Chain transfer in free radical polymerization

Fig. 57. Chain transfer in free radical polymerization with a side group of the-dead polymer. Note After termination of the radical one side chain and a free linear chain are obtained.

Catalytic Chain Transfer in Free-Radical Polymerizations... 

Gridnev Alexei A., and Ittel Steven D. Catalytic chain transfer in free-radical polymerizations. Chem. Rev. 101 no. 12 (2001) 3611-3659. 

FIGURE 12.2 The effect of the chain transfer concentration, CTA, on the polyacrylamide mass M iJ) data for reactions in which different CTA were used (top) molecular weight distribution (MWD) from SEC experiments on reaction endproducts (bottom). From Grassl B, Alb AM, Reed WF. Online monitoring of chain transfer in free radical polymerization. Macromol Chem Phys 2001 202 2518-2524. Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission. 

GrassI B, Alb AM, Reed WF. Online monitoring of chain transfer in free radical polymerization. Macromol Chem Phys 2001 202 2518-2524. 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

The control of molecular weight and end group functionality of pol3oners by chain transfer in free radical polymerization can be achieved most accurately and conveniently when the chain transfer constant has a value in the range 0.5 - 2 1.0... 

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Alkyl Co oxime complexes have been used as chain transfer catalysts in free radical polymerizations.866,867 Regioselective hydronitrosation of styrene (with NO in DMF) to PhCMe=NOH is catalyzed by Co(dmg)2(py)Cl in 83% yield.868,869 Catalytic amounts of the trivalent Co(dmg2tn)I2 (192) (X = I) generate alkyl radicals from their corresponding bromides under mild reaction conditions, allowing the selective preparation of either saturated or unsaturated radical cyclization products.870... 

The presence of lignin, resins or other extractives in the fibers may interfere with the initiation or polymerization reactions, e.g. by termination or chain transfer of free radical reactions from phenolic groups. In some cases, lignin has no adverse effect and may even be grafted . 

Although this mechanism is an oversimplification, it does give the basic idea. Chain termination is more complicated than in free radical polymerization. Coupling and disproportionation are not possible since two negative ions cannot easily come together. Termination may result from a proton transfer from a solvent or weak acid, such as water, sometimes present in just trace amounts. 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

Dithiocarbonylated ethyl xanthates, (IV), dithiophosphorylates, (V), and azo derivatives, (VI), prepared by Wilczewska [2], Destarac [3], and Charmot [4], respectively, and were effective as chain transfer agents in free radical polymerization reactions. 

Polyfunctional dithiocarbamate derivatives, (VI) and (VII), were prepared by charmot [4] and used as chain transfer reagents in free radical polymerization reactions. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

Chain transfer is of particular interest in free-radical polymerization, where it affects not only the polymerization rate, but also the molecular weight of the product (see Section 10.3). 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

The number-average degree of polymerization can be obtained from the rates of propagation (eqn 10.65) and chain breaking (sum of eqns 10.66 and 10.67) as in free-radical polymerization with termination by chain transfer to a transfer agent (see eqn 10.42) ... 

With regard to kinetics of chain breaking, the various chain-transfer reactions are analogous to those in free-radical polymerization (see Section 10.3.1) and need not be described again in detail. In cationic polymerization, the most common chain transfer is to monomer and leaves the polymer with a double bond while generating a monomeric chain carrier of the same structure as the original one [93]. 

The distribution is as in free-radical polymerization with termination by disproportionation or terminating chain transfer (eqn 10.45 in Section 10.3.4) and, with increase of the progression factor as conversion increases, in step-growth polymerization of bifimctional monomers (eqn 10.19 in Section 10.2.3). According to eqn 10.81, the progression factor is... 

Reaction (6-14a) is the initiation step, while reactions (6-14b) and (6-l4c) are atom abstraction propagation reactions. Atom abstraction reactions in free-radical polymerizations are called chain transfer reactions. They are discussed in some detail in Section 6.8. 

The following sections review the magnitudes of the various chain transfer constants, methods for measuring these parameters, and their significance in free-radical polymerizations. 

Cross-linking can occur in free-radical polymerizations because of chain transfer to polymer (Section 6.8.4) or monomer (Section 6.8.2), as well as the presence of multifunctional monomers. When it is desireable to retard or suppress crosslinking, chain transfer agents are added to the polymerization mixture. Ek]uation (7-67) shows why this is helpful. 

The above kinetic expressions illustrate some basic differences between cationic and free radical processes. In the cationic polymerization, the propagation rate is of first order with respect to the initiator concentration, whereas in free radical polymerization it is proportional to the square root of initmtor concentration (Eq. [34]). Furthermore, the molecular weight (or DP) of the polymer synthesized by the cationic process is independent of the concentration of the initiator, regardless of how termination takes place, unlike free radical polymerization where DP is inversely proportional to [I] in the absence of chain transfer (Eq. [35]). 

The basic steps in free-radical polymerization are initiation, propagation, chain transfer, and termination. 

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