Acta Polyrneriu 37 (1%) Nr. 0 51 7 f Q Fig. 3. Donor-acceptor complexes between styrene and N-PMI ( I ) and between styrene and N-@-PEMI (ZI) using t h e HANNA-ASBAUQH equation [ l l ] (Fig. 2) shows quantitatively t h a t t h e complex formed between styrene and N-PMI, N-BMI and N-/?-PEMI is 0.0714, 0.215 and 0.333 mol-1, respectively. The reason for this order is not very clear b u t our speculation is as follows: As we have emphasized on several occasions, the aromatic ring also has nucleophilic character, and we might therefore expect that, under favourable conditions, neighbouring-group participation by aryl groups might b e observed [13]. Such participations are often classified as steric hindrance phenomena between t h e t w o phenyl groups. This effect will decrease as the distance between phenyl group a n d maleimide nitrogen will increase, which in t u r n increases the K-value. The structures given i n Figure 3 can be assumed for t h e donor-acceptor complex formation between styrene and N-PMI a n d between styrene a n d N-/I-PEMI in low dielectric constant solvent. The mole ratio for both interacted monomers is 1 : 1.In acetone, which has a higher dielectric constant, there is no considerable shift observed. Such behaviour m a y be attributed t o t h e complete separation of t h e maleimides a n d styrene.