Nano-to-Macroscale Poly(methyl methacrylate) Stereocomplex Assemblies

Stereocomplexation of Stereoregular Aliphatic Polyesters: Change from Amorphous to Semicrystalline Polymers with Single Stereocenter Inversion

Stereocomplexation is a useful strategy for the enhancement of polymer properties by the co-crystallization of polymer strands with opposed chirality. Yet, with the exception of PLA, stereocomplexes of biodegradable polyesters are relatively underexplored and the relationship between polymer microstructure and stereocomplexation remains to be delineated, especially for copolymers comprising two different chiral monomers. In this work, we resolved the two enantiomers of a non-symmetric chiral anhydride (CPCA) and prepared a series of polyesters from different combinations of racemic and enantiopure epoxides and anhydrides, via metal-catalyzed ring-opening copolymerization (ROCOP). Intriguingly, we found that only specific chiral combinations between the epoxide and anhydride building blocks result in the formation of semicrystalline polymers, with a single stereocenter inversion inducing a change from amorphous to semicrystalline copolymers. Stereocomplexes of the latter were prepared by mixing an equimolar amount of the two enantiomeric copolymers, yielding materials with increased melting temperatures (ca. 20 °C higher) compared to their enantiopure constituents. Following polymer structure optimization, the stereocomplex of one specific copolymer combination exhibits a particularly high melting temperature (T_m = 238 °C).

Helix-Sense-Selective Encapsulation of Helical Poly(lactic acid)s within a Helical Cavity of Syndiotactic Poly(methyl methacrylate) with Helicity Memory

We report a highly enantio- and helix-sense-selective encapsulation of helical poly(lactic acid)s (PLAs) through a unique "helix-in-helix" superstructure formation within the helical cavity of syndiotactic poly(methyl methacrylate) (st-PMMA) with a one-handed helicity memory, which enables the separation of the enantiomeric helices of the left (M)- and right (P)-handed-PLAs. The M- and P-helical PLAs with different molar masses and a narrow molar mass distribution were prepared by the ring-opening living polymerization of the optically pure l- and d-lactides, respectively, followed by end-capping of the terminal residues of the PLAs with a 4-halobenzoate and then a C₆₀ unit, giving the C₆₀-free and C₆₀-bound M- and P-PLAs. The C₆₀-free and C₆₀-bound M- and P-PLAs formed crystalline inclusion complexes with achiral st-PMMA accompanied by a preferred-handed helix induction in the st-PMMA backbone, thereby producing helix-in-helix superstructures with the same-handedness to each other. The induced helical st-PMMAs were retained after replacement with the achiral C₆₀, indicating the memory of the induced helicity of the st-PMMAs. Both the C₆₀-free and C₆₀-bound helical PLAs were enantio- and helix-sense selectively encapsulated into the helical hollow space of the optically active M- and P-st-PMMAs with the helicity memory prepared using chiral amines. The M- and P-PLAs are preferentially encapsulated within the M- and P-st-PMMA helical cavity with the same-handedness to each other, respectively, independent of the terminal units. The C₆₀-bound PLAs were more efficiently and enantioselectively trapped in the st-PMMA compared to the C₆₀-free PLAs. The enantioselectivities were highly dependent on the molar mass of the C₆₀-bound and C₆₀-free PLAs and significantly increased as the molar mass of the PLAs increased.

DNA-Inspired Strand-Exchange for Switchable PMMA-Based Supramolecular Morphologies

Inspired by nanotechnologies based on DNA strand displacement, herein we demonstrate that synthetic helical strand exchange can be achieved through tuning of poly(methyl methacrylate) (PMMA) triple-helix stereocomplexes. To evaluate the utility and robustness of helical strand exchange, stereoregular PMMA/polyethylene glycol (PEG) block copolymers capable of undergoing crystallization driven self-assembly via stereocomplex formation were prepared. Micelles with spherical or wormlike morphologies were formed by varying the molecular weight composition of the assembling components. Significantly, PMMA strand exchange was demonstrated and utilized to reversibly switch the micelles between different morphologies. This concept of strand exchange with PMMA-based triple-helix stereocomplexes offers new opportunities to program dynamic behaviors of polymeric materials, leading to scalable synthesis of "smart" nanosystems.

Controlled Formation and Binding Selectivity of Discrete Oligo(methyl methacrylate) Stereocomplexes

The triple-helix stereocomplex of poly(methyl methacrylate) (PMMA) is a unique example of a multistranded synthetic helix that has significant utility and promise in materials science and nanotechnology. To gain a fundamental understanding of the underlying assembly process, discrete stereoregular oligomer libraries were prepared by combining stereospecific polymerization techniques with automated flash chromatography purification. Stereocomplex assembly of these discrete building blocks enabled the identification of (1) the minimum degree of polymerization required for the stereocomplex formation and (2) the dependence of the helix crystallization mode on the length of assembling precursors. More significantly, our experiments resolved binding selectivity between helical strands with similar molecular weights. This presents new opportunities for the development of next-generation polymeric materials based on a triple-helix motif.

Synthesis of Isotactic-block-Syndiotactic Poly(methyl Methacrylate) via Stereospecific Living Anionic Polymerizations in Combination with Metal-Halogen Exchange, Halogenation, and Click Reactions

Isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) form unique crystalline stereocomplexes, which are attractive from both fundamental and application viewpoints. This study is directed at the efficient synthesis of it- and st-stereoblock (it-b-st-) PMMAs via stereospecific living anionic polymerizations in combination with metal-halogen exchange, halogenation, and click reactions. The azide-capped it-PMMA was prepared by living anionic polymerization of MMA, which was initiated with t-BuMgBr in toluene at ⁻78 °C, and was followed by termination using CCl₄ as the halogenating agent in the presence of a strong Lewis base and subsequent azidation with NaN₃. The alkyne-capped st-PMMA was obtained by living anionic polymerization of MMA, which was initiated via an in situ metal-halogen exchange reaction between 1,1-diphenylhexyl lithium and an α-bromoester bearing a pendent silyl-protected alkyne group. Finally, copper-catalyzed alkyne-azide cycloaddition (CuAAC) between these complimentary pairs of polymers resulted in a high yield of it-b-st-PMMAs, with controlled molecular weights and narrow molecular weight distributions. The stereocomplexation was evaluated in CH₃CN and was affected by the block lengths and ratios.

“Arm-first” approach for the synthesis of star-shaped stereoregular polymers through living coordination polymerization

Stereoregular star polymers were synthesized through an “arm-first” approach via a living coordination polymerization mechanism for the first time using a half-sandwich scandium catalyst system.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Robust Cross-Linked Stereocomplexes and C60 Inclusion Complexes of Vinyl-Functionalized Stereoregular Polymers Derived from Chemo/Stereoselective Coordination Polymerization

The successful synthesis of highly syndiotactic polar vinyl polymers bearing the reactive pendant vinyl group on each repeat unit, which is enabled by perfectly chemoselective and highly syndiospecific coordination polymerization of divinyl polar monomers developed through this work, has allowed the construction of robust cross-linked supramolecular stereocomplexes and C60 inclusion complexes. The metal-mediated coordination polymerization of three representative polar divinyl monomers, including vinyl methacrylate (VMA), allyl methacrylate (AMA), and N,N-diallyl acrylamide (DAA) by Cs-ligated zirconocenium ester enolate catalysts under ambient conditions exhibits complete chemoselectivity and high stereoselectivity, thus producing the corresponding vinyl-functionalized polymers with high (92% rr) to quantitative (>99% rr) syndiotacticity. A combined experimental (synthetic, kinetic, and mechanistic) and theoretical (DFT) investigation has yielded a unimetallic, enantiomorphic-site-controlled propagation mechanism. Postfunctionalization of the obtained syndiotactic vinyl-functionalized polymers via the thiol-ene click and photocuring reactions readily produced the corresponding thiolated polymers and flexible cross-linked thin-film materials, respectively. Complexation of such syndiotactic vinyl-functionalized polymers with isotactic poly(methyl methacrylate) and fullerene C60 generates supramolecular crystalline helical stereocomplexes and inclusion complexes, respectively. Cross-linking of such complexes affords robust cross-linked stereocomplexes that are solvent-resistant and also exhibit considerably enhanced thermal and mechanical properties compared with the un-cross-linked stereocomplexes.

Fullerene peapod nanoparticles as an organic semiconductor-electrode interface layer

A syndiotactic poly(methyl methacrylate) bottlebrush polymer has been shown to complex with C60 fullerene and assemble into nanoparticles that can be dispersed in polar organic solvents. This composite material was used as an electrode interlayer in organic solar cell (OSC) devices leading to enhanced device performance.

Molecular mapping of poly(methyl methacrylate) super-helix stereocomplexes

In this study, by using X-ray powder diffraction profiles as blueprints, we successfully mapped the most probable molecular-level structural arrangement of the PMMA super-helix stereocomplexes through molecular dynamic simulations. Molecular-level resolution of the PMMA triple-helix supramolecule was not previously achievable by experimental methods. After constructing molecular models of stereo-regular complexes composed of linear it- and st-PMMAs, our all-atom molecular dynamics simulations identified the stereocomplex structure that best reproduces experimental diffraction profiles and thermodynamic properties as a double helix of isotactic (it-)PMMA with a helical pitch of 1.8 nm and 9 units per turn surrounded by a single helix of syndiotactic (st-)PMMA with an average helical pitch of 0.9 nm and 20 units per turn. The it-/st- complexing stoichiometry in the PMMA triple-helix is therefore 9 : 20. This presents the first all-atom model of the it-/st-PMMA triple-helix stereocomplex that accurately fits experimental X-ray diffraction profiles. In addition, the simulation results revealed the outer st-PMMA helix of the PMMA stereocomplex has a fiber diameter of at least 1.85 nm and adopts a non-ideal helical geometry. Furthermore, through dynamic simulations, surprising new sights into the effect of the structural configuration of the PMMA stereocomplex (i.e., helical pitch and direction, and tilt angle) on the physical properties of their crystal structures were obtained. Those crystal properties include X-ray diffraction profile, packing density, chain-chain spacing, chain width and cohesive energy density.

Lipase-sensitive polymeric triple-layered nanogel for "on-demand" drug delivery

We report a new strategy for differential delivery of antimicrobials to bacterial infection sites with a lipase-sensitive polymeric triple-layered nanogel (TLN) as the drug carrier. The TLN was synthesized by a convenient arm-first procedure using an amphiphilic diblock copolymer, namely, monomethoxy poly(ethylene glycol)-b-poly(ε-caprolactone), to initiate the ring-opening polymerization of the difunctional monomer 3-oxapentane-1,5-diyl bis(ethylene phosphate). The hydrophobic poly(ε-caprolactone) (PCL) segments collapsed and surrounded the polyphosphoester core, forming a hydrophobic and compact molecular fence in aqueous solution which prevented antibiotic release from the polyphosphoester core prior to reaching bacterial infection sites. However, once the TLN sensed the lipase-secreting bacteria, the PCL fence of the TLN degraded to release the antibiotic. Using Staphylococcus aureus (S. aureus) as the model bacterium and vancomycin as the model antimicrobial, we demonstrated that the TLN released almost all the encapsulated vancomycin within 24 h only in the presence of S. aureus, significantly inhibiting S. aureus growth. The TLN further delivered the drug into bacteria-infected cells and efficiently released the drug to kill intracellular bacteria. This technique can be generalized to selectively deliver a variety of antibiotics for the treatment of various infections caused by lipase-secreting bacteria and thus provides a new, safe, effective, and universal approach for the treatment of extracellular and intracellular bacterial infections.