Bisphenol A diglycidyl ether (BADGE)
is cured thermally using phthalic
acid anhydride (PhA) or hexahydrophthalic anhydride (HHPA) as hardener
in the presence of different protected N-heterocyclic
carbenes (NHCs), from which the catalytically active NHCs are generated
in situ upon heating. It is found that the curing reactions proceed
in a well-defined manner, delivering highly cross-linked, high-T
g-thermosets using low catalyst loadings (0.1–1
mol % of NHC precursor). The polymerizations can be conducted under
air without loss of activity, employing mild curing temperatures (120–160
°C) and short reaction times. By contrast, at room temperature,
polymerizations proceed only very slowly and the mixtures remain processable
for weeks, enabling formation of a true single-component composition
suitable for applications where large processing windows or storage
are required. The curing process was followed in situ by DSC as well
as by rheological measurements. On the basis of these observations,
the structure of the NHC precursor is correlated with its polymerization
activity with regard to latency, temperature profile and polymerization
kinetics. The robust and fully homogeneous system consisting of the
protected NHC, BADGE, and HHPA was successfully tuned both in terms
of activity and pot life by choosing the appropriate protected NHC
out of 12 different precatalysts. The most rapid polymerization was
effected by N,N′-bis(2,4-dimethoxyphenyl-)tetrahydropyrimidinium-2-carboxylate
(6-OMe-CO
2
), while a dimeric
zinc-based NHC-complex (6-Mes-ZnCl
2
) displayed the longest pot times.