Chemistry Reference
In-Depth Information
Commercially, large volume acetals are only produced by polymerizations of formaldehyde and
by ring opening polymerizations of trioxane. These reactions are discussed in chapters three and four.
Two such materials are manufactured in this country. One is a homopolymer of formaldehyde,
polyoxymethylene. It is sold under a trade name of Delrin. The material is end-capped to prevent
depolymerization by acetylating the terminal hydroxyl groups. The other one, a copolymer of
formaldehyde with small quantities of a comonomer, is sold under the trade name of Celcon.
Copolymerization accomplishes the same objective as end-capping. It also makes it more resistant
to attacks by bases. Polyoxymethylene is highly crystalline. This is due to easy packing of the simple,
polar chains. The crystallinity is estimated to be 60-77%. Polyoxymethylene is a strong material with
good resistance to creep, fatigue, and abrasion.
7.8 Poly(p-xylylene)s
The original preparation of
p
-xylylene was carried out by vacuum pyrolysis of
p
-xylene at
900-950
C. The intermediate,
p
-xylylene polymerizes spontaneously upon condensation on cooler
surfaces [
106
]:
Δ
n
n
vacuum
n
-xylylene as an intermediate [
106
]. This dimer converts to
a polymer under milder conditions quantitatively. Both methylene bridges cleave to form
The process was improved by using di-
p
p
-xylylene,
which is a reactive intermediate:
600
o
C
vacuuma (<1 mm Hg)
2
CH
2
CH
2
n
The molecular weights of these polymers were estimated to be as high as 500,000. The total
process is sometimes called
transport polymerization
. Poly(
p
-xylylene) films are produced commer-
T
m
of this polymer, which is crystalline, is 400
C and it carries the trade name of Perylene.
Films of poly(
cially.
-xylylene) have only fair thermal stability and are brittle, but exhibit good chemical
resistance and are very good electrical insulators. Pyrolysis of xylene in steam at 950
C yields the
dimer intermediate. The yield is reported to be 15% [
88
].
It is possible to form substituted poly(
p
-xylylene)s by starting with substituted structures. Among
the compounds that were reported were chlorinated and brominated compounds as well as some
containing alkyl, cyano, acetyl, and carboxymethyl derivatives. When the di-
p
-xylylenes are unsym-
metrically substituted, two homogeneous polymers form during pyrolysis because the two condense
with spontaneous polymerization at two different temperatures [
88
].
p
