The process of stereocomplexation between isotactic and syndiotactic PMMA in dilute solution was studied by means of differential scanning calorimetry of isolated complexed material, solution viscometry, and isothermal mixing calorimetry. The results were combined with results of investigations previously published. It appeared that essentially the same mechanism as proposed for the complexation process in bulk can account for the observed phenomena, including the previously reported association and aggregation stages following complexation. Two main processes are distinguished complexation and subsequent crystallization of complexed chain sections, leading to the formation of complex particles. With respect to crystallization, two modes are suggested: fringed-micellar and lamellar growth, depending on the so-called critical sequence length under the conditions employed and the mobility of the chain sections. These parameters appear to be influenced by various variables such as solvent quality, temperature, mixing ratio, and tacticity. Introduction Stereocomplexation of isotactic and syndiotactic poly(methyl methacry la te ) (iand s -PMMA) is possible in (dilute) solution as well as in bulk. In a companion paper the results were given of an investigation on the overall process in bulk and a mechanistic model which accounts for the observed phenomena was presented.’ With respect to dilute solution one can distinguish three kinds of solvents: strongly complexing, weakly complexing, and noncomplexing, respectively designated as solvent type A, B, and C.2p3 In addition to this it was found that the “complexing ability” of a solvent can be changed from type A to eventually t y p e C b y raising the t e m p e r a t ~ r e . ~ ~ ~ Although various investigators have tr ied to correlate the complexing abil i ty of solvents to solvent characterist ics, a satisfactory correlation was not ye t a c h i e ~ e d . ~ ~ ~ . ” In the case of strongly complexing solvents, complex particles are formed of about 10-30 nm,@-ll, consisting of a compact nucleus sur rounded b y a shell of noncomplexed chain^.^^^ In the case of an excess of one of the components this shell 0024-9297/88/2221-2203$01.50/0 consists of the component in excess, leading to steric stabilization of the complex particles.@ Schomaker et al. have given a phenomenological description of the pro~ess.~ They showed that in case of solvents of type A, three stages can be distinguished: (i) complexation: (ii) association of complexed cha in sections result ing i n the format ion of compact “complex particles”; (iii) aggregation of complex particles, eventually followed b y flocculation. As complexed material obtained from dilute solution also shows crystallinity,5JZ-l6 and as also is found that on employing a strongly complexing solvent two melting points are found,14J5 we investigated whether the same mechan ism as proposed for complexation in bulk is followed i n