Chain-transfer

It is often observed that the measured molecular weight of a polymer product made by free-radical chain polymerization is lower than the molecular weights predicted from Eq. (6.102) for termination by either coupling [Eq. (6.103)] or disproportionation [Eq. (6.104)]. Such an effect, when 

The new radical T , which results from chain transfer, can reinitiate polymerization by the reaction 

While chain transfer to monomer is negligible for most monomers, it may, however, be sig-ni cant for some monomers, such as vinyl acetate, vinyl chloride, and a-methyl substituted vinyl monomers, e.g., propylene and methyl methacrylate (MMA). For MMA the chain transfer proceeds by the reaction  

Many peroxide initiators have signi cant chain transfer reactions. Dialkyl and diacyl peroxides undergo chain transfer due to breakage of the 0-0 bond, e.g.. 

In some polymerizations, the solvent itself may act as the chain transfer agent. For example, during vinyl polymerization in solvent CCU chain transfer takes place by the reaction 

Branching is a special case of a process called chain transfer that is operational in free-radical polymerization. Chain transfer simply means the transfer of the radical from the growing polymer chain to another speeies. Effectively chain transfer eurtails polymer growth. For example, ehlorin-ated solvents are efiBcient ehain transfer agents (see Eq. 2.12). 

In many instances molecules like dihydrogen are deliberately introduced in the reaetion medium to obtain polymers of desired molecular weights (see Eq. 2.13). 

Besides reacting with ethylene or a comonomer, a growing polymer chain may also react with transfer agents (modifiers). Modifiers are chemical substances, which easily transfer an H-atom to the free radical end ofagrowing polymer chain. Bythis reaction the modifier becomes a radical itself This radical can start a new polymer chain, while the growth of the polymer chain to which the H-atom is transferred is stopped. 

The free radical on the growing polymer chain is not eliminated from the reaction but just transferred to a new molecule. 

The effectiveness of a modifier depends on its chemical structure, concentration, temperature, and pressure. A concentration-independent measure for its effectiveness is the chain transfer constant, defined as the ratio of kinetic coefficients for the transfer reaction to this substance and radical chain propagation reaction. Usually the effectiveness of chain transfer agents is increased with rising temperature and reduced pressure. The chain transfer constant of modifiers falls from aldehydes, which are more effective than ketones or esters, to hydrocarbons. Unsaturated hydrocarbons typically have higher transfer constants than saturated hydrocarbons and a strong effect on polymer density must be considered because of the ability to copolymerize that give a higher frequency of short-chain branches in the polymer. 

Even the polymer itself can react as a chain transfer agent. In the latter case, one has to distinguish between intramolecular and intermolecular chain transfer. 

Intramolecular chain transfer leads to short-chain branching with mainly butyl groups at the branches  

The chain length in free radical polymerisations is usually lower than would be expected from the mechanism of termination. The reason for this discrepancy is that the growing polymer chain can transfer the radical to other species, leading to termination of one chain, and thus generating a new radical that will react further. The following transfer mechanisms may occur  

Transfer to monomer This type of transfer involves abstraction of hydrogen radicals from the monomer and is negligible in the copolymerisation of MMA and ethylene glycol dimethacrylate (EDMA) [15]. 

Transfer to initiator This type of transfer involves transfer of radicals to the initiator. Organic peroxides are particularly susceptible to this kind of transfer whereas azo-initiators are not as reactive in this respect [10]. 

Transfer to solvent A number of solvents are reactive towards free radicals. Halogenated solvents belong to this group and CCI4 is particularly reactive. They react with the growing polymer chain by abstraction of a chlorine radical and the resulting solvent radical can then initiate a new chain or terminate a growing chain [10]. 

As anticipated for a chemically controlled reaction, CO2 has only a minor influence on the rate coefficient for chain-transfer to DDM and to the MMA tri-mer in MMA and styrene homo- and copolymerizations. Going from bulk polymerization to solution polymerization with 40 wt% CO2 present enhances Cx by about 10%, but leaves the associated activation volume, AV (Cx), unchanged [48]. As pointed out in the previous section, the observed lowering of kp,app upon increasing CO2 content is no true kinetic effect, and the propagation rate coefficient kp,kin most likely remains unaffected by the presence of CO2. Thus, ktr for DDM and for the MMA trimer should not be significantly varied by the presence of CO2. 

In addition to the data summarized above, the CO2 influence on Cx for CoPhBF in MMA polymerizations has recently been studied by Davis and coworkers [57] at, however, quite different reaction conditions, i.e. 50°C, 150 MPa, with a CO2 content of 80%. An enhancement of Cx by approximately one order of magnitude compared to polymerizations in solution in toluene was reported. 

40 wt% CO2. This value is close to the activation volume of kj observed for MMA bulk polymerizations [58, 59]. 

The results in this section indicate that chemically controlled transfer reactions, such as with DDM or with the MMA trimer, are adequately represented by the rate coefficients reported for bulk polymerization. Catalytic chain transfer processes, as with CoPhBF, are speeded up by the presence of CO2. 

Propagation rate is moderately influenced by CO2. Reductions by up to about 40% compared to the propagation rate of the respective bulk polymerizations have been found. Such effects have been considered to be due to local monomer concentration at the free-radical site being different from the overall monomer concentration determined by analysis in the case of solution polymerizations with significant amounts of CO2. Studies into reactivity ratios provide no evi- 

It is important to note that the free radical is not destroyed in the reaction it is merely transferred, and if the new species is sufficiently active, another chain will emanate from the new center. This is known as chain transfer and is a reaction resulting in the exchange of an active center between molecules during a bimolecular collision. Several types of chain transfer have been identified. 

Transfer to monomer. The two important reactions in this group both involve hydrogen abstraction. Two competitive alternatives exist in the first group 

If the radical formed in reaction (II) is virtually unstabilized by resonance, then the reaction with the parent unreactive monomer may produce little chain propagation due to the tendency for stabilization to occur by removal of hydrogen from the monomer. This leads to rapid chain termination and is known as degradative transfer. Allylic monomers are particularly prone to this type of reaction 

A second group of transfer reactions can occur by hydrogen abstraction from the pendant group. The relevant kinetic expression is 

Transfer to initiator. Organic peroxides, whrai used as initiators, are particulaily susceptible to chain transfer. Azo initiators are not vulnerable in this respect and are more useful when a kinetic analysis is required. For peroxides 

A remarkable amount of experimental data show that the Mg/Ti catalysts characteristically provide polymers with lower molecular weight as compared to non-supported catalysts38,91 126,121 128). This is true for ethylene and propylene polymerization and, in principle, may be the result of a considerable increase of the constants for the chain transfer rates with monomer k , hydrogen kf and organoaluminum k 1, although experimental data are rather scarce (see Table 6). Spontaneous P-elimination k,Sp is not considered important at normal polymerization temperatures 126,129,130,131). 

In the absence of hydrogen and under normal polymerization conditions and at normal ethylene concentrations, it was found that with Mg/Ti catalysts the chain transfer to monomer predominates 128). This has recently been confirmed by Kashiwa 38) in the case of propylene polymerization with the TiCl4/EB/MgCl2 -AlEt3/EB catalytic system. As a consequence of the increased chain transfer rate, mean lifetimes of growing polymer chains produced with Mg/Ti are considerably shorter than those observed with unsupported catalysts. Kashiwa, for example, quoted a value of 2-3 sec for the lifetime of polypropylene growing chains obtained at 50 °C with the above mentioned catalysts, as compared with 4-10 min for those obtained at 60-70 °C with conventional catalysts. 

from the above results and from those concerning the propagation rate constant values, it may be concluded that the support becomes part of the catalyst system. The presence of Mg ions in the second coordination sphere of Ti ions results in a definite modification of the active center reactivity. Unfortunately, no comparable 

Q = Mw/Mn = Polymer polydispersity index ROT Quenching with tritium labeled alcohol 1 1/mol sec 

Friedel-Crafts (or Lewis) acid in this work TiCU electron pair donor, hereafter electron donor in this work triethyl amine 

Chain transfer to monomer can be either monomolecular and zero order in monomer 

HM= = dead polymer formed by all types of chain transfer HM X = chlorine-terminated polymer (from HX) 

Similar treatment of (10.27), with the assumption that is independent of temperature, results in 

Example 3. For a typical free-radical addition reaction, Ep — E,/2 5 kcal/mol and Ej 30 kcal/mol. Estimate the changes in rp and x for a typical homogeneous, ftee-radical addition polymerization on going from 60 to 70°C. 

Solution. The effective activation energy for polymerization is -1-20 kcal/moL Inserting this into (10.30) along with Ti — 273 -l- 60 = 333 K, — 273 + 70 = 343 K, and R = 1.99 cal/raol-K gives 

In practice, another t3 5e of reaction sometimes occurs in free-radical addition polymerizations. These chain-tranffer reactions kill a growing chain radical and start a new one in its place 

chain transfer results in shorter chains, and if reactions 10.32 are not too frequent compared to the propagation reaction and don t have very low rate 

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Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

The molecules used in the study described in Fig. 2.15 were model compounds characterized by a high degree of uniformity. When branching is encountered, it is generally in a far less uniform way. As a matter of fact, traces of impurities or random chain transfer during polymer preparation may result in a small amount of unsuspected branching in samples of ostensibly linear molecules. Such adventitious branched molecules can have an effect on viscosity which far exceeds their numerical abundance. It is quite possible that anomalous experimental results may be due to such effects. 

Elsewhere in this chapter we shall see that other reactions-notably, chain transfer and chain inhibition-also need to be considered to give a more fully developed picture of chain-growth polymerization, but we shall omit these for the time being. Much of the argumentation of this chapter is based on the kinetics of these three mechanistic steps. We shall describe the rates of the three general kinds of reactions by the notation Rj, Rp, and R for initiation, propagation, and termination, respectively. 

In the next three sections we consider initiation, termination, and propagation steps in the free-radical mechanism for addition polymerization. One should bear in mind that two additional steps, inhibition and chain transfer, are being ignored at this point. We shall take up these latter topics in Sec. 6.8. 

Since the radical lifetime provides the final piece of information needed to independently evaluate the three primary kinetic constants-remember, we are still neglecting chain transfer-the next order of business is a consideration of the measurement of r. 

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. 

It is apparent from these reactions how chain transfer lowers the molecular weight of a chain-growth polymer. The effect of chain transfer on the rate of polymerization depends on the rate at which the new radicals reinitiate polymerization ... 

If the rate constant kj is comparable to kp, the substitution of a polymer radical with a new radical has little or no effect on the rate of polymerization. If kj hp, the rate of polymerization will be decreased by chain transfer. 

The kinetic chain length has a slightly different definition in the presence of chain transfer. Instead of being simply the ratio Rp/R, it is redefined to be the rate of propagation relative to the rates of all other steps that compete with propagation specifically, termination and transfer (subscript tr) ... 

It is apparent from this expression that the larger the sum of chain transfer term becomes, the smaller will be... 

The magnitude of the individual terms in the summation depends on both th( specific chain transfer constants and the concentrations of the reactants undei consideration. The former are characteristics of the system and hence quantitie over which we have little control the latter can often be adjusted to study particular effect. For example, chain transfer constants are generally obtainec under conditions of low conversion to polymer where the concentration o polymer is low enough to ignore the transfer to polymer. We shall return belov to the case of high conversions where this is not true. 

If an experimental system is investigated in which only one molecule ii significantly involved in transfer, then the chain transfer constant to tha... 

This suggests that polymerizations should be conducted at different ratios of [SX]/[M] and the molecular weight measured for each. Equation (6.89) shows that a plot of l/E j. versus [SX]/[M] should be a straight line of slope sx Figure 6.8 shows this type of plot for the polymerization of styrene at 100°C in the presence of four different solvents. The fact that all show a common intercept as required by Eq. (6.89) shows that the rate of initiation is unaffected by the nature of the solvent. The following example examines chain transfer constants evaluated in this situation. 

Estimate the chain transfer constants for styrene to isopropylbenzene, ethylbenzene, toluene, and benzene from the data presented in Fig. 6.8. Comment... 

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 to initiator or monomer cannot always be ignored. It may be possible, however, to evaluate the transfer constants to these substances by investigating a polymerization without added solvent or in the presence of a solvent for which Cgj is known to be negligibly small. In this case the transfer constants Cjj and Cj determined from experiments in which (via... 

Fairly extensive tables of chain transfer constants have been assembled on the basis of investigations of this sort. For example, the values of acryla-... 

As noted above, chain transfer to polymer does not interfere with the determination of other transfer constants, since the latter are evaluated at low conversions. In polymer synthesis, however, high conversions are desirable and extensive chain transfer can have a dramatic effect on the properties of the product. This comes about since chain transfer to polymer introduces branching into the product ... 

A moment s reflection reveals that the effect on v of transfer to polymer is different from the effects discussed above inasmuch as the overall degree of polymerization is not decreased by such transfers. Although transfer to polymer is shown in one version of Eq. (6.84), the present discussion suggests that this particular transfer is not pertinent to the effect described. Investigation of chain transfer to polymer is best handled by examining the extent of branching in the product. We shall not pursue the matter of evaluating the transfer constants, but shall consider instead two specific examples of transfer to polymer. 

We conclude this section by noting an extreme case of chain transfer, a reaction which produces radicals of such low reactivity that polymerization is effectively suppressed. Reagents that accomplish this are added to commercial monomers to prevent their premature polymerization during storage. These substances are called either retarders or inhibitors, depending on the degree of protection they afford. Such chemicals must be removed from monomers prior to use, and failure to achieve complete purification can considerably affect the polymerization reaction. 

Inhibitors are characterized by inhibition constants which are defined as the ratio of the rate constant for transfer to inhibitor to the propagation constant for the monomer in analogy with Eq. (6.87) for chain transfer constants. For styrene at 50°C the inhibition constant of p-benzoquinone is 518, and that for O2 is 1.5 X 10. The Polymer Handbook (Ref. 3) is an excellent source for these and most other rate constants discussed in this chapter. 

Use of chain transfer agents may be indicated to regulate u and thus avoid some of the difficulties mentioned in items (3) and (4). 

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. 

Chain transfer is far more important than in the anionic case, so we do not encounter living polymers in cationic systems. 

Chain transfer reactions to monomer and/or solvent also occur and lower the kinetic chain length without affecting the rate of polymerization ... 

Gregg and Mayot studied the chain transfer between styrene and carbon tetrachloride at 60 and 100°C. A sample of their data is given below for each of these temperatures ... 

Evaluate the chain transfer constant (assuming that no other transfer... 

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