Isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) were synthesized by anionic polymerization in toluene at -78 °C with t-C 4 H 9 MgBr and t-C 4 H 9 Li/(C 2 H 5 ) 3 Al, respectively, as initiators. Polymers were assembled by the stepwise immersion of a 9-MHz quartz-crystal microbalance (QCM) as a substrate into organic solutions at ambient temperature. Quantitative QCM analysis revealed that acetonitrile was the best solvent for assembly on the substrate compared to acetone and N,N-dimethylformamide (DMF). The analysis also showed linear growth of the assembly except during the initial two steps (possibly due to direct influence of the gold QCM substrate), indicating that the amount of assembly can be easily regulated at each step. A 5 min immersion of the QCM into each PMMA solution was enough to obtain stepwise assembly. The ratios of st-/it-PMMA assembled, the static contact angle of the assembly at each step, and its reflection absorption spectrum (RAS) strongly implied the stepwise formation of a stereocomplex between it-and st-PMMAs in the assembly. The amount of stereocomplex assembled was significantly affected by the addition of water to the organic solutions, the PMMA concentration, and its molecular weight, as well as the solvent species. Atomic force microscopic (AFM) examination of the assembly revealed a smooth surface. The assembly was formed at the step from the physically adsorbed it-PMMA to st-PMMA. A possible assembling mechanism was also proposed.

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ABSTRACT:The poly(methyl methacrylate)s (PMMAs) differing in their tacticity were investigated by liquid chromatography at the critical adsorption point (LC CAP). The set of chromatographic conditions (eluent composition determining its adsorption strength, column packing nature and temperature) was identified for given highly stereoregular PMMAs, under which the exclusion of polymer chain from the pores (entropic effect) was just compensated by the adsorption of the polymer chain on the column packing surface (enthalpic effect). The result was that no separation according to the solute molar mass took place. Using bare silica gel column packing and tetrahydrofuran (THF)/n-hexane mixture as eluent at 30°C, syndiotactic (st-) PMMAs were more readily adsorbed on the column packing surface than isotactic (it-) PMMAs. The critical adsorption point (CAP) for st-PMMAs was reached with the eluent composition THF/n-hexane (83.2/16.8, w/w). Under the same conditions, however, it-PMMAs were eluted in the size exclusion chromatography (SEC) mode and the separation was achieved for mixtures of it-PMMA and st-PMMA of lower molar mass. With the same bare silica gel as the stationary phase and chloroform (CHC13)/ethanol or CHCl3/THF mixtures as eluents at 30°C, st-PMMAs were less adsorbed on column packing and they eluted in the SEC mode while the CAP was reached for it-PMMAs, i.e., CHC13/ethanol (97.2/2.8, w/w) or CHCl3/THF (87.7/12.3, w/w). By using latter non-aggregating mixed eluents containing chloroform, the satisfactory separation ofst-plus it-PMMA mixtures were achieved even for polymers with molar masses exceeding IOOOOOgmol-1 . The results indicate the potential of liquid chromatographic procedures that combine exclusion and adsorption mechanisms in separation of macromolecules according to their stereoregularity. Further, the advantages of the LC CAP mixed eluents is demonstrated that are composed solely of the solvents for polymers under study.KEY WORDS Poly(methyl methacrylate) / Tacticity / Isotactic Poly(methyl methacrylate) / Syndiotactic Poly(methyl methacrylate) / Liquid Chromatography at Critical Adsorption Point / Separation /

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Isotactic (it-) poly(methyl methacrylate) (PMMA) and syndiotactic (st-) poly(ethyl methacrylate) (PEMA) and poly(propyl methacrylate) (PPMA) were synthesized by anionic polymerization in toluene at -78 °C with t-C4H9MgBr and t-C4H9Li/(C2H5)3Al, respectively, as initiators/coordinators. Combined it-and stpolymers were assembled by the alternate immersion of a 9 MHz quartz-crystal microbalance (QCM) as a substrate into their acetonitrile solutions at ambient temperature. QCM analysis showed constant growth of the assembly except in the first two steps. The assembled ratios between st-PEMA and st-PPMA with it-PMMA (st-/it-) were 2.5 × 0.4 and 1.8 × 0.5, respectively, indicating stepwise stereocomplex formation. The static contact angle of the assembly at each step and its reflection absorption spectrum also implied complex formation. The amount of stereocomplex assembled was significantly affected by the st-polymer concentration and its molecular weight. Atomic force microscopy observation showed the assemblies to have a molecularly smooth surface.