We have recently reported the differentiation of isotactic, syndiotactic, and atactic polymethyl methacrylates by the film-balance technique.' Now we wish to describe the behavior of a stereoblock polymethyl methacrylate* when spread as a monomolecular film or monolayer.Various methods for the polymerization of methyl methacrylate to crystalline polymers of stereoisomeric structure are Physical properties,2 x-ray diffraction studies,3 infrared absorption correlations,6 and nuclear spin resonance spectroscopy6 leave little doubt as to the predominant steric structure of syndiotactic (Type I ) and isotactic (Type 11) polymethyl methacrylate.A third stereoisomer, the stereoblock (Type 111), is produced when mixtures of polar and nonpolar solvents are used as reaction media.V The detailed structure of the polymer is still subject to investigation. It gives an x-ray diffraction pattern identical to that of an intimate physical mixture of the isotactic and syndiotactic f~r m s ,~J yet it does not appear to separate into individual components by fractionation. On alkaline hydrolysis, the stereoblock behaves differently from a physical mixture.* The stereoblock polymer may thus consist of blocks of units with isotactic configuration followed by blocks of units with syndiotactic configuration.2 It has been assumed that, in the presence of mixtures of solvents, the metal counter ion at times may become complesed either with the propagating center to give isotactic placements or with the solvent to give syndiotactic placements, provided that the rate of formation and decomposition of the solvent complex is slower than the rate of polymerization.6 A more sophisticated statement of the argument considers the equilibrium of solvated-nonsolvated reactive centers in such a mixture of solvents.9 However, depending on the magnitude of the difference in the rate of formation and decomposition of the counter ion solvent complex and the rate of polymerization, part of the chains may grow solvated, accompanied through their entire lifetime by a counter ion a t the reactive center (isotactic placements), and another part may grow in the absence of complexing interference from the counter ion (syndiotactic placements). Such a scheme may give a difficultly separable mixture of isotactic, syndiotactic, and possibly some stereoblock polymers. In this regard, Miller and co-~orkers,~ apparently, have prepared polymethyl in CHCI,, M, = 2.4 x 106 by light scattering in CHBr,, and a J value = 63. The isotactic and syndiotactic samples were prepared and purified as described previously.1