Page 392 - Elements of Chemical Reaction Engineering Ebook

Sec. 7.3 Polymerization 363
2. Transfer to another species:

Chain (R,+Cl
transfer
3. Transfer of the radical to the solvent:

Rj+S k, > Pj+Rl

The species involved in the various chain transfer reactions such
as CC13 * and C6H5CH2- are all assumed to have the same reactivity as
R1. In other words, all the Rl's produced in chain transfer reactionls are
taken to be the same. However, in some cases the chain transfer agent
may be too large or unreactive to propagate the chain. The choice of
solvent in which to carry out the polymerization is important. For
example, the solvent transfer specific reaction rate k, is 10,000 times
greater in CC14 than in benzene.
The specific reaction rates in chain transfer are all assume:d to
be independent of the chain length. We also note that while the radi-
cals R, produced in each of the chain transfer steps are different, they
function in essentially the same manner as the radical R1 in the prop-
agation step to form radical R,.

Termination. Termination to form dead polymer occurs primarily by two
mechanisms:

1. Addition (coupling) of two growing polymers:

2!. Termination by disproportionation:
I R,+Rk kd_, PJ+Pk 1 1
I

for example,
H H
I I
(C'H,),C(CH,CHCl),CH,C + C-CH,(CH,CHCl),(CH,),C -
I I I
CN c1 c1 CN
Termination
H H
I I
(CH3)2C(CH2CHC1jJ CH + C=CH(CH2CHCl)k(CH& C +
I I 1 I
CN c1 c1 CN