Kinetics of Photoinitiated Reactions

Kinetics of Photoinitiated Reactions 4.3.1 Kinetics of Free Radical Photopolymerization... 

Initiation of UV Hybrid Curing Kinetics of Photoinitiated Reactions... 

In this work, the kinetics of these reactions are closely examined by monitoring photopolymerizations initiated by a two-component system consisting of a conventional photoinitiator, such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and TED. By examining the polymerization kinetics in detail, further understanding of the complex initiation and termination reactions can be achieved. The monomers discussed in this manuscript are 2-hydroxyethyl methacrylate (HEMA), which forms a linear polymer upon polymerization, and diethylene glycol dimethacrylate (DEGDMA), which forms a crosslinked network upon polymerization. 

In contrast, RT-FTIR spectroscopy allows a rapid and quantitative measurement of the conversion of specific reactive functional groups under variable conditions as light intensity, photoinitiator concentration, coating thickness, etc., which are closer matched to those in technical coating and printing processes. In the past, RT-FTIR spectroscopy has been successfully used to study the kinetics of photopolymerization reactions in dependence on the irradiation conditions and other experimental parameters, the reactivity of monomers and oligomers and the efficiency of newly developed photoinitiator systems. 

The above-mentioned phenomenological model is usually used to describe the kinetics of autocatalytic reactions that are characterized by a maximum reaction rate between 20 and 40% conversion. The shape of the photoinitiated polymerization rate curves... 

In particular, above said may be applied also to photoinitiated radical (co)polymerization of mono- and polyfimctional monomers. Two diamet-rically-opposite concepts are used to deseribe the kinetics of photoinitiated (co)polymeiization. The first one is the concept of diffusion-controlled reactions, based on the assumption about the diffusion control of elementary acts of macromolecular chain propagation and decay [14]. The second concept is the radieal polymerization model, based on the notion of mi-... 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Representative Kinetic Data on Reactions of Photoinitiator Free Radicals... 

In the present ehapter we consider the inter- or intramolecular photoinduced electron transfer phenomenon. We mainly focus on photoinduced electron transfer processes that lead to the photoinitiation of polymerization, and on processes initiated by photoredueed or photooxidized excited states. We concentrate especially on a description of the kinetic schemes, a description of the reactions that follow the primary proeess of eleetron transfer, and the characteristics of intermediates formed after electron transfer. Understanding the complexity of the processes of photo-initiated polymerization requires a thorough analysis of the examples illustrating the meehanistie aspects of the formation of free radicals with the ability to start polymerization. 

For processes in which the rate of polymerization is not limited by the rate of electron transfer, the equation describing the rate of polymerization, one can obtain during the analysis of simple kinetics a scheme of photoinitiated polymerization. A mechanism describing a photoinitiated polymerization via PET (not considering the kinetics of free-radical formation, because this process does not affect the rates of polymerization), that contains all the major reaction steps, can be represented by Eqs. (34)-(39). 

Quantitative study of kinetics of radicals accumulation has required solution of auxiliary problem - definition of the rate of photoinitiation Win. In the case of solid-phase reactions there are experimental difficulties in solution of this problem. Measurement of Win according to consumption of inhibitor is complicated by possible photochemical reactions of inhibitor itself and specific solid-phase effects of kinetic stop type and so on [10]. Measurement of Win according to initial rate of radicals accumulation is also tactless in solid polymer, as the latter may be much lower than Wm [164]. 

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 ... 

The investigation [20a] of the photoinitiated reaction of Cp Rh(CO)2 with neopentane in liquid krypton by low-temperature IR flash kinetic spectroscopy gave the results that are consistent with a pre-equilibrium mechanism. According to this mechanism, an initially formed transient krypton complex Cp Rh-(CO)(Kr) is in rapid equilibrium with a transient (uninserted) alkane complex... 

The kinetics of ultrafast polymerization of acrylic monomers exposed to UV radiation or laser beams has been investigated by IR spectroscopy. An 8 fold increase of the cure speed was observed by using diphenoxybenzophenone as photoinitiator instead of benzophenone. llie reactivity of polyurethane-acrylate or epoxyacrylate systems was markedly improved by adding acrylic monomers that contain carbamate or oxazolidone groups and which impart both hardness and flexibility to the cured polymer. Time-resolved infirared spectroscopy was used to directly record the actual polymerization profile for reactions taking place within a fraction of a second upon UV or laser exposure. Comparison with other techniques of real-time analysis show the distinct advantages of this method for an accurate evaluation of the important kinetic parameters and of the dark polymerization which develops just after the irradiation. 

Kinetic Study of Photoinitiated Polymerization Reactions by Real-Time Infrared Spectroscopy... 

In this review article we present some typical examples showing how effective RTIR spectroscopy proved to be to evaluate the important kinetic parameters of photoinitiated radical and cationic polymerizations, and to assess the influence of some chemical and physical parameters on both the rate and the extent of the light-induced curing reactions. The photopolymerization of different types of monomer mixtures will also be examined, as RTIR spectroscopy is a unique tool to follow the kinetics of... 

One of the common features of all UV-curable systems is the rapidity at which the polymerization takes place under intense illumination, usually less than one second. Therefore it is difficult to accurately follow the kinetics of such ultrafast reactions, which is a prerequisite for a better understanding and control of the curing process. Moreover, evaluation of the kinetic parameters (rate of polymerization, kinetic chain length, propagation and termination rate constant) is essential in order to compare the reactivity of different photosensitive resins and assess the performance of novel photoinitiators and monomers. 

Many other examples can be sited. Following are some basic kinetic considerations of the reactions of photoinitiators and photoinitiating processes. Based on Beer s Law, the fi action of absorbed light by a solution (or a light curable composition) can be expressed as follows ... 

Recently Fouassier and coworkers, presented a general photo-thermal methods for studying both kinetic and thermodinamic properties of the photopolymerization processes. Photoacoustic and thermal lensing spectroscopies allow the determination, of triplet quantum yields and energy levels of photoinitiators. Beyond the possibility of determining easily and accurately bond dissociation energies of coinitiators, the methods provide important information on their reactivity. The application of photoacoustics was extended by Fouassier and coworkers, to the study of the initiation step. A specific data treatment was developed to determine the rate constants and the enthalpy of the reaction of addition of a radical to a monomer unit. 

Scott et a/., carried out Fourier transform infrared and electron spin resonance spectroscopic investigations of the photopolymerization of vinyl ester resins These consisted of studying reaction kinetics during photopolymerization of bis glycol methacrylate/styrene blends using commercial photoinitiators, Irgacure 819 and Lucirin TPO ... 

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