium-salt photoinitiator. Two kinetic parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system using an auto-catalytic kinetics model as shown in the following equation ... 

There are special features in photochemical reactions where side reactions become the major track and make a well planned synthesis unsuccessful. Sometimes the photoactive compound in the reaction mixture which could be applied in stoichiometric or catalytic (photosensitizer) amounts may also be the photoinitiator of a radical chain reaction. Again, the role of the solvent has to be considered carefully. If hydrogen abstraction reactions can be induced (e.g. when reactive free radicals are produced), solvents labile to this process must be excluded. Many saturated hydrocarbons, ethers, and alcohols (which normally are useful solvents because of their optical properties) have to be avoided in these... 

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

Some nickel compounds, such as nickel dibutyl dithiocarbamate (NiDBC) and nickel acetophenone dioxime (NiOx), have been regarded as quenchers up to now. But neither of these two acts as quencher for carbonyl or singlet oxygen. Rather, they rapidly convert hydroperoxides into non-free-radical products at high temperatures, and thus remove the primary photoinitiators. These reactions are catalytic in the case of NiDBC, but stoichiometric in the case of NiOx. 

Epoxy adhesives use a catalytic cure mechanism. The catalyst is a byproduct from the reaction of the photoinitiator to UV light. By definition, a catalyst is something that promotes a chemical reaction, but is not consumed in the reaction. One consequence of this is that UV-curing epoxy adhesives can exhibit a shadow curing capability— material that is not directly exposed to UV light will cure, sooner or later. 

There are a few reports in the previous literature of QAI in solution 1,2). In a prototypical exanq>le, the photoinitiated conversion of hexamethyl Dewar benzene ( DB) to hexamethylbenzene (HMB) in the presence of a catalytic amount of an electron-accepting photosensitizer was found to have a quantum yield much greater than unity (/). The chain reaction mechanism is thought to proceed as follows ... 

Two other patents have also appeared describing the use of alternate free radical sources as reducing agents for diaryliodonium and triarylsulfonium salts Crivello and his coworkers have discovered several other types of reactions which can be used to thermally induce cationic polymerization using onium salt photoinitiators. Diaryliodonium salts are thermally decomposed in the presence of catalytic amounts of copper compounds An organocopper compound is proposed as an intermediate which undergoes electrophilic arylation of cationally polymerizable monomers as shown in Scheme 14 to initiate polymerization. 

Thiols and thiolate anions are very good nucleophiles. Various click reactions have been developed based on the nucleophilic attack of thiols on to the electrophilic substrates such as epoxides, isocyanates, halides, and Michael acceptors. The reactions are generally initiated by bases, which are added in catalytic amounts or are produced in catalytic amounts by photolatent bases that act as photoinitiators for these reactions [48]. The rates of these reactions are dependent on the substrates... 

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