Thermodynamics of the
Carbon Dioxide–Epoxide
Copolymerization and Kinetics of the Metal-Free Degradation: A Computational
Study
- Publication date
- Publisher
Abstract
The copolymerization reactions of carbon dioxide and
epoxides to
give polycarbonates were examined by density functional theory (DFT),
and chemically accurate thermochemical data (benchmarked to experimental
values) were obtained via composite <i>ab initio</i> methods.
All of the examples studied, i.e., formation of poly(ethylene carbonate),
poly(propylene carbonate), poly(chloropropylene carbonate), poly(styrene
carbonate), poly(cyclohexene carbonate), and poly(indene carbonate),
exhibited enthalpies of polymerization of 21–23 kcal/mol, with
the exception of poly(cyclopentene carbonate) (15.8 kcal/mol) which
suffers both ring strain and intramolecular steric repulsion caused
by the cyclopentane ring fused to the polymer chain. The metal-free
carbonate backbiting reaction by a free anionic polycarbonate strand
is inhibited by bulky groups at the methine carbon but is accelerated
by resonance stabilization of the pentavalent transition state in
the case involving poly(styrene carbonate). Nucleophilic attack at
the methylene carbon of a substituted epoxide has a lower barrier
than for the corresponding reaction involving ethylene oxide due to
charges being distributed onto the pendant groups. The undesired backbiting
reaction to afford cyclic organic carbonates observed under polymerization
conditions for many systems due to the low activation barrier (Δ<i>G</i><sup>‡</sup> = 18–25 kcal/mol) was negligible
for poly(cyclohexene carbonate) because, in this instance, it must
overcome an additional endergonic conformational change (Δ<i>G</i> = 4.7 kcal/mol) before traversing the activation barrier
(Δ<i>G</i><sup>‡</sup> = 21.1 kcal/mol) to
cyclization. Backbiting from an alkoxide chain end is proposed to
proceed via a tetrahedral alkoxide intermediate, where formation of
this intermediate is barrierless. Further reaction of this intermediate
to the cyclic carbonate has a free energy barrier 10 kcal/mol less
than the carbonate chain end backbiting reaction