Chemistry Reference
In-Depth Information
This is a very important process in the free-radical, high-pressure polymeriza-
tion of this monomer. It produces short branches (ethyl, butyl, etc.) of a type and
frequency that depend on the particular pressure and temperature in the reactor.
Branched polyethylene from this process has lower crystallinity than its linear
counterpart. As a consequence it tends to be less rigid and tougher and forms
clearer films. Branches in free radical polyethylene vary more in length and are
more clustered than those produced by copolymerization of ethylene and higher
olefins by organometallic catalysts (Chapter 11). The conventional term for the
latter materials is
linear low density polyethylene
(LLDPE), while the former type
is called is called
low density polyethylene
(LDPE). There are many varieties
within each polymer type, but in general, LLDPEs are tougher than LDPEs. They
can be extruded into thinner packaging films and normally require more power to
extrude at the same output rates.
8.8.5
Allylic Transfer
Chain transfer to monomer was described in general terms in
Section 8.8.2
.
Such reactions are particularly favored with allylic monomers that have the
structure
HH
CH
2
C
C
H
X
8-4
with a C
H bond alpha to the double bond. The polymer radical is reactive, and
the radical formed by transfer to the monomer is particularly unreactive. For allyl
acetate, for example, transfer to monomer involves the reaction
a
H
C
CH
2
CH
2
O
C
CH
3
CH
2
CH
2
H
C
O
C
CH
3
CH
2
O
CH
2
O
CH
C
+
+
O
O
CH
2
CH
2
CO
CO
CH
3
CH
3
(8-90)
Allylic transfer is also variously named
degradative chain transfer, autoinhibi-
tion
,or
allylic termination
. The stable radical derived from the monomer by
reactions like (8-90) are slow to reinitiate and prone to terminate. Low-molecular-
weight products are therefore formed at slow rates and small concentrations of
allyl monomers can inhibit or
retard the polymerization of more reactive
monomers.
