Chemistry Reference
In-Depth Information
It was suggested that initiators, like dibutylzinc, that lack active hydrogens should be placed into a
special category [
96
]. They can initiate polymerizations of some lactones. One of them is
-
caprolactone. Polymers form that are inversely proportional in molecular weights to the catalyst
concentrations [
112
]. The same is true of stannic tetraacrylate. High molecular weight poly(
e
-
caprolactone), as high as 100,000 forms. Addition of compounds that may serve as source of active
hydrogens is not necessary [
95
]. This group of initiators also includes dimethylcadmium,
methylmagnesium bromide, and a few others that are effective in polymerizations of
e
d
-valerolactone,
e
-caprolactone, and their alkyl substituted derivatives. The polymers that form are high in molecular
weight, some as high as 250,000 [
113
].
Another group consists of zinc and lead salts, stannous esters, phosphines, and alkyl titanates. This
group does require additions of compounds with active hydrogens. Such additives can be polyols,
polyamines, or carboxylic acid compounds [
95
]. Molecular weight control is difficult with the
catalysts belonging to the first group. This second group, on the other hand, not only allows control
over the molecular weights, but also over the nature of the end groups [
95
].
Weymouth and coworkers carried out kinetic and mechanistic studies of heterocyclic carbene
mediated zwitterionic polymerization of cyclic esters [
96
]. Based on their results they proposed the
following ring-opening mechanism:
O
O
O
O
O
n
O
O
O
O
O
O
O
O
R
R
R
R
N
N
N
N
+
O
O
R
R
n
N
N
O
O
O
From the kinetic studies they were able to conclude that in the heterocyclic carbene initiated
polymerization of lactide, the rate of initiation is slower than the rate of propagation. Also, the rate of
propagation is much faster than chain termination via cyclization.
5.9 Polymerizations of Lactams
Polymerizations of lactams produce important commercial polymers. The polymerization reactions,
therefore, received considerable attention. Lactam molecules polymerize by three different
mechanisms: cationic, anionic, and a hydrolytic one (by water or water releasing substances).
The lactam ring is strongly resonance stabilized and the carbonyl activity is low. Nevertheless, the
ring-opening polymerizations start with small amounts of initiators through
-acylation reactions.
Fairly high temperatures, however, are needed, often above 200
C. In all such reactions, one
molecule acts as the acylating agent or as an electrophile while the other one acts as a nucleophile
and undergoes the acylation.
Generally, the initiators activate the inactive amide groups causing them to react with other
lactams through successive transamidations that result in formations of polyamides. Both acids and
bases catalyze the transamidation reactions. The additions of electrophiles affect increases in the
electrophilicity of the carbonyl carbon of the acylating lactam. The nucleophiles, on the other hand,
increase the nucleophilic character of the lactam substrate (if they are bases).
trans
