Poly(methylmethacrylate) (PMMA) is a widely used polymer commonly known as either acrylic glass or just acrylic. The PMMA stereocomplex—an assembly of three helical strands of PMMA—has applications in ultrathin films, microcellular foams, dialyzers, thermoplastic elastomers and ion gels. Now, Takehiro Kawauchi and co-workers1 from the Japan Science and Technology agency and collaborators at Nagoya University report on a new method to selectively prepare the PMMA stereocomplex.

Fig. 1: A chiral PMMA stereocomplex, in which isotactic polymer double helix (yellow) is included in a single helix of syndiotactic PMMA. Note that the single helix and double helix have the same handedness.

The monomer from which PMMA is made (methylmethacrylate) has a stereogenic centre—a carbon with four different substituents which result in non-superimposable mirror images. When the polymer is assembled, several possibilities result from relative stereochemistry between monomer units—known as the tacticity of the polymer. In the PMMA stereocomplex, the two important arrangements are isotactic—in which the polymer is assembled from numerous monomers all with the same stereochemistry—and syndiotactic—in which the stereochemistry of each monomer unit is alternated along the polymer chain. Recently, this group has shown that the PMMA stereocomplex resulted from the assembly of two isotactic polymer strands, assembled into a double helix, enclosed in one helical syndiotactic strand.

“We were inspired by the helical structures seen in nature, such as DNA,” says Kawauchi. “We wanted to show that control of the handedness of helical assembly was possible even with simple synthetic polymers such as PMMA.”

First, Kawauchi and co-wrorkers showed that, in the presence of simple chiral alcohols, a helix of syndiotactic PMMA could assemble in a stereochemically controlled fashion around a number of C60guests resulting in a ‘peapod-like’ structure. Next, they showed that the C60 guests were slowly displaced by a double helix of isotactic PMMA, forming the PMMA stereocomplex. An optically active syndiotactic/ C60 complex was then shown to interact selectively with one helical form of the isotactic double helix.

“We expect that the optically active syndiotactic PMMA helix may act as a selective host for other chiral molecules,” says Kawauchi. “This may lead to its use as, for example, a chiral stationary phases for enantioseperation. However, PMMA is a commodity plastic, and thus these materials could present a significant cost saving.”