Cyclic oligomers of engineering plastics such as polycarbonate and aromatic polyesters have been known for some time, being formed at very low levels during commercial manufacture. At GE Global Research, we have devised methods for high-yielding and selective preparation of such cyclic oligomers, that are amenable to commercial manufacture. The mixtures of cyclic oligomers have melting points significantly lower than individual ring sizes, and have very low viscosities in the melt. Ring-opening polymerization using appropriate initiators affords rapid reactions (2-6 min) without measurable exotherm, providing very high-molecular-weight polymers and without reaction byproducts. These materials have been scaled to commercial quantities, and have shown to be extremely useful for fabrication of glass and carbon fiber composites with high-fiber fractions. (C) 2008 Wiley Periodicals, Inc.