Chemistry Reference
In-Depth Information
O
RH
O
O
hydrogen
abstraction
O
O
O
O
O
O
O
3.11 Donor-Acceptor Complexes in Copolymerization
Polar interactions of electron donor monomers with electron acceptor monomers lead to strong
tendencies toward formations of alternating copolymers. Also, some alternating copolymerizations
might even result from compounds that by themselves are not capable of conventional polymerization.
An example is copolymerization of dioxene and maleic anhydride. Two reaction mechanisms were
proposed. One suggests that the interactions of donor monomers with acceptor radicals or acceptor
monomers with donor radicals lead to decreased energies of activation for cross-propagations [
168
].
The transition state is stabilized by a partial electron transfer between the donor and acceptor species
[
169
]. The second mechanism suggests that the interactions result in formations of charge-transfer
complexes [
170
]. An electron is completely transferred from the donor monomer to the acceptor
monomer. After the transfer, the complex converts to a diradical that subsequently polymerizes by
intermolecular coupling. For instance, while many believe that the Diels-Alder reaction takes place by
a concerted mechanism, the intermediate was postulated by some to be a charge-transfer complex.
An electron is transferred from the donor to the acceptor and a charge-transfer complex forms [
171
].
This can be illustrated on a Diels-Alder reaction between butadiene and maleic anhydride:
O
O
O
+
O
+
O
O
donor
acceptor
charge transfer complex
O
Δ
O
+
Diels Alder adduct
O
If the reaction mixture is irradiated with high energy radiation, such as gamma rays, instead of
being heated, an alternating copolymer forms. The complex converts to a diradical [
1
,
171
] that
homopolymerizes:
O
O
O
O
O
O
O
γ-rays
polymerization
O
+
n
O
