R7.1 Polymerization

Termination
Initiation
Propagation
Transfer
Termination
363
Termination.
mechanisms:
Chap.
Termination to form dead polymer occurs primarily by two
1. Addition (coupling) of two growing polymers:
2. Termination by disproportionation:
For example,
R j
The steps in free-radical polymerization reaction and the corresponding
rate laws are summarized in Table R7.1-1. For the polymerization
of styrene at 80�C
initiated by 2,2-azobisisobutyronitrile, the
rate constants4
are
k0
� 1.4 � 10�3
s�1
R j
kp
� 4.4 � 102
dm3/mol�s
ks
� 2.9 � 10�3
dm3/mol�s
� Rk ⎯⎯→ P j
km
� 3.2 � 10�2
dm 3/mol�s
kta
� 1.2 � 108
dm3/mol�s
ktd
� 0
Typical initial concentrations for the solution polymerization of styrene are
0.01 M for the initiator, 3 M for the monomer, and 7 M for the solvent.
R7.1.2.2 Developing the Rate Laws for the Net Rate
of Reaction
We begin by considering the rate of formation of the initiator radical I.
Because there will always be scavenging or recombining of the primary radicals,
only a certain fraction f will be successful in initiating polymer chains.
Because each reaction step is assumed to be elementary, the rate law for the
formation of the initiator free radicals, r If, is
�k
4 D. C. Timm and J. W. Rachow, ACS Symp. Ser. 133, 122 (1974).
k
td
k
ta
�Rk ⎯⎯→ P j�
H H
( CH3) 2CCH ( 2CHCl)
j CH2C � C�CH2( CH2CHCl) k ( CH3) 2C CN Cl Cl CN
H
H
(CH 3) 2C(CH 2CHCl) j CH � C�CH(CH 2CHCl) k(CH 3) 2 C
P
k
CN Cl Cl
CN