R7.1 Polymerization

383
Chap.
In many instances termination of anionic polymerization is brought about by adding
a base to neutralize the propagating end of the polymer chain.
Other Useful Definitions. The number-average kinetic chain length, VN,
is
the ratio of the rate of the propagation rate to the rate of termination:
r p
V N � ----
(R7.1-49)
rt Most often, the PSSH is used so that rt
� ri:
r p
V N � ---ri
The long-chain approximation holds when VN
is large.
Excellent examples that will reinforce and expand the principles discussed
in this section can be found in Holland and Anthony, 7 and the reader is
encouraged to consult this text as the next step in studying polymer reaction
engineering.
For the free-radical polymerization in which termination is by transfer to
the monomer or a chain transfer agent and by addition, the kinetic chain length is
For termination by combination
Mn � 2V N MM and for termination by disproportionation
7
V N
r p
--rt
k p MR� ktmMR� kt R� ( ) 2 kCt R� ------------------------------------------------------------------
� � C
k p M
� � � ----------------------------------------------
V N
k p M
ktmM ( 2kt koI 2 f ) 12 / � -------------------------------------------------------------------
� �kctC
Mn �
V N MM ktmM ktR� � �kctC
(R7-1.50)
C. D. Holland and R. G. Anthony, Fundamentals of Chemical Reaction Engineering,
2nd ed. (Upper Saddle River, N.J.: Prentice Hall, 1977) p. 457.