Styrene polymerization kinetics

In order to develop a sound optimization policy, a good understanding of styrene polymerization kinetics is necessary. In the following section the general kinetic scheme of styrene homopolymerization is introduced. 

CUTTER and DREXLER Simulation of Styrene Polymerization Kinetics... 

In studies of the polymerization kinetics of triaUyl citrate [6299-73-6] the cyclization constant was found to be intermediate between that of diaUyl succinate and DAP (86). Copolymerization reactivity ratios with vinyl monomers have been reported (87). At 60°C with benzoyl peroxide as initiator, triaUyl citrate retards polymerization of styrene, acrylonitrile, vinyl choloride, and vinyl acetate. Properties of polyfunctional aUyl esters are given in Table 7 some of these esters have sharp odors and cause skin irritation. 

An alternating copolymer of a-methyl styrene and oxygen as an active polymer was recently reported [20]. When a-methyl styrene and AIBN are pressurized with O2, poly-a-methylstyreneperoxide is obtained. Polymerization kinetic studies have shown that the oligoperoxides mentioned above were as reactive as benzoyl peroxide, which is a commercial peroxidic initiator. Table 1 compares the overall rate constants of some oligoperoxides with that of benzoyl peroxide. 

It should be mentioned that the predicted curve at highest benzene level in Figure 13 agrees with classical kinetics (no diffusion-control). It is not clear therefore why measured data at even higher benzene concentrations do not agree with classical kinetics. There may be some subtle chemical interactions at these high solvent levels. Duerksen(lT) fomd similar effects with styrene polymerization in benzene and had to correct kp for solvent. 

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 formation of inter- and intrapolymer complexes has also been shown to affect the polymerization kinetics. For example, Ferguson and Shah (1968) investigated the influence of intrapolymer complexation on the kinetics of AA in the presence of copolymer matrices composed of either A-vinylpyrrolidone and acrylamide or A--vi nyl pyrrol idone and styrene. The polymerization rate reaches a maximum in the vicinity of AA to VP ratio equal to one for the VP/AAm matrix. This maximum in the polymerization rate is most pronounced in the presence of copolymer with the highest content of VP. When the hydrophilic acrylamide is replaced with the more hydrophobic styrene monomer in the copolymer matrix, the observed maximum in AA polymerization rate occurred at a lower than equimolar ratio of AA to VP. The hydrophilic groups of VP were interacting with the hydrophobic nucleus consisting of the styrene units in the VP/St copolymer, and were thus unable to participate in the formation of the complex unlike in the case of VP/AAm copolymer matrix. 

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. 

Polymerization inhibitors miscellaneous, 23 383 in styrene manufacture, 23 338 Polymerization initiators alkyllithiums as, 74 251 cerium application, 5 687 peroxydicarbonates as, 74 290 Polymerization kinetics, in PVC polymerization, 25 666-667 Polymerization mechanism, for low density polyethylene, 20 218 Polymerization methods, choice of,... 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

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... 

Model 4. As a result Lam et al. concluded that Model 3 best describes the plasma polymerization kinetics of styrene. 

Kinetics of the propagation of styrene polymerization initiated by SrS2 in THF without added SrB2... 

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

I. Kende and M. Azori Kinetics of inhibition of styrene polymerization by nitro compounds. IUPAC Symposium on Macromolecular Chemistry, Moscow 1960, Section II 31. 

Such hydrophilic macromonomers (DPn=7-9) were radically homopolymer-ized and copolymerized with styrene [78] using AIBN as an initiator at 60 °C in deuterated DMSO in order to follow the kinetics directly by NMR analysis. The macromonomer was found to be less reactive than styrene (rM=0.9 for the macromonomer and rs=1.3 for styrene). Polymerization led to amphiphilic graft copolymers with a polystyrene backbone and poly(vinyl alcohol) branches. The hydrophilic macromonomer was also used in emulsion polymerization and copolymerized onto seed polystyrene particles in order to incorporate it at the interface. 

Szwarc and coworkers (232) concluded from kinetic studies of sodium catalyzed polymerizations of vinyl pyridine and styrene that propagation involves two consecutive steps. In the first step, monomer complexes with the catalyst. In the rate-determining second step, the complex rearranges to yield product. For styrene polymerization, the steps were formulated as follows ... 

Table I. Kinetic Behavior of Styrene Polymerization Initiated by Radiation Under Wet (Radical) and Dry (Ionic) Conditions...

The polymerization kinetics is governed by the droplet size. Tang et al. found that the polymerization of styrene miniemulsions created by the microfluidizer was faster than that of miniemulsions created by the omnimixer [64]. This behavior can mainly be attributed to the different droplet size prior to polymerization. In the first case, the droplets are smaller than in the second case [65]. Fontenot and Schork observed similar behavior for MMA miniemulsions. With increasing shear and increasing concentration of surfactant, the polymerization rate increases [22]. This again can be explained by different sizes of the initial droplets. 

About thirty years ago, all cases of polymerization kinetics used to be solved as statinary reactions. Hayes and Pepper [27] were the first to call attention to the non-stationary character of ionic polymerizations. They noticed the premature decay of styrene polymerization initiated by H2S04 (see Fig. 8). This was a simple case of non-stationarity caused by the slow decay of rapidly generated active centres [27, 28]. They assumed that the polymerization proceeds according to a rather conventional scheme represented in simplified form (without transfer) by the reactions... 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

If initiation is faster or comparable to propagation and termination is negligible, kinetic plots are straight in semilogarithmic coordinates. Initiation is faster than propagation and not kinetically detectable in polymerizations of isobutene and styrene initiated by cumyl derivatives because the initiator is more easily ionized than the propagating species. However, if the initiator is less easily ionized than the propagating species as in a-methyl-styrene polymerizations initiated by cumyl derivatives, and in isobutene polymerizations initiated by /-butyl derivatives (cf., also Section III. A.5), then initiation may be incomplete and the overall polymerization rate will increase continuously. 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

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