Styrene polymerization thermal initiation

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. 

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. 

To test our model, we set up small and large-scale tests for thermally-initiated polymerization of styrene. 

In order to test this computer model, we conducted experiments on thermally Initiated styrene polymerization In sealed pressure vessels. We only measured pressures and temperatures In these experiments. We conducted our tests in two phases. 

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. 

Figure 13.7 illustrates stability regimes for the thermally initiated polymerization of styrene for laminar flow in a single tube. Design and operating variables... 

Thermal initiation makes an appreciable contribution to the polymerization rate for styrene at very low initiator concentrations, as we have pointed out earlier. Since the rate Rp includes contributions from thermal as well as from catalytic initiation, the second term in Eq. (36) remains valid provided the thermal initiation involves monoradicals. Diradical initiation, if it occurred, would introduce a deviation, since it produces no chain ends. 

In thermal polymerization where the rate of initiation may also vary with composition, an abnormal cross initiation rate may introduce a further contribution to nonadditive behavior. The only system investigated quantitatively is styrene-methyl methacrylate, rates of thermal copolymerization of which were measured by Walling. The rate ratios appearing in Eq. (26) are known for this system from studies on the individual monomers, from copolymer composition studies, and from the copolymerization rate at fixed initiation rate. Hence a single measurement of the thermal copolymerization rate yields a value for Ri. Knowing hm and ki22 from the thermal initiation rates for either monomer alone (Chap. IV), the bimolecular cross initiation rate constant kii2 may be calculated. At 60°C it was found to be 2.8 times that... 

The appearance of polymerized monomer droplets indicates that polymerization is initiated both in the monomer droplets and in monomer-containing micelles. This result is completely different from that obtained in the EP of styrene under identical conditions, where no monomer droplets polymerize. Similar experiments with 1,3,5-trivinylbenzene also yielded polymerized monomer droplets as by-products [77]. The amount of polymerized 1,4-DVB droplets further increased when PPS was replaced by an oil soluble initiator, such as, AIBN [83] or, when the EP was thermally initiated [84]. Figure 5 compares electron micrographs of the polymers formed by thermally (90 °C) initiated EP of 1,4-DVB and S. 

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. 

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

The same initial polymerization rate and degree of polymerization as in Problem 3-15 are obtained at 27°C for a particular AIBN thermal-initiated polymerization of styrene. Calculate the Rp and X values at 77°C. 

Styrene monomer and a styrene/butadiene copolymer are fed to the first reaction zone. The polymerization is initiated either thermally or chemically. Many chemical initiators are available such as ferf-butyl peroxybenzoate and ferf-butyl peracetate. Conditions are established to prevent a phase inversion or the formation of discrete rubber particles in the first reaction zone. The conversion in the first reaction zone should be 5-12%. An important function of the first reaction zone is to provide an opportunity for grafting of the styrene monomer to the elastomer (8). 

Mayo, F. R. Chain transfer in the polymerization of styrene. VIII. Chain transfer with bromobenzenc and mechanism of thermal initiation. J. Am. Chem. Soc. 75, 6133 (1953). 

Catalysis of Thermal Initiation of Styrene Emulsion Polymerization by Emulsifiers... 

Although cyclobutanes with varying substitution patterns are known, cyclopropanes present a much wider variety and much greater ease of synthesis. Ethyl 2-(p-methoxyphenyl)-l-cyanocyclopropanecarboxylate has been shown to thermally initiate the diradical polymerization of acrylonitrile [138]. In the presence of zinc chloride as activator, it also initiates the diradical polymerization of styrene [139]. On the other hand, this same initiator also initiates the thermal cationic polymerization of AT-vinylcarbazole [140]. This direction of tetra- and trimethylene chemistry is currently under active investigation. 

Vinyl monomers, such as styrene, methyl methacrylate, vinyl acetate, vinyl chloride or acrylonitrile are preferably polymerized by chain polymerization techniques initiated by free radicals. Suitable free radicals can be handily achieved from unstable chemicals like peroxides (benzoyl peroxide, dicumil peroxide) or di-azo reagents (e.g. 2,2 -azo-bis-isobutyronitrile, AIBN) which are dissolved in monomer and usually thermally decompose at temperature range of 40-120 °C. Alternatively, suitable radicals for polymerization can also be activated without addition of external initiators, by just applying ultraviolet light (wave length 200-350 nm) or ultrasound (15,33,34) onto monomer. 

The rate of the thermally initiated methyl methacrylate polymerization amounts to only about 1 % of the rate measured with styrene. It can be increased by the presence of heavy metal atoms [16] which can change the multiplicity of the diradical and thus also its reactivity. 

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