Design Principles for Rational Polyurethane Catalyst Development

Tertiary amine catalysts are essential
components in manufacturing
polyurethane materials. The low-emission requirements for indoor applications
are typically achieved by employing tertiary amines with catalytically
active N,N-dimethyl groups as the
base catalyst and a longer alkyl substituent with a reactive end,
that is, alcohol or amine, to incorporate it in the polyurethane matrix. N,N-dimethyl groups are, however, oxidized
when exposed to air and lead to undesired formaldehyde emissions.
Here, we employ modern quantum chemical methods to understand design
principles how the structure of tertiary amine catalysts having N,N-dimethyl groups can be modified to
avoid this source of formaldehyde formation but still preserve their
catalytic activity. We found the pyrrolidine derivative of commonly
used N,N-dimethylated catalysts
to be the most promising candidate and developed design principles
to rationalize why longer alkyl chains or larger ring sizes inhibit
the catalytic activity. The computationally predicted catalyst performances
were confirmed experimentally in model polyurethane systems for selected
amine catalysts, and emission measurements showed that the formaldehyde
emission was completely suppressed when pyrrolidine derivative was
used as a catalyst. Our results further illustrate how condensed phase
reactions can be predicted using quantum chemical methods and that
to account for steric hindrance near the reaction center, it was also
necessary to include conformational energy contributions in the calculated
activation free energies.