Chemistry Reference
In-Depth Information
Chapter 5
Ring-Opening Polymerizations
5.1 Chemistry of Ring-Opening Polymerizations
Formation of polymers through ring-opening reactions of cyclic compounds is an important process
in polymer chemistry. In such polymerizations, chain-growth takes place through successive
additions of the opened structures to the polymer chain:
R
X
n
RX
n
An example of the above is a ring-opening polymerization of ethylene oxide that results in
formation of poly(ethylene oxide), a polyether:
O
O
n
n
The cyclic monomers that undergo ring-opening polymerizations are quite diverse. Among them
are cyclic alkenes, lactones, lactams, and many heterocyclics with more than one heteroatom in the
ring. Such polymerizations are ionic in character and may exhibit characteristics that are typical of
ionic chain-growth polymerizations (e.g., effect of counterion and solvent). It would, however, be
wrong to assume that these polymerizations necessarily take place by chain-propagating mechanisms.
Actually, many such reactions are step-growth in nature, with the polymer size increasing slowly
throughout the whole course of the process. There are, on the other hand, some cyclic monomers that
do polymerize in a typical chain-growth manner.
5.2 Kinetics of Ring-Opening Polymerization
There is general similarity between the kinetics of many ring-opening polymerization and those of
step-growth polymerizations that are discussed in Chap.
7
.
Some kinetic expressions in ring-opening
polymerizations, on the other hand, resemble ionic chain-growth reactions.
There are several forms of the rate law that describe the cationic ring-opening polymerization.
For living or polymerizations without termination, one can write
R
p
¼ k
p
½
M
½
M
