Chemistry Reference
In-Depth Information
fumarates, maleic anhydride, dialkyl maleates, maleimides).
32
The
intermediacy of a CTC complex is supported by UV spectroscopy. Its
stability is correlated with the complementary electronic behavior of the
two ethylenic comonomers overlapping the highest occupied molecular
orbital (HOMO) of the donor with the lowest unoccupied molecular or-
bital (LUMO) of the acceptor, as proposed in Mulliken's theory.
33
The
reactivity in copolymerization is obviously affected by the ionization po-
tential (IP) of the donor monomer and by the electron anity of the
acceptor.
34
Typically allyloxy derivatives exhibit an IP value much lower
than that of the vinyl analogs (IP
=
9.62 eV for allyl ethyl ether versus
IP
=
8.93 eV and 8.83 eV for methyl vinyl ether and ethyl vinyl ether,
respectively).
35
The formation of alternated sequences can also be explained by the
preponderance of the cross-propagation steps, with reactivity ratios of the
Lewis-Mayo model, r
1
and r
2
for each comonomer, equal or close to
zero.
36
Both mechanisms can occur simultaneously,
leading to co-
polymers having the same structure (Scheme 5).
37
Allyloxy compounds, as for most of a-olefins, are known to undergo
ecient degenerative transfer reactions that compete with the con-
ventional propagation mechanism. The various reactions possibly
accurring after incorporation of an allylether monomer are detailed
in Scheme 6: elimination of an alkoxy radical (equation 1), dis-
proportionation (equation 4), and hydrogen abstraction at the CH
2
group
of the allyloxy moiety by the propagating free-radical with the formation
of a stable chain transfer allyl radical CH
2
ΒΌ
CH-
d
CH-OR which cause
inecient reinitiation retarding the polymerization (equation 2). This
latter radical may dissociate further, inducing the formation of acrolein
(equation 3).
38
Thus, these side reactions lead to is not only results in the
reduction of the kinetic chain length, hence to low MW polymers, and
eventually generate chain branching.
39
Free-radical cyclopolymerization of diallyl or mixed allyl-vinyl and allyl-
acrylic derivatives has been investigated.
40
Particularly, Jansen has
studied the photo-initiated copolymerization of various commercial
vinylethers as well as allylethers with fumarates or maleates. Real-time
infrared spectroscopy with advanced multivariate statistical techniques
was used for the determination of copolymerization reactivity ratios of
Formation of CTC
Homopolymerisation of CTC
D
A
+D
+
or
AD
P
AD
D
A
Initiation
Cross-propagation
PA
+D
+D
D
PD
+A
Alternating
copolymer
PI
PI
+A
+A
A
PD
PA
+D
Scheme 5 Possible mechanisms of alternating copolymerization of vinyl ethers (D) with
acceptor comonomers (A).
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