Styrene thermally initiated

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... 

The total rate of initiation R (mol/Ls) is the summation of styrene thermal initiation and chemical initiator decomposition, i.e. 

The total rate of initiation (mol/Ls) involving bifunctional initiator as well as styrene thermal initiation R can thus be expressed as... 

Thermal initiation of styrene has been shown to be third order in monomer. The average rate constants for third order initiation determined by Hui and Hamielec is k = 105 34 e(,j8iaT) (M V).- "0 The rate constant for formation of the Mayo dimer determined in trapping experiments with nitroxidcs (Scheme 3.63) or acid (Scheme 3.64) as kn = 104 4 (M ls 1)j21 is substantially higher than is... 

An issue when making the second (and subsequent) blocks from styrenic monomers is that thermal initiation or an added initiator will provide a homopolymer impurity. 

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. 

The thermally-initiated styrene system is considerably simpler than most industrial applications. Though these experiments provided useful guidelines, it was difficult to develop broadly applicable design criteria without carefully evaluating a broad range of monomer, polymer and initiator systems. Hence we extended our kinetic model to some other monomer systems such as styrene and methyl methacrylate using common initiators such as benzoyl peroxide (BPO) and... 

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 11, Styrene thermal polymerization at 140°C, initial conversion —0%...

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

Results for styrene - yield Ea 21 kcal. Since Ep — Et/2 was found previously to be 6.5 kcal., we conclude that the activation energy Ei for thermal initiation in styrene is 29 kcal., which would be quite acceptable for the process (21), already rejected on other grounds. For methyl methacrylate, Ea—l kcal. and Ep — Et/2 = b kcal. Hence Ei = 22 kcal. These initiation reactions are very much slower than is normal for other reactions with similar activation energies. The extraordinarily low frequency factors Ai apparently are responsible. For methyl methacrylate, Ai is less than unity. Interpreted as a bimo-lecular process, this would imply initiation at only one collision in about 10 of those occurring with the requisite energy ... 

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. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

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 example, Melville [26] studied the ultrasonically induced polymerisation of monomers such as styrene, methyl methacrylate and vinyl acetate in the presence and absence of polymethyl methacrylate and found that the polymerisation rates ( 1 % conversion/h) were not substantially increased by the presence of polymer. He concluded, in contrast to Driscoll, that the degradation of polymer was not the major source of radical production. Using hydroquinone as an inhibitor, he was able to deduce, from retardation times, that the rate of radical production was 2 X 10 mol dm s. A typical value for radical production using as an example the thermal initiation of AZBN (10 mol dm ) at 60 °C is 2 x 10" mol dm s" ... 

For the thermally initiated (potassium persulphate) emulsion polymerisation of styrene, we have observed [73] a twofold increase in the initial polymerisation rate in the presence of ultrasound (20 kHz), the increase being dependent upon the level of surfactant employed. Several workers have suggested that possible explanations for the observed increase in rate are ... 

Recently Biggs [74] has investigated the emulsion polymerisation of styrene using ultrasonic irradiation as the initiation source (i. e. in the absence of a chemical initiator). Similar to Lorimer and Mason using a thermally initiated system, Biggs found both a marked increase in monomer conversion rate as a function of time as the ultrasonic intensity was increased but remarkable constancy in the resultant latex particle... 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

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. 

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... 

It thus seems that sodium dodecyl sulfate, sodium tetrapropylene benzene sulfonate, and potassium octadecanoate do accelerate the thermal initiation reaction of styrene but that Triton X-100 and sodium dodecyl benzene sulfonate do not although the latter is effective in accelerating thermal initiation of alkyl methacrylates (3 ). ... 

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