Chain transfer agent concentration effect

A laboratory scale, continuous process for the polymerization of acrylamide in aqueous solution is described. The reaction conditions can be held constant within narrow limits and the effect of small changes in individual variables, such as temperature, initiator concentration, and chain transfer agent concentration, can be quantitatively ascertained. Some experimental results are presented showing the effect of these factors on the molecular weight of the polymer. The data are examined vis-a-vis some theoretically derived equations. 

Fig. 6-4 Effect of chain transfer agent concentration (5) on polystyrene degree of polymerization [8].

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

In mbber production, the thiol acts as a chain transfer agent, in which it functions as a hydrogen atom donor to one mbber chain, effectively finishing chain growth for that polymer chain. The sulfur-based radical then either terminates with another radical species or initiates another chain. The thiol is used up in this process. The length of the mbber polymer chain is a function of the thiol concentration. The higher the concentration, the shorter the mbber chain and the softer the mbber. An array of thiols have subsequendy been utilized in the production of many different polymers. Some of these apphcations are as foUow ... 

A number of different materials were used as chain transfer agents to control molecular weight. These results are shown in Table 6.1. The effect of varying concentration of t-butyl alcohol and reaction temperature is shown in Figure 6.1. The results are consistent with normal free radical polymerizations. Polymer output was characterized by inherent viscosity and ZST tests. 

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. 

DP is the average number of monomer (or repeat) units per polymer chain and so is directly related to molecular weight (or viscosity). This relationship shows that we must have control over variables that have a significant effect on propagation, chain transfer, and termination to achieve the desired polymer properties. What are these variables They are the same as those we have discussed throughout this chapter temperature, reactant monomer concentrations, concentrations of chain transfer agents or other impurities that affect polymerization, initiator or catalyst concentration, residence time, etc. 

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. 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

None of the above reactions terminates the kinetic chain. All are treated as chain transfer reactions since there is reinitiation of new propagating chains. The relative extents of the various termination reactions depend on the monomer, identity and concentrations of the initiator components, temperature, and other reaction conditions. There are considerable differences in the efficiencies of chain transfer to different Group I-III metal components for example, diethylzinc is much more effective in chain transfer compared to triethylaluminum. Molecular hydrogen is a highly effective chain-transfer agent and is commonly used for molecular weight control in the industrial production of polypropylene. 

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