Polymerization kinetics thermal initiated

Theoretical Aspects. Ultraviolet light-cured inks are cured by free radical-initiated vinyl addition polymerization. The photochemical initiation of vinyl polymerization has been the subject of many investigations dating back more than 30 years, to the first systematic studies of polymerization kinetics. Photochemical initiation offered a reproducible source of radicals that is not dependent upon temperature as is the thermal decomposition of free radical initiators. However, despite these early studies of its mechanism and kinetics, only recently has photochemical initiation become of practical interest, mainly because of the recent development of ultraviolet lamps suitable for production curing of printing inks and coatings. 

Initiator decomposition can be triggered in a variety of ways. The most conunon method for industrial free radical polymerization is thermal initiation (typically using azo or peroxy initiating species), while photoinitiation is more popular for laboratory scale kinetic studies. In either case. Equation 1.6 describes the decomposition of initiator into two radical species, which may or may not have equal reactivities, depending on the choice of initiator [1,2]. The concentration of initiator can then be calculated by ... 

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. 

In conclusion, we have reviewed how our kinetic model did simulate the experiments for the thermally-initiated styrene polymerization. The results of our kinetic model compared closely with some published isothermal experiments on thermally-initiated styrene and on styrene and MMA using initiators. These experiments and other modeling efforts have provided us with useful guidelines in analyzing more complex systems. With such modeling efforts, we can assess the hazards of a polymer reaction system at various tempera-atures and initiator concentrations by knowing certain physical, chemical and kinetic parameters. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

The effect of the nitrone stmcture on the kinetics of the styrene polymerization has been reported. Of all the nitrones tested, those of the C-PBN type (Fig. 2.29, family 4) are the most efficient regarding polymerization rate, control of molecular weight, and polydispersity. Electrophilic substitution of the phenyl group of PBN by either an electrodonor or an electroacceptor group has only a minor effect on the polymerization kinetics. The polymerization rate is not governed by the thermal polymerization of styrene but by the alkoxyamine formed in situ during the pre-reaction step. The initiation efficiency is, however, very low, consistent with a limited conversion of the nitrone into nitroxide or alkoxyamine. 

In order to tailor the physical properties of the polyaromatic networks obtained by thermal curing, it is important to ascertain the relationship between structure and physical properties for both the starting oligomer and its resultant network. We therefore sought a reactive group whose mechanism of thermal initiation, kinetics, and thermodynamics of polymerization are known. 

This is one of the reasons we decided to prepare oligomers containing styrene-type functional groups. Styrene s thermal initiation mechanism is fairly well understood, and the same is true for the kinetics and thermodynamics of its radical polymerization. In addition, thermal and radical polymerization of styrene is much faster than any of the other previous classes of reactive groups and at the same time, the microstructure of the crosslinking points is known. 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

The addition of small amounts of radical scavenger (such as benzoquinone and diphenylpicrylhydrazyl) led to the appearance of induction periods in the kinetic curves. The duration of the induction periods are proportional to the concentration of the radical scavenger. The presence of atmospheric oxygen slightly slowed the polymerization. These observations indicate that the polymerization proceeds by a radical mechanism. The radicals are formed from the y-radiolysis of the monomers. By comparison to the ESR spectrum of the radicals formed by thermal initiation with azobisisobutyroni-trile in the presence of a spin trap, the radical formed is... 

For the bulk polymerization of styrene using thermal initiation, the kinetic model of Hui and Hamielec (13) was used. The flow model (Harkness (1)) takes radial variations in temperature and concentration into account and the velocity profile was calculated at every axial point based on the radial viscosity at that point. The system equations were solved using the method of lines with a Gear routine for solving the resulting set of ordinary differential equations. 

A distinctive characteristic of styrene polymerization is its thermal selfinitiation at high temperatures (without the presence of a chemical initiator). The mechanism of styrene thermal initiation was first described by Mayo [12]. The kinetics of thermal initiation were described by Weickert and Thiele [13] as a second-order reaction, while Hui and Hamielec [14], Husain and Hamielec... 

It is worth noting that the dimer and trimer generated in reactions (8) and (9) can react with polymeric radicals as a chain transfer agent, and therefore their effect on the polymer molecular weight should not be neglected the quantitative estimation of the concentration of these byproducts depends on the fact that whether the rate of thermal initiation is a second- or third-order reaction of monomer concentration. More kinetic information for such transfer reactions can be found in a number of publications [14-19]. Nevertheless, detailed kinetic studies on such Diels-Alder byproducts remain scarce. Katzenmayer [20], Olaj et al. [21,22], and Kirchner and Riederle [23] have published some quantitative results on this matter. 

A mathematical model for styrene polymerization, based on free-radical kinetics, accounts for changes in termination coefficient with increasing conversion by an empirical function of viscosity at the polymerization temperature. Solution of the differential equations results in an expression that calculates the weight fraction of polymer of selected chain lengths. Conversions, and number, weight, and Z molecular-weight averages are also predicted as a function of time. The model was tested on peroxide-initiated suspension polymerizations and also on batch and continuous thermally initiated bulk polymerizations. 

This section is principally devoted to the preparation of thermally sensitive hydrogel particles using the batch polymerization process. The effect of each reactant and parameter (initiator, temperature, cross-linker agent) on the polymerization process (polymerization kinetic, conversion, final particle size, morphology, water-soluble polymer, etc.) is presented and discussed. For the... 

The presence of the nitroxide radical was confirmed through EPR and XH NMR spectroscopic methods. The copolymer GPC trace (Mn=33,100, Mw/Mn=1.37) was symmetrical with no evidence of unreacted macroinitiator or homopolymer of St resulting from either thermal initiation or from disproportionation of the pSt from the TEMPO chain end [231]. The kinetic results showed a first-order relationship between monomer conversion and time and the molecular weights increased linearly with conversion, indicating the polymerization proceeded with minimal termination or chain transfer reactions. The presence of the pEAD block produces an amphiphilic copolymer with a biodegradable block that may be useful for biomedical applications. 

Peculiarly there is no relationship between the concentration of the growing species and the amount of p-ClC6H4N PFfl used. This is probably due to the irreproduci-bility of the thermal decomposition of the diazonium salt. In experiments 1-3 (cf. Table 8.3) the amount of liberated PF5 was apparently different, although the amount of pClC6H4N+PF was kept constant. This observation weakens the authors29 earlier statement concerning the effect of initiator concentration (expressed in terms of precursor concentration) on the polymerization kinetics and molecular weights. 

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