Polymerization kinetics polymer molecular weights

Since the publication by the discoverers (3) of chromium oxide catalysts a considerable number of papers devoted to this subject have appeared. Most of them (20-72) deal either with the study of the chromium species on the catalyst surface or with the problem of which of this species is responsible for polymerization. Fewer results have been published on the study of processes determining the polymer molecular weight (78-77) and kinetics of polymerization (78-99). A few papers describe nascent morphology of the polymer formed (100-103). 

Figure 1 Is a flow sheet showing some significant aspects of the Iterative analysis. The first step In the program Is to Input data for about 50 physical, chemical and kinetic properties of the reactants. Each loop of this analysis Is conducted at a specified solution temperature T K. Some of the variables computed In each loop are the monomer conversion, polymer concentration, monomer and polymer volume fractions, effective polymer molecular weight, cumulative number average molecular weight, cumulative weight average molecular weight, solution viscosity, polymerization rate, ratio of polymerization rates between the current and previous steps, the total pressure and the partial pressures of the monomer, the solvent, and the nitrogen.

In this chapter we have discussed methods of polymerization, the resulting molecular weight distribution, and the interplay between the chemistry of the monomer and the type of polymer that will be produced. We also briefly introduced some of the commercial methods of producing polymers and the role that the type of polymerization has on the choices made in commercial applications. In the following chapters we will build on this framework to explore the role of physical chemical processes, such as the thermodynamic and kinetic processes involved in polymer manufacture. We will also gain an understanding of structural properties of polymers and the means to explore these properties. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

Using the same toluene-benzoyl peroxide system Nakatsuka (105) measured polymerization rate and molecular weight as functions of temperature (40° and 58°) and of the concentration of.three retarders p-nitrophenol, 2,4-dinitrophenol and picric acid. Results were consistent with a kinetic scheme postulating (among other things) bimolecular initiation involving peroxide and monomer and spontaneous unimolecular termination of growing polymer chains. 

One way to distinguish between the two possibilities is to study the isotope effect on the kinetics of vinyl acetate polymerization and on the polymer molecular weight. The deuterium isotope eftect has been ascribed to the difference in the zero point energies of the stretching vibrations of the C-H and C-D bond (11). The rate of a reaction in which deuterium is transferred is slower than that of the corresponding reaction for hydrogen, since the C-D bond has a lower zero point energy. 

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. 

Johnston and Pepper conclude that phosphines initiate near ideal living polymerizations. However, when the authors turned to amine initiators they found that, although macrozwitterions were formed, the polymerization kinetics were very different. At comparable reagent concentrations room temperature rates were at least one thousand times slower, but paradoxically increased as temperature was reduced. Arrhenius plots indicated that by -100°C amine and phosphine polymerization rates would be equal. Polymer molecular weights were much higher than would have been expected had initiation been complete, and were uninfluenced by polymerization conditions. It is believed that molecular weights are determined by traces of weak acid transfer agents present in the monomer. 

It seems that formation of dimers and trimers from soybean oil is the slow step of polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean oil molecule itself, a large molecule with low activity, due to mid chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. We propose further investigation on the possible combination of two polymerization mechanisms for the polymerization of soybean oil in SCCO2. Kinetic study of soybean oil polymerization currently is carried out in our laboratory. 

Macroradicals are isolated I rom each other in emulsion polymerizations because they grow in particles, which can accommodate either one or zero radicals at any instant. The distinguishing feature of the kinetics of such reactions is that the polymerization rate and polymer molecular weight are proportional to the number of particles, as distinct from free-radical polymerizations in bulk, solution or suspension. An interesting consequence is that the rate of polymerization will be inversely proportional to the particle size. This holds affixed final polymer content, which is the way such reactions are usually performed. Polymer molecular weight may also be affected by particle size under the same conditions. 

There is little information on the mechanism or kinetics of formation of the amorphous polymer produced concurrently with the crystalline polymer. In general, polymerization rates and molecular weights are lower, but there is no clear relationship between rate of formation of amorphous polymer and catalyst composition. Catalysts from TiCl4, VOCI3 or VCI4 which tend to produce colloidally dispersed or appreciably soluble catalysts give higher amounts of amorphous polymer and in some instances little or no crystalline material is produced. There is a tendency with most catalysts for the amount of amorphous polymer to increase with increase in metal—alkyl concentration. 

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