| Summary: | Biodegradable aliphatic polycarbonates are important components of non-toxic
thermoplastic elastomers, which have a variety of medical applications. Industrially,
aliphatic polycarbonates derived from six-membered cyclic carbonates such as
trimethylene carbonate (TMC or 1,3-dioxan-2-one) are produced via ring-opening
polymerization (ROP) processes in the presence of a tin catalyst. It is worth mentioning
that TMC is readily obtained by transesterification of 1,3-propanediol with various
reagents including phosgene and its derivatives. Therefore, it has been of great interest
to investigate greener routes for the production of this important class of polymers.
Toward this goal, the synthesis of aliphatic polycarbonates via the metal catalyzed
alternative coupling of oxetanes and carbon dioxide represents an attractive alternative.
The use of an abundant, inexpensive, non-toxic, and biorenewable resource, carbon
dioxide, makes this method very valuable. Furthermore, in this reaction, the sixmembered
cyclic carbonate byproduct, TMC, can also be ring-opened and transformed
into the same polycarbonate. For over a decade, the Darensbourg research group has successfully utilized metal salen complexes as catalysts for the epoxide/CO2
copolymerization process. Hence, this dissertation focuses on the examination of these
complexes as catalysts for the oxetane/CO2 copolymerization reaction and the further
elucidation of its mechanism.
Chromium(III) salen derivatives in the presence of an azide ion initiator were
determined to be very effective catalysts for the coupling of oxetanes and carbon dioxide
providing polycarbonates with minimal amounts of ether linkages. Kinetic and
mechanistic investigations performed on this process suggested that copolymer
formation proceeded by two routes. These are the direct enchainment of oxetane and
CO2, and the intermediacy of trimethylene carbonate, which was observed as a minor
product of the coupling reaction. Anion initiators which are good leaving groups, e.g.
bromide and iodide, are effective at affording TMC, and hence, more polycarbonate can
be formed by the ROP of preformed trimethylene carbonate. Research efforts at tuning
the selectivity of the oxetane/CO2 coupling process for TMC and/or polycarbonate
produced from the homopolymerization of preformed TMC have been performed using
cobalt(II) salen derivatives along with anion initiators. Lastly, investigations of this
process involving 3-methoxy-methyl-3-methyloxetane will be presented.
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