Chemistry Reference
In-Depth Information
Hydrogen transfer can also take place to the acetate moiety:
O
O
R
+
RH
+
O
O
O
O
The above described chain transferring is called
degradative chain transferring
. Other monomers,
such as methyl methacrylate and methacrylonitrile, also contain allylic carbon-hydrogen bonds. They
fail to undergo extensive degradative chain transferring, however, and do form high-molecular-
weight polymers. This is believed to be due to lower reactivity of the propagating radicals that
form from these monomers [
5
].
Yamasaki et al. [
152
], reported that they successfully performed the radical polymerization of
allylbiguanide hydrochloride in a concentrated, acid solution using either hydrochloric acid or
phosphoric acid in the presence of a radical initiator at 50
C. The polymer was precipitated from
the reaction solution through the addition of an excess amount of acetone. The molecular weight
average of the product was 10,340-113,200, with a low polydispersity 1.04-1.68.
In spite of degradative chain transferring, polyallyl compounds can be readily polymerized by a
free-radical mechanism into three-dimensional lattices. High DP is not necessary to achieve growth in
three dimensions. An example of such polyallyl compounds is triallyl phosphate:
O
O
P
3
Many other polyallyl derivatives are offered commercially for use in cross-linked films and are
described in the trade literature.
3.9
Inhibition and Retardation
Free-radical polymerizations are subject to inhibition and retardation from side reactions with various
molecules [
54
]. Such polymerization suppressors are classified according to the effect that they exert upon
the reaction.
Inhibitors
are compounds that react very rapidly with every initiating free radical as it forms.
This prevents any polymerization reaction from taking place until the inhibitor is completely consumed in
the process. The reactions of inhibitors with initiating radicals result in formations of new free radicals. The
newly formed free radicals, however, are too stable to initiate chain growths. As a result, well-defined
induction periods exist. After the inhibitors are used up, polymerizations proceed at normal rates.
Retarders
are compounds that also react with initiating radicals. They do not react, however, as
energetically as do the inhibitors, so some initiating radicals escape and start chain growth. This
affects the general rate of the reaction and slows it down. There is no induction period and retarders
are active throughout the course of the polymerization.
