Ethylene terephthalate cyclic oligomers (ETCs) have been successfully polymerized to a high molecular weight poly(ethylene terephthalate) (PET) employing the advantages of the low viscosity of cyclic oligomers and lack of chemical emissions during polymerization. Using ring-opening polymerization of ETCs with organically modified montmorillonite (OMMT), we intend to ascertain the possibility of preparing high performance PET/clay nanocomposites. Due to the low molecular weight and viscosity, ETCs are successfully intercalated to the clay gallerys, what is evidenced by XRD showing a down-shift of basal plane peak of layered silicate along with TEM investigation. Subsequent ring-opening polymerization of ETCs in-between silicate layers yielded a PET matrix of high molecular weight along with high disruption of layered silicate structure and homogeneous dispersion of the latter in the matrix. Although co-existence of exfoliation and intercalation states of silicate layers after polymerization of ETCs rather than perfect exfoliation was observed, a dramatic increase in d-spacing along with fast polymerization presents us a great potential of cyclic oligomer process in producing a thermoplastic polymer-clay nanocomposites of extremely well-dispersed silicate nanoplatelets and the corresponding high performances. (c) 2005 Elsevier Ltd. All rights reserved.