Stereoregular High-Density Bottlebrush Polymer and Its Organic Nanocrystal Stereocomplex through Triple-Helix Formation

Bottlebrush Polymers Based on RAFT and the "C1" Polymerization Method: Controlled Synthesis and Application in Anticancer Drug Delivery

In this work, we reported a strategy to synthesize well-defined bottlebrush polymers. Diazoacetate macromonomers of polystyrene (1-PS_n) with controlled molecular weights were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The diazo can tolerate the RAFT polymerization conditions and remained on the chain end of the yielded PS macromonomer. The terminal diazo groups of the macromonomer were polymerized by the allyl PdCl/L catalyst to afford well-defined bottlebrush polymers ((1-PS_n)_ms) carrying a side chain on each backbone atom. Meanwhile, an amphiphilic bottlebrush polymer containing brush-shaped PS and polyethylene glycol (PEG) was synthesized by polymerization of the diazoacetate macromonomer of PEG (2-PEG) using Pd(II)-terminated (1-PS_n)_m as the macroinitiator. The yielded amphiphilic (1-PS₃₀)₅₀-b-(2-PEG)₁₀₀ could self assemble into a well-defined core-shell micelle in aqueous solutions. The hydrodynamic diameter of the micelle was ca. 146 nm and had good biocompatibility. These results indicate the micelles have great potential in drug delivery.

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.

Grubbs' and Schrock's Catalysts, Ring Opening Metathesis Polymerization and Molecular Brushes-Synthesis, Characterization, Properties and Applications

In this review, molecular brushes and other macromolecular architectures bearing a bottlebrush segment where the main chain is synthesized by ring opening metathesis polymerization (ROMP) mediated by Mo or Ru metal complexes are considered. A brief review of metathesis and ROMP is presented in order to understand the problems and the solutions provided through the years. The synthetic strategies towards bottlebrush copolymers are demonstrated and each one discussed separately. The initiators/catalysts for the synthesis of the backbone with ROMP are discussed. Syntheses of molecular brushes are presented. The most interesting properties of the bottlebrushes are detailed. Finally, the applications studied by different groups are presented.

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.

Supramolecular Graft Copolymerization of a Polyester by Guest-Selective Encapsulation of a Self-Assembled Capsule

Repeating guest units of polyesters poly-(R)-2 were selectively encapsulated by capsule 1(BF₄ )₄ to produce supramolecular graft polymers. The encapsulation of the guest units was confirmed by ¹ H NMR spectroscopy. The graft polymer structures were confirmed by the increase in the hydrodynamic radii and the solution viscosities of the polyesters upon complexation of the capsule. After the capsule was formed, atomic force microscopy showed extension of the polyester chains. The introduction of the graft chains onto poly-(R)-2 resulted in the main chain of the polymer having an M-helical morphology. The complexation of copolymers poly-[(R)-2-co-(S)-2] by the capsule gave rise to the unique chiral amplification known as the majority-rules effect.

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.