Chemistry Reference
In-Depth Information
The reaction is exothermic and requires cooling to maintain the temperature between 100
and 108
C.
The monomer can also be prepared from ethylene:
30 - 50
o
C
+
Cl
2
Cl
Cl
500
o
C
Cl
+
HCl
kaolin
The reaction of dehydrochlorination is carried out at elevated pressure of about 3 atm.
Free-radical polymerization of vinyl chloride was studied extensively. For reactions that are
carried out in bulk the following observations were made [
292
]:
1. The polymer is insoluble in the monomer and precipitates out during the polymerization.
2. The polymerization rate accelerates from the start of the reaction. Vinyl chloride is a relatively
unreactive monomer. The main sites of initiation occur in the continuous monomer phase.
3. The molecular weight of the product does not depend upon conversion nor does it depend upon the
concentration of the initiator.
4. The molecular weight of the polymer increases as the temperature of the polymerization decreases.
The maximum for this relationship, however, is at 30
C.
There is autoacceleration in bulk polymerization rate of vinyl chloride [
293
]. It was suggested by
Schindler and Breitenbach [
294
] that the acceleration is due to trapped radicals that are present in the
precipitated polymer swollen by monomer molecules. This influences the rate of the termination that
decreases progressively with the extent of the reaction, while the propagation rate remains constant.
The autocatalytic effect in vinyl chloride bulk polymerizations, however, depends on the type of
initiator used [
295
]. Thus, when 2, 2
0
-azobisisobutyronitrile initiates the polymerization, the autocat-
alytic effect can be observed up to 80% of conversion. Yet, when benzoyl peroxide initiates the
reaction, it only occurs up to 20-30% of conversion.
When vinyl chloride is polymerized in solution, there is no autoacceleration. Also, a major feature
of vinyl chloride free-radical polymerization is chain transferring to monomer [
296
]. This is
supported by experimental evidence [
297
,
298
]. In addition, the growing radical chains can terminate
by chain transferring to “dead” polymer molecules. The propagations then proceed from the polymer
backbone [
297
]. Such new growth radicals, however, are probably short lived as they are destroyed by
transfer to monomer [
299
].
The
13
C NMR spectroscopy of poly(vinyl chloride), which was reduced with tributyltin hydride,
showed that the original polymer contained a number of short four-carbon branches [
300
].
This, however, may not be typical of all poly(vinyl chloride) polymers formed by free-radical
polymerization. It conflicts with other evidence from
13
C NMR spectroscopy that chloromethyl
groups are the principal short chain branches in poly(vinyl chloride) [
301
,
302
]. The pendant
chloromethyl groups were found to occur with a frequency of 2-3/1,000 carbons. The formation
of these branches, as seen by Bovey and coworkers, depends upon head to head additions of
monomers during the polymer formation. Such additions are followed by 1,2 chlorine shifts with
subsequent propagations [
301
,
302
]. Evidence from still other studies also shows that some head to
head placement occurs in the growth reaction [
303
]. It was suggested that this may be not only
