Chain transfer with solvents

When vinyl and aHyl monomers undergo chain transfer with solvents, so-caHed telomers may form, generaHy of rather low molecular weight (1). 

The gegen ion, K+, has been dismissed from the scheme in consideration of the comparatively high dielectric constant of the liquid ammonia medium.) The associated kinetic equations are equivalent to Eqs. (44) and (45). It will be observed that the termination step (51) is essentially a chain transfer with solvent. A similar process has... 

When chain transfer with solvent occurs, what effect does this have on the average DP ... 

Poly(styrene)s containing acylperoxide groups are thus obtained by selective photolysis of the azo moieties at 350 or 371 nm. These prepolymers are successively used as macronitiators for the free radical polymerization of vinyl chloride at 70 °C. Styrene/vinyl chloride block copolymers are thus produced [55] by the above two-step route, although relevant amounts (50-60%) of poly(styrene) and poly(vinyl chloride), due to both low peroxide content ( 0.6 groups per macromolecule of polystyrene) and chain transfer with solvent and monomer, are also pre t. 

Termination by chain transfer with solvent or epoxy resin... 

Although solution polymerisation is able to control and retards auto acceleration, the solvent is rarely intent and the product of lower Molecular weight gets obtained by chain transfer with the solvent. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

Solution Polymerization. By adding a solvent to the monomer-polymer mixtnre, heat removal can be improved dramatically over bnlk reactions. The solvent mnst be removed after the polymerization is completed, however, which leads to a primary disadvantage of solution polymerization. Another problem associated with radical chain polymerizations carried out in solution is associated with chain transfer to the solvent. As we saw in Section 3.3.1.2, chain transfer can significantly affect the molecular weight of the final polymer. This is particnlarly trne in solntion polymerization, where there are many solvent molecules present. In fact, chain transfer to solvent often dominates over chain transfer to other types of molecnles, so that Eq. (3.79) reduces to... 

In contrast to chain transfer to solvent which would be prevalent from the initial stages of a polymerization due to the solvent s high concentration, chain transfer to polymer often does not compete noticeably with propagation until the end of the polymerization when monomer is depleted. In addition, chain transfer and termination reactions generally have higher activation energies than propagation, and therefore can be... 

If a solvent that is able to release a proton is used, however, it can react with the active site. Ammonia is an example of such a protic solvent and the reaction results in the formation of a negatively charged NH2 ion, which can initiate the polymerization of a new chain. In other words, we have chain transfer to solvent (Figure 3-32). What do you think would happen if we used an inert or non-protic solvent (one that does not readily release a proton) ... 

The ratio A,j/Ap is the chain-transfer coefficient, Ca. Similar coefficients, Cg, Cm and Q, may be defined for chain transfer to solvent, monomer and initiator, respectively. In commercial systems, the value of Ca is chosen to be >1 so that only small concentrations of XA are required in order to have a marked effect on the value of DP. For example, if 0.1% of XA is added with respect to monomer and DPq is 1000, then, if Ca is 1, the DP is reduced to 500. Also, since the agent is consumed slowly and there is no effect on the rate of... 

It was found by Burnett and Melville36 in 1947 that the radical polymerization of vinyl acetate was retarded in aromatic solvents. This retardation effect was confirmed by several researchers37-42. It is characterized by three features all of which cannot be simultaneously explained by the conventional kinetic scheme involving degradative chain transfer to solvent. (1) The rate of polymerization is markedly reduced in comparison with that in many aliphatic solvents. (2) The order with respect to initiator remains close to one-half over a wide range of initiator concentration. 

Also, Priddy and Pirc used a chain transfer solvent (ethylbenzene) in a CSTR operating at > 99% monomer conversion, and at high polymer solids (40-50 w/w). Under these conditions, they found that chain transfer to solvent (CTS) was extremely high (2) since the high monomer conversions achieved under steady state (SS) operation resulted in a large ratio (typically, 500 1) of solvent to monomer. Gatske [75] has shown that the CTS increases exponentially with conversion as shown in Fig. 6. 

ATRP reactions are very versatile with a high tolerance toward the presence of functional groups, such as the allyl, amino, epoxy, hydroxy, and vinyl groups, on both monomer and initiator. The reactions can be carried out either in bulk or in solution and also in heterogeneous systems. If solvents are used, chain transfer to solvent should be low, but the effect on the catalyst must be considraed. 

Polymerization in solution follows conventional kinetics except for certain solvent-specific side reactions. At monomer concentrations above 2-2.5 M, the reaction order with respect to monomer and initiator has been found to be 1.0 and 0.5, respectively [35]. In DMF, however, a monomer reaction order greater than expected was explained by chain transfer to solvent followed by slow reinitiating by the DMF radical [43]. At higher monomer concentrations, however, the monomer has the effect of adding a nonsolvent to the reaction mixture. Under these conditions, the reaction orders with respect to initiator and monomer can deviate from the expected values. Vidotto et al. [35] found that the reaction became heterogeneous at... 

In this process the choice of the solvent s chain transfer constant is very important because this influences the molecular weight to a considerable extent. Because of chain transfer with the solvent and because of the lower monomer concentration, the molecular weight of polymers prepared by solution polymerization is usually lower than that of the corresponding bulk polymers. 

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