This work focuses on the synthesis of polyrotaxanes with high molecular weight template poly(ethylene glycol) PEG (20 kg mol) having various and well-defined amounts of α-cyclodextrins (α-CD) per chain from 3 up to 125. is the complexation degree of the polyrotaxane defined to be the average number of cyclodextrin molecules per template chain. The usual route has been used for high values of , while sparsely complexed polyrotaxanes have been synthesized with an original one pot synthesis in water. Furthermore, a systematic study was carried out to understand and control the complexation degree of the polyrotaxane as a function of the complexation time, the temperature and the initial ratio of α-CD to template polymer. It has been shown that a high temperature thermal plateau leads to the formation of very sparsely complexed (low ) pseudo-polyrotaxanes for which, the threaded α-CD act like nuclei and generate a favourable driving force for the final complexation at lower temperature.