Chemistry Reference
In-Depth Information
involves the use of photosensitizers that undergo excitation at an irradiation fre-
quency that is not effective on the monomer or initiator of direct interest.
Transfer of energy from the excited photosensitizer takes place at a frequency at
which the recipient molecule can absorb and subsequently decompose.
Benzophenone is a common photosensitizer in radical reactions.
8.5.4
Other Initiation Methods
Ionizing radiation like
γ
rays and fast electrons are sometimes used to initiate
polymerizations and also to cross-link polymers. The interactions between such
radiations and matter result in a complex series of chemical events that culminate
generally in the production of free radicals. In scrupulously dry systems ionic
polymerization may also take place along with or to the exclusion of radical
processes.
Some pure monomers undergo initiation when heated. The subsequent poly-
merization is free radical in character. Styrene exhibits significant thermal initia-
tion at temperatures of 100
C or more. Methyl methacrylate also self-initiates but
at a slower rate. Low-molecular-weight vinyl polymers can often be made simply
by heating the monomers, but the molecular weight control is not very close and
initiation in some cases at least may be from thermal homolysis of impurities in
the reaction mixture.
8.5.5
Initiator Efficiency
Initiators are not used efficiently in free-radical polymerizations. A significant
proportion of the primary radicals that are generated are not captured by
monomers, and the initiator efficiency
f
in
Eq. (8-10)
is normally in the range
0.2
0.7 for most
initiators
in homogeneous
reaction systems.
It will be
lower yet
in polymerizations in which the initiator may not be very well
dispersed.
Two major reactions are involved in the wastage of primary radicals. These
are induced decomposition of initiator by radicals and side reactions in the solvent
cage. The mechanisms of induced decompositions depend on the structure of the
initiator molecule. For benzoyl peroxide the reaction involved could be an
S
N
2
attack of propating macroradicals on the O
a
O bond:
M
n
+
COO
C
M
n
O
C
O
O
O
(8-49)
O
+
C
O
