Polymer transfer reactions

Transfer to polymer may occur by addition of the growing radical end to a double bond of the polymer chain, or it may occur by II atom abstraction from an active H atom of the chain. H atoms in the a position to a double bond (C=C or C=0) or an ether linkage are easily abstractable and thus lend themselves to polymer transfer reactions. The transfer reaction to polymer may be intramolecular or it may be intermolecular. Both have been demonstrated. In the case of intermolecular transfer the demonstration has been made of the productioTi of graft polymers. The method is to polymerize a monomer in the presence of inert polymer of a different composition. " The final product will contain the inert polymer with the new polymer grafted onto it. 

In a zero-one system in which a radical has just entered a polymer particle containing one polymer radical, and is terminated instantaneously, the number fraction distribution in the absence of a polymer transfer reaction is given by ... 

It is evident from Eq. 85 that the condition =l/(kp[M]pFe) < Cm is needed to apply the CLD method to emulsion polymerization. Note that the radical entry rate may be increased through the radical exit. Even when these conditions are satisfied, a higher polymer concentration than for the corresponding bulk polymerization may result in more occurrences of the polymer transfer reaction. 

The effect of the polymer transfer reaction on the apphcability of the CLD method can be examined by applying the MC simulation method [298]. Figure 15 shows the MC simulation results for the condition Cm=5xl0", s= l/(kp[M]pFe) =5x10 , and for the polymer transfer constant Cfp=5xl0", when the total number of monomeric imits bound into polymer chains in a polymer particle np=lxl0 . 

Fig. 15 Monte Carlo simulation results for emulsion polymerization that involves polymer transfer reactions, under the conditions Cm=5xl0" =5x10" and Cfp=5xl0" without radical desorption...

Figure 16a shows the development of average branching density in emulsion polymerization and in a corresponding bulk polymerization, both of which involve the polymer transfer reactions [301]. 

The limited space effects present rather difficult problems to account for in the conventional deterministic approach. The problem can be highlighted by considering the following hypothetical example. Suppose that there are three polymer chains and three radicals (R ), and that polymer transfer reactions are about to occur [269]. Within these polymer chains, suppose that one chain is much larger than the other two, as shown in Fig. 17. 

Fig. 17 Hypothetical example that illustrates the differences between the polymer transfer reactions that occur in bulk polymerization a and in emulsion polymerization b...

On the other hand, suppose that each of these polymer molecules is isolated into different particles, and that each particle contains one radical, as shown in Fig. 17b. If the radical causes the polymer transfer reaction, the partner must be the polymer molecule that happens to exist in the same particle (so it cannot partner a larger polymer molecule that exists in a different polymer particle). As a consequence, the expected size of the polymer molecule attacked by a radical is smaller for emulsion systems than for the homogeneous model shown in Fig. 17a. 

Radical transfer. Free-radical transfer reactions with monomer, polymer, and added transfer agents can take place in the particles. The polymer transfer reaction will be more important in emulsion polymerization because of the high concentration of polymer in the particles. 

In a study of chain-transfer constants of the monomeric vinyl acetate it was found that the formation of nonhydrolyzable branches is virtually negligible while hydrolyzable branches are formed at position 1 of Structure 1 by a terminal double-bond reaction rather than by a polymer-transfer reaction. The long nonhydrolyzable branches in poly(vinyl alcohol) are, presumably formed almost exclusively by a polymer transfer mechanism [35]. 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

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. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

The increase in the temperature reduces the viscosity of the polymerization medium which increases the termination reactions. This is attributed to an increase in chain transfer reactions higher than that of propagation reactions [16,51]. Consequently, the weight-average molecular weight of the formed polymer decreases. 

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination. Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination processes and <30% of the chain ends carry initiator residues [59]. 

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