Chemistry Reference
In-Depth Information
(A) They react with monomers continuing the ROP.
(B) They undergo an intermolecular condensation step, which in this chapter will
be called chain extension and not chain growth to distinguish it from the
propagation step of the ROP.
(C) They undergo an intramolecular condensation step meaning cyclization by
''end-biting''.
In a KCP, cyclics cannot be formed by ''back-biting'', and thus the formation
of cyclic oligomers and polymers is indicative for the intermediate formation
of linear chains having two reactive end groups. In other words, the formation of
cyclics is indicative for simultaneous ROP and (poly)condensation steps. If side
reactions are absent, and if the linear chains are mobile enough for cyclization,
cyclics will be the only reaction products, and combined ROP and polyconden-
sation is an efficient strategy for the preparation of cyclic polymers in high yields.
The average molecular weight of the cyclic polymers depends on the kinetic
scenario. For a short discussion of the correlation between ring size and kinetic
course of the entire polymerization process, the four reaction rates defined in
Eqs. (
15.1
)-(
15.4
) will be helpful.
v
in
¼ k
in
½
½
15
:
1
Þ
initiationrate of the ROP
½
v
pr
=k
pr
a-
ð
x
-b
15
:
2
Þ
propagation rate of the ROP
2
v
ce
=k
ce
a-
ð
x
-b
ð
15
:
3
Þ
rate of chain extension by intermolecular condensation
v
cy
=k
cy
a
ð
x-b
½
ð
15
:
4
Þ
rate of cyclization by ''end-biting''.
Two extreme cases should be discussed to illustrate the relationship between
average ring size and these four reaction rates: (a) formation of high molar mass
cyclic polymers, and (b) formation of low molar mass cyclics. Favorable for (a) is
high initial monomer concentration and v
in
[ v
pr
, so that all chains are initiated
almost simultaneously. A high concentration of difunctional linear chains favors,
in turn, a high y
ce
/v
cy
ratio according to the Ruggli-Ziegler Dilution Principle.
Furthermore, a relatively high ratio of v
pr
/vc
y
is helpful, so that in the optimum
case almost all monomers were consumed by the ROP yielding long chains before
cyclization begins. Favorable for (b) is a low initial monomer concentration and
v
pr
[ v
in
. This kinetic situation has the consequence that only few chains are
initiated per time unit, their molar concentration will be low thus favoring rela-
tively high v
cy
/v
pr
and v
cy
/v
ce
ratios.
