Model styrene polymerization

Fig. 22. Ten polymer chain structures taken into account when modeling styrene polymerization initiated by A-R-B where A and B are peroxide groups having different decomposition rates...

Fig. 22. Ten polymer chain structures taken into account when modeling styrene polymerization...

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. 

Example 13.9 Illustrate temperature and molecular weight changes in a tubular reactor by constructing a simple model of styrene polymerization in a tube. 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

A number of the mechanistic features proposed by Williams and Hayes were incorporated into a theoretical model developed by Denaro et al. to explain the kinetics of styrene polymerization in a 2 MHz discharge. Initiation was proposed to proceed through the collision of electrons with the polymer film... 

A detailed model of an industrially important reaction, styrene polymerization, is given in... 

An example of a design and optimization study using a fairly sophisticated model for styrene polymerization is given in... 

Inaba et al. prepared a series of model styrene/butyl acrylate copolymer latexes with glass transition temperatures at room temperature. The functional monomer 2-(3-isopropenylphenyl)-2-methylethylisocyanate (TMI) was used as monomer/crosslinking agent for further film formation. A small amount of methacrylic acid was introduced in some formulations in order to enhance the crosslinking reaction. A redox initiation system was used to reduce premature crosslinking during the polymerization [82]. 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

The transfer constants in styrene polymerizations are higher than predicted from model studies using diarylmethylium cations at -70° C (cf., Chapter 2) [289], In the model studies, (p-MeOPh)PhCH+ adds to styrene approximately 300 times faster than it reacts with anisole, 107 times faster than with toluene, and 10 ° times faster than with benzene. However, if the slightly higher activation enthalpy of reaction with anisole compared to styrene (AAHt = 12 kJ/mol) [289,322] is taken into account, extrapolation of their relative rate constants from - 70 to 0° C still indicates that anisole is 30 times more reactive than styrene. This is in contrast to the similar reactivities (Cx 1 at 0° C) calculated from polymerization studies [317]. That is, the p-methoxydiphenylcarbenium ion is apparently... 

Much less information is available on transfer constants in polymerizations of other alkenes. It appears that the transfer constant to anisole in isobutene polymerizations is smaller than in styrene polymerizations, and much closer to values predicted by model studies Cx 5 x 10 3 [323], Calculated Cx values are relatively independent of temperature. 

Controlled polymerization requires that the initiation rate is at least comparable to that of propagation. Initiation in controlled/living carbocationic systems is usually carried out using models of growing species in their dormant state (e.g., the adducts of a monomer with protonic acids). This enables a similar set of equilibria to be established between carbocations and dormant species for initiation and for propagation. For example, 1-phenylethyl halides have similar reactivity as the macromolecular dormant species in styrene polymerizations, and I-alkoxyethyl derivatives are as reactive as the macromolecular species in the polymerization of vinyl ethers [Eq. (38)] ... 

There is further evidence that radical termination reactions are diffusion-controlled. For many polymers, the rate of polymerization shows a sudden increase when the fraction of polymer produced reaches values near 15 to 30 per cent. In the case of methyl methacrylate, Matheson et al. found that, at 30 C and 15 per cent conversion, kt has decreased 160 fold, while kp has not changed appreciably. Vaughan " has proposed a simple diffusion model which is in reasonable accord with the data on styrene polymerization at high conversions. 

Systems Where Radical Desorption is Negligible. Styrene and methyl methacrylate emulsion polymerization are examples of systems where radical desorption can be neglected. In Figures 4 and 5 are shown comparisons between experimental and theoretical conversion histories in methyl methacrylate and styrene polymerization. The solid curves represent the model, and it appears that there is excellent agreement between theory and experiment. The values of the rate constants used for the theoretical simulations are reported in previous publications (, 3). The dashed curves represent the corresponding theoretical curves in the calculation of which gel-effect has been neglected, that is, ktp is kept constant at a value for low viscosity solutions. It appears that neglecting gel-effect in the simulation of styrene... 

It must be stated that up to now models that can describe detailed styrene polymerization including all kinds of initiation step are rare. The work of Dhib ei al. [31] is so far the most comprehensive in this respect. It is a common practice tc fit the model to experimental data under different reaction process conditions. 

In this part of the chapter, a model of acid-mediated styrene polymerization will be developed and discussed. The purpose of the modeling work was twofold ... 

Since polystyrene is one of the oldest commercial polymers with over 9 million tonnes/yr of sales, there have been thousands of patents issued covering all aspects of its manufacture and property enhancement. The styrene monomer readily polymerizes to polystyrene either thermally or with free-radical initiators (see Chapter 6 on free-radical polymerization and Chapter 8 on nitroxide-mediated polymerization). Commercial processes for the manufacture of polystyrene are described in Chapter 3 while process modelling and optimization of styrene polymerization is examined in Chapter 5. Styrene also can be polymerized via anionic and Ziegler-Natta chemistries using organometallic initiators. Using free radical and anionic polymerization chemistries, the... 

---