Glass transition temperature (Tg) is an important intrinsic parameter of polymers

which greatly affects their properties and application. Tg of a polymer could be changed through copolymerization.It is well known that Tg of a copolymer usually deviates from the additivity of that of the homopolymers.Tg of a copolymer was affected not only by the composition but also by the sequence distribution of the copolymer.Many researchers have proposed various equations to describe the copolymer Tg composition relationship based on free volume theory as well as thermodynamic theory of glass transition.Among them

Johnston and Barton considered the diad effect

and better fitting result between the theory predicted and experimental data was obtained.Whereas they failed when the Tg composition curve is asymmetric or even S-shaped.When triad sequence effect was taken into consideration

the fitting was greatly improved.Unfortunately

there are 6 intractable parameters in the equation which greatly holdback its application.Ham and Uematsu proposed their methods to reduce the number of intractable parameters to 1

which increased the facility of the triad sequence equation.Whereas the manners they proposed were unreasonable.It is found that all the equations proposed by now were based on the additivity of contributions of different monomers or sequences.In this paper

we propose an equation based on the additivity of contributions of different bonds concerning the substitution on the adjacent C atoms.For the case of a headtotail binary copolymer composed of mono-substituted vinyl monomers

the bonds were classified into 8 kinds when the substitutions on C1

C2 and C3 were concerned.Based on the thermodynamic theory of glass transition

the equation is described as below under the assumption of steady-state concentration. Tg=nAAATgA+nBBBTgB+2(nABA-nAAB)Tg［AB］+3nAABTg［AAB］+3nBBATg［BBA］Where nijk represents the mole fraction of triad sequence ijk

Tg［x］is Tg of periodic copolymer poly［x］.nijk could be obtained experimentally or calculated from the monomer feed compositions and reactivity ratios.The equation could be applied to investigate the relation between copolymer Tgs and compositions conveniently because that it contains no mere fitting parameter.If the periodic copolymers poly［AAB］ and poly［BBA］ cannot be acquired

there are only two intractable parameters in the equation.Even if the alternating copolymer poly［AB］ also cannot be acquired

there are only three adjustable parameters

far less than the original six in the Ham equation.More over

Tgs of them could be predicted by using the equation and Tg data of random copolymers for this case.The new equation was applied to MMA-St and E-VAc random copolymers

and excellent fitting was obtained.Whereas large deviations pronounced when only diad sequence effect was taken into consideration.Especially for the case of E-VAc copolymers

the predicted Tgs show opposite derivation compared with the experimental results.Tgs of corresponding periodic copolymers were also evaluated.This equation may be extended to describe multi-copolymers or block

graft copolymers only if no phase separation appears in their bulk.Also the fact of Tg changed with tacticity

such as PMMA

may be explained by using this equation.