Chemistry Reference
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in PE or another oligo-R. The combination between secondary carbon rad-
icals on PE substrates terminates the overall free-radical grafting, causing an
increase in molecular weight of the resultant graft polymer.
For PP, the graft mechanism of MA is more complicated and still con-
troversial
27
because of b-scission of polypropylene substrate (Scheme 9.1).
The grafting probably occurs on tertiary carbon radicals rather than on
secondary carbon radicals at the chain end of fragments due to b-scission
(Scheme 9.1). Heinen et al.
28
determined, on the basis of
13
C NMR spec-
troscopy, two species of the graft: one is the single succinic anhydride ring on
a tertiary carbon of PP substrate (3) and the other is the itaconic anhydride
terminal (4) (Figure 9.14). The species 3 can be formed through addition of
an MA monomer to a tertiary carbon radical of a PP chain followed by a ready
intra- or intermolecular hydrogen transfer from a PP chain with methine
carbons, which is quite different from PE chains with only methylene car-
bons, because the hydrogen on a tertiary carbon is abstracted more readily
than that on a secondary carbon.
30
The formation of these species has been
also confirmed by
13
C NMR spectroscopy of several industrial MA-grafted PP
products.
31
A mechanism for generating the species 4 is that the single
succinic anhydride radical on a tertiary carbon in a PP chain undergoes
intramolecular hydrogen abstraction via a six-membered transition state
followed by b-scission, and finally by disproportionation (Scheme 9.2);
27
this
is different from the original mechanism proposed by Heinen et al.
28
The
mechanism in Scheme 9.2 expects that the formation of the itaconic an-
hydride terminals 4 is accompanied by a decrease in molecular weight of the
backbone polymer. This agrees well with the inverse relationship between
the molecular weight and the content of the itaconic anhydride terminals 4,
which was observed from NMR examination of industrial products of MA
grafted PP with different graft contents of MA.
31
Roover et al.
32
proposed the formation of single or oligo-grafts on the
secondary carbon terminals, from IR spectroscopy. Although with IR spec-
troscopy it is fairly dicult to determine exactly the structure of such grafts,
it is possible to evaluate qualitatively the grafting of MA.
Grafting of maleic anhydrides on poly(ethylene-co-propylene) can be
understood on the basis of the graft structures on polyethylene and poly-
propylene described above, depending on the mode of sequence of ethylene
O
O
O
O
O
O
O
O
O
3
4
Figure 9.14 Graft structures in PP, determined by
13
C NMR.
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