The ROMP toolbox upgraded

A new echelon of precision polypentenamers: highly isotactic branching on every five carbons

A systematic study shows that bulky allylic substituents on cyclopentene monomers affords a highly precise microstructure after ring-opening metathesis polymerization.

Empowering mechanochemistry with multi-mechanophore polymer architectures

Multi-mechanophore polymers provide advantages in characterization and function relative to chain-centered, single mechanophore polymers.

Effects of Grafting Density on Block Polymer Self-Assembly: From Linear to Bottlebrush

Grafting density is an important structural parameter that exerts significant influences over the physical properties of architecturally complex polymers. In this report, the physical consequences of varying the grafting density (z) were studied in the context of block polymer self-assembly. Well-defined block polymers spanning the linear, comb, and bottlebrush regimes (0 ≤ z ≤ 1) were prepared via grafting-through ring-opening-metathesis polymerization. ω-Norbornenyl poly(d,l-lactide) and polystyrene macromonomers were copolymerized with discrete comonomers in different feed ratios, enabling precise control over both the grafting density and molecular weight. Small-angle X-ray scattering experiments demonstrate that these graft block polymers self-assemble into long-range-ordered lamellar structures. For 17 series of block polymers with variable z, the scaling of the lamellar period with the total backbone degree of polymerization (d* ∼ N_bb^α) was studied. The scaling exponent α monotonically decreases with decreasing z and exhibits an apparent transition at z ≈ 0.2, suggesting significant changes in the chain conformations. Comparison of two block polymer systems, one that is strongly segregated for all z (System I) and one that experiences weak segregation at low z (System II), indicates that the observed trends are primarily caused by the polymer architectures, not segregation effects. A model is proposed in which the characteristic ratio (C_∞), a proxy for the backbone stiffness, scales with N_bb as a function of the grafting density: C_∞ ∼ N_bb^f(z). The scaling behavior disclosed herein provides valuable insights into conformational changes with grafting density, thus introducing opportunities for block polymer and material design.

Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions

Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP). One-pot copolymerization of an ω-norbornenyl macromonomer and a discrete norbornenyl comonomer (diluent) provides opportunities to control the backbone sequence and therefore the side chain distribution. Toward sequence control, the homopolymerization kinetics of 23 diluents were studied, representing diverse variations in the stereochemistry, anchor groups, and substituents. These modifications tuned the homopolymerization rate constants over 2 orders of magnitude (0.36 M^-1 s^-1 < k_homo < 82 M^-1 s^-1). Rate trends were identified and elucidated by complementary mechanistic and density functional theory (DFT) studies. Building on this foundation, complex architectures were achieved through copolymerizations of selected diluents with a poly(d,l-lactide) (PLA), polydimethylsiloxane (PDMS), or polystyrene (PS) macromonomer. The cross-propagation rate constants were obtained by nonlinear least-squares fitting of the instantaneous comonomer concentrations according to the Mayo-Lewis terminal model. In-depth kinetic analyses indicate a wide range of accessible macromonomer/diluent reactivity ratios (0.08 < r₁/r₂ < 20), corresponding to blocky, gradient, or random backbone sequences. We further demonstrated the versatility of this copolymerization approach by synthesizing AB graft diblock polymers with tapered, uniform, and inverse-tapered molecular "shapes." Small-angle X-ray scattering analysis of the self-assembled structures illustrates effects of the graft distribution on the domain spacing and backbone conformation. Collectively, the insights provided herein into the ROMP mechanism, monomer design, and homo- and copolymerization rate trends offer a general strategy for the design and synthesis of graft polymers with arbitrary architectures. Controlled copolymerization therefore expands the parameter space for molecular and materials design.

Fluorescent ROMP Monomers and Copolymers for Biomedical Applications

The synthesis and characterization of a series of green, blue and red-fluorescent exo-oxanorbornene acid and imide monomers carrying nitrobenzofurazan, coumarin, and Rhodamin B, respectively, as fluorophores is presented. These monomers carry oxanorbornene as polymerizable unit, and were readily copolymerized with bioactive functional oxanorbornene monomers by ring-opening metathesis polymerization (ROMP), as demonstrated by gel permeation chromatography and NMR spectroscopy. Due to the ease of synthesis of these monomers, and their cost-effectiveness compared many to other fluorescent probes, they are useful for biomaterials applications.

Light-Mediated Reversible Assembly of Polymeric Colloids

This contribution highlights the functionalization of colloidal particles featuring high-symmetry patches with telechelic block copolymers and subsequent reversible self-assembly of the resulting particles into longer chain and branched structures using host-guest complexation. The 3-(trimethoxysilyl)propyl methacrylate (TPM)-based anisotropic particles, obtained through a cluster-encapsulation process, consist of poly(styrene) patches and are site-specifically functionalized with block copolymers bearing pendant viologen or azobenzene motifs. Key to the design is the engineering of heterotelechelic α-hydroxy-ω-formyl-poly(norbornene)s via ring-opening metathesis polymerization (ROMP). The block copolymers feature both main chain anchor points to the particle surface, as well as orthogonal reactive sites for cyanine dye conjugation. The polymeric particles undergo directed and reversible supramolecular assembly in the presence of the host cucurbit[8]uril.

Glycosidase Inhibition by Multivalent Presentation of Heparan Sulfate Saccharides on Bottlebrush Polymers

We report herein the first-time exploration of the attachment of well-defined saccharide units onto a synthetic polymer backbone for the inhibition of a glycosidase. More specifically, glycopolymers endowed with heparan sulfate (HS) disaccharides were established to inhibit the glycosidase, heparanase, with an IC50 value in the low nanomolar range (1.05 ± 0.02 nm), a thousand-fold amplification over its monovalent counterpart. The monomeric moieties of these glycopolymers were designed in silico to manipulate the well-established glycotope of heparanase into an inhitope. Studies concluded that (1) the glycopolymers are hydrolytic stable toward heparanase, (2) longer polymer length provides greater inhibition, and (3) increased local saccharide density (monoantennary vs diantennary) is negligible due to hindered active site of heparanase. Furthermore, HS oligosaccharide and polysaccharide controls illustrate the enhanced potency of a multivalent scaffold. Overall, the results on these studies of the multivalent presentation of saccharides on bottlebrush polymers serve as the platform for the design of potent glycosidase inhibitors and have potential to be applied to other HS-degrading proteins.

Copolymerization of Norbornene and Norbornadiene Using a cis-Selective Bimetallic W-Based Catalytic System

The bimetallic cluster Na[W₂(μ-Cl)₃Cl₄(THF)₂]·(THF)₃ ({W₂}, {W ³ W}⁶⁺, a'²e'⁴), which features a triple metal-metal bond, is a highly efficient room-temperature initiator for ring opening metathesis polymerization (ROMP) of norbornene (NBE) and norbornadiene (NBD), providing high-cis polymers. In this work, {W₂} was used for the copolymerization of the aforementioned monomers, yielding statistical poly(norbornene)/poly(norbornadiene) PNBE/PNBD copolymers of high molecular weight and high-cis content. The composition of the polymer chain was estimated by ¹³C CPMAS NMR data and it was found that the ratio of PNBE/PNBD segments in the polymer chain was relative to the monomer molar ratio in the reaction mixture. The thermal properties of all copolymers were similar, resembled the properties of PNBD homopolymer and indicated a high degree of cross-linking. The morphology of all materials in this study was smooth and non-porous; copolymers with higher PNBE content featured a corrugated morphology. Glass transition temperatures were lower for the copolymers than for the homopolymers, providing a strong indication that those materials featured a branched-shaped structure. This conclusion was further supported by viscosity measurements of copolymers solutions in THF. The molecular structure of those materials can be controlled, potentially leading to well-defined star polymers via the "core-first" synthesis method. Therefore, {W₂} is not only a cost-efficient, practical, highly active, and cis-stereoselective ROMP-initiator, but it can also be used for the synthesis of more complex macromolecular structures.

Structural Color for Additive Manufacturing: 3D-Printed Photonic Crystals from Block Copolymers

The incorporation of structural color into 3D printed parts is reported, presenting an alternative to the need for pigments or dyes for colored parts produced through additive manufacturing. Thermoplastic build materials composed of dendritic block copolymers were designed, synthesized, and used to additively manufacture plastic parts exhibiting structural color. The reflection properties of the photonic crystals arise from the periodic nanostructure formed through block copolymer self-assembly during polymer processing. The wavelength of reflected light could be tuned across the visible spectrum by synthetically controlling the block copolymer molecular weight and manufacture parts that reflected violet, green, or orange light with the capacity to serve as selective optical filters and light guides.

Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization

This study investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization. Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative ¹³ C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, this study also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, the results suggested to us that chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) occurred when reaction times were extended.

The mechanism of activation of amidobenzylidene ruthenium chelates - latent catalysts of olefin metathesis

Amidobenzylidene ruthenium chelates - latent catalysts of olefin metathesis can be easily activated by the addition of Brønsted or Lewis acids. Their activation in the presence of hydrogen chloride involves the formation of catalytically active trans-dichloro carbamatobenzylidene ruthenium chelates.

Expanding the ROMP Toolbox: Synthesis of Air-Stable Benzonorbornadiene Polymers by Aryne Chemistry

Benzonorbornadiene polymers synthesized by ring-opening metathesis polymerization (ROMP) are typically prone to oxidation at the benzylic/allylic position under ambient conditions. Accordingly, the use of benzonorbornadiene polymers in practical applications has remained limited. In this manuscript, we report the synthesis of poly(benzonorbornadiene) polymers using a strategic blend of benzyne chemistry and ROMP. Through a comparative study, we show that substitution at the benzylic/allylic position prevents oxidative deformation, yet does not inhibit polymerization by common ruthenium catalysts with good control over molecular weight dispersity. We expect the benzyne/ROMP reaction sequence will allow easy access to air-stable benzonorbornadiene polymers for various applications.

Stereospecific reductive coupling polymerization of bis(benzylic gem-dibromide)s via formation of a trans-CC bond

Stereospecific polymers are constructed by stereospecific reductive coupling polymerization of bis(benzylic gem-dibromide)s via successive formation of a trans CC bond.

Synthesis of vanadium–alkylidene complexes and their use as catalysts for ring opening metathesis polymerization

Synthesis of vanadium–alkylidene complexes and some reactions have been reviewed; highly active, thermally robust, cis specific ROMP of cyclic olefins has been attained by ligand modification in (imido)vanadium(v)–alkylidene catalysts.

Enzyme-Responsive Nanoparticles for the Treatment of Disease

Nanomedicine for cancer therapy seeks to treat malignancies through the selective accumulation of therapeutics in diseased tissue. Nanoparticles offer the convenience of high drug loading capacities and can be readily decorated with targeting moieties, drugs, and/or diagnostics. Our lab has pioneered a new tissue targeting strategy where enhanced accumulation of nanomaterials occurs as a result of morphology changes to the material in response to overexpressed enzymes in diseased tissues. Herein, we describe the general strategy for the preparation of these enzyme-responsive nanoparticles (ER-NPs) for therapeutic applications.

Synthesis and characterization of naphthalimide-functionalized polynorbornenes

ABSTRACT

Highly fluorescent and photostable (2-alkyl)-1H-benzo[de]isoquinoline-1,3(2H)-diones with a polymerizable norbornene scaffold have been synthesized and polymerized using ring-opening metathesis polymerization. The monomers presented herein could be polymerized in a living fashion, using different comonomers and different monomer ratios. All obtained materials showed good film-forming properties and bright fluorescence caused by the incorporated push-pull chromophores. Additionally, one of the monomers containing a methylpiperazine functionality showed protonation-dependent photoinduced electron transfer which opens up interesting applications for logic gates and sensing.

GRAPHICAL ABSTRACT