CHAIN TRANSFER

Termination in a free-radical polymerization normally occurs by collision between two active centers attached to polymer chains, but the chain length of the product in many systems is lower than one would expect if this was the mechanism solely responsible for limiting the kinetic chain length v. Usually xn will lie within the expected limits of v (disproportionation) and 2v (combination), but not always, and Flory (1953) found that attenuation of chain growth takes place if there is premature termination of the propagating chain by a transfer of activity to another species through a collision. This is a competitive process involving the abstraction of an atom by a chain carrier from an inactive molecule XY with replaceable atoms and is dependent on the strength of the X–Y bond.

v_p^*

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68 Polymers: Chemistry and Physics of Modern Materials

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 propaga-tion due to the tendency for stabilizapropaga-tion 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

where abstraction of the α-hydrogen leads to a resonance-stabilized allylic radical capable only of bimolecular combination with another allyl radical. This is effec-tively an auto-inhibition by the monomer. Propylene also reacts in this manner and both monomers are reluctant to polymerize by a free-radical mechanism.

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

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

(3.19) Transfer to polymer. The transfer reaction with a polymer chain leads to branch-ing rather than initiation of a new chain so that the average molar mass is relatively unaffected. The long- and short-chain branching detected in polyethylene is believed to arise from this mode of transfer.

M_m +XY M_mX +Y

R H₂C CHX RH H2C C X RH2CHX +

+ (I)

(II)

R+H₂C CHCH₂OCOCH₃ RH H+ ₂C CH CH OCOCH₃

v_tr =k_tr^M[M][M ]^•

v_tr =k_tr^I[I][M ]^•

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Free-Radical Addition Polymerization 69

Transfer to modifier. Molar masses can be controlled by addition of a known and efficient chain transfer agent such as an alkyl mercaptan.

Mercaptans are commonly used because the S—H bond is weaker and more susceptible to chain transfer than a C—H bond.

Transfer to solvent. A significant decrease in polymer chain length is often found when polymerizations are carried out in solution rather than in the undiluted state, and this variation is a function of both the extent of dilution and the type of solvent used. The effectiveness of a solvent in a transfer reaction depends largely on the amount present, the strength of the bond involved in the abstraction step, and the stability of the solvent radical formed. With the exception of fluorine, halogen atoms are easily transferred, and the reaction of styrene in CCl4 is a good example of this chain transfer.

When the solvent is present in significant quantity, step (I) is of minor impor-tance and the resulting polymer contains four chlorine atoms which can be detected by analysis.

Hydrogen is normally the atom abstracted and, as radical stability enhances the transfer reaction, we find that toluene, which forms a primary radical, is less efficient than ethyl benzene, which forms a secondary radical. Both are inferior to isopropyl benzene, which forms a tertiary radical. All are much better than t-butyl benzene, whose radical is unstable, so that virtually no chain transfer takes place in this solvent. It is interesting to note that even benzene acts as a chain transfer agent on a modest scale.

The kinetic expression is

(3.20) CH₂CHX +RSH CH₂CH₂X+RS

RS + H₂C CHX RSCH₂CHX

CH2 CH+ C₆H₅

CCl4 CH2CHCl C₆H₅

CCl3

+

(I)

+ CH

C₆H₅ Cl₃CCH₂ H₂C CHC₆H₅

CCl₃

) I I (

CH+ C₆H₅

CCl₄ CHCl

C₆H₅ CCl₃

Cl₃C Cl₃C +

(III)

v_tr =k_tr^S[S][M ]^•

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70 Polymers: Chemistry and Physics of Modern Materials

3.9.1 CONSEQUENCES OF CHAIN TRANSFER

The primary effect is a decrease in the polymer chain length, but other less obvious occurrences can be detected. If ktr is much larger than kp, then a very small polymer is formed with xn between 2 and 5. This is known as telomerization. The chain reinitiation process can also be slower than the propagation reaction, and a decrease in vp is observed. However, the influence on xn is most important, and it can be estimated by considering all the transfer processes in a form known as the Mayo equation:

1/xn = (1/xn)₀ + Cs[S]/[M] (3.21) This is a simplified form in which the main assumption is that solvent transfer predominates and all other terms are included in (1/xn)0. The chain transfer constant Cs is then .

A plot of 1/x_n against {[S]/[M]} for a variety of agents is shown in Figure 3.3.

The slope is a measure of C_s and the intercept is (1/x_n)₀. If the activation energy of the process is required, log C_s can be plotted against 1/T.