Chemistry Reference
In-Depth Information
Equation (7-4)
represents a step-growth polymerization in which the overall
reaction is a succession of etherifications:
-
H
2
O
HOCH
2
CH
2
CH
2
CH
2
OH + HOCH
2
CH
2
CH
2
CH
2
OH
HOCH
2
CH
2
CH
2
OCH
2
CH
2
CH
2
OH
HOCH
2
CH
2
CH
2
CH
2
OH
-H
2
O
(7-6)
HOCH
2
CH
2
CH
2
CH
2
OCH
2
CH
2
CH
2
CH
2
OCH
2
CH
2
CH
2
CH
2
OH
CH
2
CH
2
CH
2
CH
2
-OH
HO
CH
2
CH
2
CH
2
CH
2
-O
x
Each monomer addition involves the reaction between two hydroxyl groups,
and a monomer can react as readily with a functional group on the end of a poly-
mer as with another monomer. Initially, however, a monomer is far more likely to
encounter another monomer than any other reactive molecule because these are
the most prevalent species. The first stages of the reaction produce dimers which
are likely in turn to yield trimers or tetramers by reacting with monomers or other
dimers. There is thus a gradual increase in the average size of the species in the
reaction mixture, and the molecular weight and yield of polymer both increase as
the reaction proceeds.
The chain-growth polymerization of
Eq. (7-5)
actually represents a sequence
of monomer reactions which is initiated by a small concentration of a strong acid:
-
HClO
4
+
HO
ClO
4
+
(7-7)
O
The kinetic chain is propagated by successive additions of monomer to the
active site generated in reaction
(7-7)
.
-
ClO
4
-
ClO
4
HO
+
HOCH
2
CH
2
CH
2
CH
2
+
O
+
O
ClO
4
-
O
CH
2
O
+
n
4
(7-8)
The reactive site is regenerated on the oxygen atom of each new monomer as
it is added to the growing macrocation, and this polymerization will continue until
the existence of the propagating cation is terminated by reaction with adventitious
impurities. The reaction rate of monomers with each other is negligible compared
to the rate of addition of monomer to the cationic chain end. Polymers can grow
to very large sizes in the presence of much monomer. The amount of polymer
increases as the reaction proceeds but its molecular weight can be essentially con-
stant as long as the initiator and monomer concentrations are not seriously
depleted. [In practice, low-molecular-weight versions of this polymer are made
by cationic polymerization of tetrahydrofuran and used mainly as flexible seg-
ments in polyurethanes (p. 19).]
We turn now to a review of step-growth polymerizations. Chain-growth poly-
merizations are the subject of subsequent chapters.
