Microwave driven trimethylsilylation of phenol-formaldehyde resins: Extent of reactions determined by ¹³C and ²⁹Si n.m.r.

A phenol-formaldehyde (PF) resite resin and phenol:2,6-di-t-butylphenol-formaldehyde (DTBPF) co-resite were reacted under conditions of microwave heating with N-(trimethylsilyl)imidazole (TMSI). Low degrees of reaction were observed for the PF resin (19%) and for the DTBPF co-resite (0.3-0.4%) as examples of macromolecular systems with very limited porosity. ¹³C SPE MAS n.m.r. was an effective method to determine the degree of silylation and ¹³C CP MAS n.m.r. gave very similar results. When a different reagent mix (1:1:1 N,O-bis(trimethylsilyl)acetamide:N(trimethylsilyl)imidazole: trimethylchlorosilane) was used both ¹³C and ²⁹Si SPE n.m.r. were in agreement that 100% silylation of the OH groups had been achieved for the PF resin. Thus, with proper choice of conditions and reagents, microwave-driven silylation is a very effective method of OH analysis even for systems of significantly more limited porosity compared with coals.