We present comprehensive experimental and theoretical studies of poly(methyl methacrylate) (PMMA) ablation by single pulses of 9.17 µm laser radiation in a wide range of fluences. Ablation was carried out in a quartz cell through which nitrogen under atmospheric pressure was pumped. The composition of the ablation products was analysed with the use of an automatic diffusion battery. Both the irradiated surface and the substrate with the deposited ablation products were examined by transmission electron microscopy. Different aspects and stages of ablation have been investigated: laser heating and vaporization of the PMMA surface, mechanisms of ablation, dynamics of the laser-induced plume, nanoparticle formation in the plume and/or ejection from the irradiated surface. It has been found that the size distribution of nanoparticles formed during ablation changes its form from a single peak to a bimodal shape with increasing laser fluence. The transformation of the size distribution is analysed with the help of thermal and gasdynamic modelling which gives a basis for insight into the mechanisms and dynamics of ablation.