Chemistry Reference
In-Depth Information
The remaining materials were methane, ethane, propane, and a number of unsaturated and
saturated higher hydrocarbons. The quantity of ethylene increased slightly with an increase in
the temperature of pyrolysis. Formation of propylene can occur in two ways. The first one is
through intramolecular radical transfer to the second carbon, followed by a decomposition
reaction. This, however, is not believed to contribute significantly to the quantity of propylene
obtained, based on theoretical consideration [
454
]. The second one can occur as a result of
scissions of C-C bonds located in the positions
to the terminal double bonds. Such double
bonds probably form in large quantities during decomposition and this process is thought to be
the main source of propylene [
454
]:
It was observed that
b
isotactic polypropylene decomposes
thermally by a mechanism that varies at
different temperatures and conditions [
455
]. Thus, at 340
C the major volatile product is propane,
while at 380
Citis
n
-pentane, and at 420
C it is propylene. The propane is believed to originate from
some weak spot in the polymeric chain. Formation of
-pentane involves a radical abstraction and a
six-membered ring formation in a backbiting process. Propylene may come from a free-radical
depolymerization process or a cyclic six-membered ring formation involving a terminal double
bond [
455
].
The thermal
n
was the subject of several studies [
456
,
457
]. The
scheme for polyisoprene and polybutadiene degradation was postulated in part by Golub and Garguila
[
458
,
460
]. It is based on infra-red spectra and NMR studies of the products:
degradation of diene polymers
(H)
(H)
399
o
C
(H)
(H)
+
(H)
(H)
+
(H)
+
+
(H)
(H)
(H)
cyclized rubber
(H)
+
