Benzoxazine-based phenolic resin has recently attracted a great desl of attention due to its versatile properties．One of its unique properties is volumetric expansion or near-zero shrinkage uponpolymerization．However

the cause for the expansion or near-zero shrinkage is not well understood．In this paper．systematic investigation on the cure shrinkage of three benzoxazines based on bisphenol A

p-cresol and 2．4-diehlorophenol respectively has been made This research has laid particular emphasis on the effect of the Dhenlic portion of the benzoxazine molecule on volumetric expansionl

wich is different from that of Ishida’s work

namely

the effect of the amine moiety of the benzoxazine molecule on volumetric expansion The cufe reactions of these benzoxazines are carried OUt without the aid of catalysts above 140℃．By means of gravimetric and dilatometric methods etc

the studies on volumetric shrinkage

density versus cure time plots

isothermal cure shrinkage and density versus temperature plots have been performed．The cure shrinkage of benzoxazine is demonstrated from the point of view of polymerization for the first time．The volumetric changes of benzoxazines during core reaction process as well as their effects on volumetric expansion are discussed．The results of apparent volumetric shrinkage show that both mono-benzoxazine and bis-benzoxazine precursors exhibit apparent volumetric expansion after polymerization

that is

the densities of monomer are larger than those of polymers at room temperature Moreover

the apparent volumetric expansion of crystalline monomer after polymerization is larger than that of amorphous monomer．The density versus cure time plots give another point of view of the volumetric expansion of beuzoxazines atroom temperature

that is

the higher the cure temperature is

the longer the core time last

the larger the volumetric expansion of benzoxazines will be．The analysis of isothermal cure shrinkage reveals that these bensoxazines exhibit volumetric shrinkage while curing isothermally at elevated temperature．Furthermore

the isothermal cure shrinkages of bis-benzoxazine precursors are smaller than those of monobenzoxazine precursots Results of the density versus temperature plots depict the full view of these benzoxazines as they undergo thermal expansion

shrinkage by curing

and shrinkage by cooling from the cure temperature to room temperature

which is in consistent with the results of apparent volumetric shrinkage and isothermal cure shrinkage．The thermal expansion coefficient of benzoxazine polymer is much smaller than that of monomer. From the deviated point of the density versus temperature plots of bensoxazine monomers

we can predict the initial polymerization temperature of benzoxazines．To sum up

these three kinds of benzoxazinesexhibit apparent volumetric expansion after polymerization and volumetric shrinkage while curingisothermallyl The volumetric changes of different benzoxazines at different cure reaction temperatures as well as with different cure times are evidently distinctive．The larger the extent of polymerization is

the larger the volumetric expansion of benzoxazines will be Therefore

this volumetric expansion phenomenon of benzoxazines is a consequence of the molecular packing rather than the ring-opening polymerization mechanism and the dimension of benzoxazine is associated with its molecular structure

extent of polymerization and crosslinking density．