Polymerization, photoinitiated, kinetics

Kinetics and Mechanism of Methyl Methacrylate Polymerization Photoinitiated by Benzophenones in Tetrahydrofuran... 

More detailed information on the activation pathway, which distinguishes the polymeric photoinitiators from the corresponding low-molecular-weight structural models, has been obtained by photophysical measurements [22,27,28] in terms of quantum yield and average lifetime of the triplet excited state of benzophenone moieties in the above systems. However, in order to get a better comprehension of this point, it is necessary to introduce some basic concepts about the kinetic treatment of a photoinitiated chain polymerization. 

The kinetic data (Table 12), determined in the presence of the above polymeric photoinitiators, clearly indicate the following order of activity poly (UP36-co-DAPA) = poly(ABP-co-DEEA)>poly(ABP-co-DAPA)>poly(UP36-c i-DEEA) > poly( VBP-co-DEEA) poly( ABP-co-DMAS) > poly(VBP-co-DMAS). 

The data in Figures 23 and 24 clearly show that the efficiency of the BP-TAA as a polymerization initiator couple depends on both the benzophenone structure and the structure of the tertiary amine. Since the trend observed in Figure 24 is characteristic for kinetic phenomena in what is known as the inverted Marcus region [12-14, 110, 111], and since benzophenone triplet quenching cannot display this specific kinetic phenomena, one concludes that the rate of polymerization photoinitiated by... 

The kinetics of cationic polymerizations are considerably more complex than those of the free-radical polymerizations, and kinetic data is difficult to interpret becanse of several reasons (92,138-140). For example, in cationic polymerizations the identity and proximity of the coimterion has a marked effect on the reactivity of the active center. An active center that is encumbered by a closely associated counterion has a dramatically lower reactivity (typically an order of magnitnde lower) than an active center that is separated from the counterion. As described in the section on photoinitiation, this consideration has lead to the development of large, nonnucleophilic coimterions however the reactivity of a cationic active center still depends on the proximity of the counterion. Therefore, at any given time, a variety of propagating species may exist, ranging from ion pairs to separated ions. For this reason, an effective propagation rate constant which inclndes contributions from all propagating species is usually adopted, as described below. Secondly, unlike free-radical polymerizations, the steady-state approximation for active center concentration is not valid since the cationic active centers are not reactive toward one another, and the rate of active center... 

Photoinitiation Photoinitiation [6] takes advantage of initiators that can form radical species upon UV irradiation. Unlike thermal initiation, which produces a relatively small supply of radicals throughout the course of a polymerization, photoinitiation can provide a burst of radicals when desired. This makes photoinitiation an ideal candidate for kinetic experiments or surface-initiated polymerization because the production of radicals is limited to the area that is irradiated at the time of irradiation. Furthermore, the concentration of radicals, p, produced by a given number of photons can be easily calculated as follows ... 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Not all initiating radicals (/ ) succeed in initiating polymerization, recombination of these radicals in the solvent can decrease the efficiency (/) to a value lower than 1. Detailed kinetic treatment of photoinitiation processes are discussed by Oster and Yang [3]. 

Photoinitiation is an excellent method for studying the pre- and posteffects of free radical polymerization, and from the ratio of the specific rate constant (kx) in non-steady-state conditions, together with steady-state kinetics, the absolute values of propagation (kp) and termination (k,) rate constants for radical polymerization can be obtained. 

Bamford and coworkers [24] also investigated the kinetics and mechanism of free radical polymerization of bulk MMA photoinitiated by Mn2(CO)io or Re2(CO)io in the presence of a series of fluoro-olefms such as ... 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Photopolymerization of MMA was also carried out in the presence of visible light (440 nm) using /3-PCPY as the photoinitiator at 30°C [20]. The initiator and monomer exponent values were calculated as 0.5 and 1.0, respectively, showing ideal kinetics. An average value of kp /kt was 4.07 x 10 L-mol -s . Kinetic data and ESR studies indicated that the overall polymerization takes place by a radical mechanism via triplet carbene formation, which acts as the sources of the initiating radical. 

In order to formulate an answer to the obviously important question of the length of this interval of acceleration and to ascertain under what conditions it may be long enough to observe experimentally, we shall examine the non-steady-state interval from the point of view of reaction kinetics. Let us suppose, however, that the polymerization is photoinitiated, with or without the aid of a sensitizer. It is then possible to commence the generation of radicals abruptly by exposure of the polymerization cell to the active radiation (usually in the near ultraviolet), and the considerable period required for temperature equilibration in an otherwise initiated polymerization can be avoided. Then the rate of generation of radicals (see p. 114) will be 2//a s, and the rate of their destruction 2kt [M ]. Hence... 

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. 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

Stationary and Non-Stationary Kinetics of the Photoinitiated Polymerization Yu.G. Medvedevskikh, A.R. Kytsya,... 

ESR (electron spin resonance) and optical absorption spectroscopy at low temperatures were used to analyse the individual reaction steps of the optical and thermal polymerization reactions and their kinetics. The reaction steps are the photoinitiation, the chain propagation and chain termination reactions. 

As reported in Table 15, the kinetic data clearly indicate that the photoinitiation activity of poly(BMOA-co-MtA) is not substantially affected by the content of BMOA co-units along the polymer chain and is of the same order of magnitude as that found for the model compound BMOAc. The absence of a polymer effect in the above photoinitiators has been interpreted [84] in terms of a photodegradation mechanism of the macromolecules involving the ftee radical species anchored to the main chain, even in the presence of acrylic monomers, analogous to what is reported in Scheme 18. Moreover, the induction period of the HDDA/BA photoinduced polymerization increases, on decreasing the content of... 

A detailed analysis of the kinetic data of Table 21 clearly shows that all the polymeric systems display higher photoinitiation activity than that of the corresponding low-molecular-weight analogues MBAc and MBEE [84]. The above results confirm that a positive polymer effect on activity is also present in polymeric systems working with a photofragmentation mechanism. 

The comparison of the kinetic data reported in Tables 26 and 28 clearly shows that poly(VBPO-co-MMA)s are more active in the photoinitiated polymerization of the HDDA/BA equimolar mixture than poly(MAPO-co-MMA)s, notwithstanding their uncomplete solubility in the acrylic formulation. In particular, the remarkable shortening of the induction period (to) causes a sharp increase of the overall polymerization efficiency in poly(VBPO-co-MMA)s, as indicated by ti/2 values which are about one third of those for poly(MAK)-co-MMA)s. 

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. 

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