Chain transfer to solvents

Chain transfer to solvent is the best investigated reaction among all chain transfer reactions. The first evidence of chain transfer to solvent based on the formation of low molecular weight polymers and on the direct detection of solvent fragments in macromolecules was obtained more than half a century ago for various combinations of monomers and solvents. These include the polymerization of butadiene in toluene [9] or styrene in liquid ammonia [10]. Later on, chain transfer to aromatic solvents was reported for many other systems. Therefore, the most important is not a qualitative result (whether chain transfer to solvent takes place or not) but rather quantitative one (to what extent it goes). That is why this reaction deserves to be considered in detail. 

The effects of chain transfer to solvent on molecular masses and MWDs of polymers formed in nonterminating polymerizations have been theoretically studied in a number of papers [11-18]. A detailed review is given in Ref. [19]. Most studies have examined common types of polymerization, that is, homopolymerization by monofunctional initiators (of the type RMt with a single active center). The calculations have been based on the kinetic scheme 3.1 that was proposed initially by Higginson and Wooding [11]. 

In this scheme, S is the solvent, S is an intermediate active species arising due to chain transfer, and R 1) and P(l) are the growing and dead macromolecules, respectively, containing I monomer units. Later on, a similar scheme was independently proposed in Refs [12, 15] for spontaneous transfer 

It should be said that in spite of that chemistry of chain transfer to solvent and spontaneous transfer is different, the two mechanisms are identical both in mathematics and in results at ksp = k S. 

Instantaneous primary initiation was assumed in all papers. In combination with the absence of chain termination, this means that the total concentration of active species R + S is constant from the very beginning of polymerization and is equal to the concentration of initiator Jq. As was discussed previously, this is satisfied for 

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Figure 6.8 Effect of chain transfer to solvent according to Eq. (6.89) for polystyrene at 100°C. Solvents used were ethyl benzene ( ), isopropylbenzene (o), toluene (- ), and benzene (°). [Data from R. A. Gregg and F. R. Mayo, Discuss. Faraday Soc. 2 328 (1947).]...

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

Chain-transfer constants, 25 571t Chain-transfer rate constants, 19 832 Chain-transfer rates, 19 839 Chain transfer to solvent (CTS), 23 385 Chalcanthite, 7 772 Chalcogenide glasses, 12 575, 584 semiconductivity in, 12 587 Chalcogenides acidic, 12 190-191 gallium, 12 359 in photocatalysis, 19 75 plutonium, 19 691 zirconium, 26 641... 

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) ... 

B. When there is no termination, but there is chain transfer to solvent (Equation 4-48). 

Similarly, for chain-breaking predominantly by chain transfer to solvent or an agent, the chain-breaking rate and radical chain length are... 

In a companion paper Price and Akkapeddi [22] report the kinetics of base initiated polymerization of epoxides in DMSO and hexamethyl-phosphoramide (HMPT). The initiator is potassium t-butoxide. Second order rate coefficients for (R,S)—PO were about double those for (+)—(R) or (—)—(S) monomer. They conclude that the steric factor favouring alternation of isotactic and syndiotactic placement of the t-BuEO also influences PO. Chain transfer to solvent (DMSO) was also studied. For PO polymerization in DMSO they obtain k = 1.5 x 10 exp(—17,200/RT). However, due to some erratic results they are not very confident about the accuracy. In HMPT rates are about three fold faster than in DMSO k = 7.3 X 10 exp(—16,300/RT). Three other epoxides were also studied in HMPT EO, k = 2.75 x 10 exp(-13,300/RT) t-BuEO, fe = 2.0 x 10 exp(-17,100/RT) phenylglycidyl ether (PGE), fc = 5.4 x 10" ... 

Solution Easier to control heat and mass transport Wide range of accessible molecular weights Easier to transport reagents and products Needs agitation Solvent removal and recycling Chain transfer to solvent may lead to undesirable effects Inefficient heat and mass transport at high conversions and/or concentrations... 

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... 

Modify the Mayo equation (6.148) to take into account the effect of degrada-tive chain transfer on the number-average degree of polymerization. For simplicity, assume that only the chain transfer to solvent is degradative (i.e., the new radical formed does not initiate polymerization). 

Also because of the unusual nature of this solvent, chain transfer to solvent is important in this system (see later). 

Extensive chain transfer to solvent occurs by the reaction... 

If the chain transfer to solvent occurs through the reaction shown above, the regenerated active chain should contain a CHD2 group at the beginning of the chain ... 

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. 

Another possible explanation for the solvent effect might be based on the difference in the chain transfer rate from the propagating radical to solvent and on the reinitiation rate by the resulting solvent radical. Let us discuss the effect of the solvent transfer reaction on kp under the following three aspects (1) likelihood of chain transfer to solvent, (2) stability of the resulting solvent radical, (3) polarity of the radical. 

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