Access to Bicyclo[4.2.0]octene Monomers To Explore the Scope of Alternating Ring-Opening Metathesis Polymerization

Glycopolymers Prepared by Alternating Ring-Opening Metathesis Polymerization Provide Access to Distinct, Multivalent Structures for the Probing of Biological Activity

A myriad of biological processes are facilitated by ligand-receptor interactions. The low affinities of these interactions are typically enhanced by multivalent engagements to promote binding. However, each biological interaction requires a unique display and orientation of ligands. Therefore, the availability and diversity of synthetic multivalent probes are invaluable to the investigation of ligand-receptor binding interactions. Here, we report glycopolymers prepared from bicyclo[4.2.0]oct-6-ene-7-carboxamide and 4,7-dihydro-1,3-dioxepin or cyclohexene. These glycopolymers, synthesized by alternating ring-opening metathesis polymerization, display precise ligand spacing as well as the option of a hydrophobic or acetal-functionalized polymer backbone. Small-angle X-ray scattering (SAXS) data analysis revealed that these [4.2.0] glycopolymers adopted distinct conformations in solution. In aqueous media, [4.2.0]-dioxepin glycopolymers formed swollen polymer chains with rod-like, flexible structures while [4.2.0]-cyclohexene glycopolymers assumed compact, globular structures. To illustrate how these glycopolymers could aid in the exploration of ligand-receptor interactions, we incorporated the [4.2.0] glycopolymers into a biological assay to assess their potential as activators of acrosomal exocytosis (AE) in mouse sperm. The results of the biological assay confirmed that the differing structures of the [4.2.0] glycopolymers would evoke distinct biological responses; [4.2.0]-cyclohexene glycopolymers induced AE in mouse sperm while [4.2.0]-dioxepin glycopolymers did not. Herein, we provide two options for glycopolymers with low to moderate molecular weight dispersities and low cytotoxicity that can be implemented into biological assays based on the desired hydrophobicity, rigidity, and structural conformation of the polymer probe.

The Fluorocarbene Exploit: Enforcing Alternation in Ring-Opening Metathesis Polymerization

Fluoroalkenes are known to be notoriously reluctant substrates for olefin metathesis due to the generation of thermodynamically stable Fischer-type fluorocarbene intermediates, which invariably fail to undergo further reaction. In the present disclosure, we find that fluorine substitution on the sp² carbon also strictly suppresses homopolymerization of norbornene derivatives (NBEs), and this can be harnessed to achieve alternating ring-opening metathesis polymerization (ROMP) with an appropriately electron-rich comonomer. Dihydrofuran (DHF) is thereby shown to undergo alternating ROMP with fluorinated norbornenes, the perfectly alternating structure of the resulting copolymer having been unambiguously elucidated by ¹H, ¹⁹F, and ¹³C NMR analyses. Furthermore, we find that the degradability of the resultant copolymers in acidic media via hydrolysis of enol ether moieties in the backbone can be predictably modulated by the number of fluorine atoms present in the NBE comonomer, affording an opportunity to engage with the desirable physical properties of fluorinated polymers while limiting their attendant environmental degradability issues.

Cascade Alternating Metathesis Cyclopolymerization of Diynes and Dihydrofuran

Ruthenium alkoxymethylidene complexes have recently come into view as competent species for metathesis copolymerization reactions when coupled with appropriate comonomer targets. Here, we explore the ability of Fischer-type carbenes to participate in cascade alternating metathesis cyclopolymerization (CAMC) through facile terminal alkyne addition. The combination of diyne monomers and an equal feed ratio of low-strain dihydrofuran leads to a controlled chain-growth copolymerization with high degrees of alternation (>97% alternating diads) and produces degradable polymer materials with low dispersities and targetable molecular weights. When combined with enyne monomers, this method is amenable to the synthesis of alternating diblock copolymers that can be fully degraded to short oligomer fragments under aqueous acidic conditions. This work furthers the potential for the generation of functional metathesis materials via Fischer-type ruthenium alkylidenes.

Substituent Effects Provide Access to Tetrasubstituted Ring-Opening Olefin Metathesis of Bicyclo[4.2.0]oct-6-enes

Herein, we report the origin of unexpected reactivity of bicyclo[4.2.0]oct-6-ene substrates containing an α,β-unsaturated amide moiety in ruthenium-catalyzed alternating ring-opening metathesis polymerization reactions. Specifically, compared with control substrates bearing an ester, alkyl ketone, nitrile, or tertiary amide substituent, α,β-unsaturated substrates with a weakly acidic proton showed increased rates of ring-opening metathesis mediated by Grubbs-type ruthenium catalysts. ¹H NMR and IR spectral analyses indicated that deprotonation of the α,β-unsaturated amide substrates resulted in stronger coordination of the carbonyl group to the ruthenium metal center. Principal component analysis identified ring strain and the electron density on the carbonyl oxygen (based on structures optimized by means of ωB97X-D/6311+G(2df,2p) calculations) as the two key contributors to fast ring-opening metathesis of the bicyclo[4.2.0]oct-6-enes; whereas the dipole moment, conjugation, and energy of the highest occupied molecular orbital had little to no effect on the reaction rate. We conclude that alternating ring-opening metathesis polymerization reactions of bicyclo[4.2.0]oct-6-enes with unstrained cycloalkenes require an ionizable proton for efficient generation of alternating polymers.

Cationic Amphiphilic Alternating Copolymers with Tunable Morphology

A series of ionic amphiphilic alternating copolymers were characterized via SAXS, TEM and DLS to help understand factors that could potentially affect self-assembly, including the degree of polymerization, the length of hydrophobic spacers between ionic units, the distance between charged groups and polymer backbone, solvent envrioment and counterions.

Alternating Cascade Metathesis Polymerization of Enynes and Cyclic Enol Ethers with Active Ruthenium Fischer Carbenes

Ruthenium alkoxymethylidene complexes have rarely been demonstrated as active species in metathesis reactions and are frequently regarded as inert. Herein, we highlight the ability of these Fischer-type carbenes to participate in cascade alternating ring-opening metathesis polymerization through their efficient alkyne addition reactions. When enyne monomers are combined with low-strain cyclic vinyl ethers, a controlled chain-growth copolymerization occurs that exhibits high degrees of alternation (>90% alternating diads) and produces degradable poly(vinyl ether) materials with low dispersities and targetable molecular weights. This new method is amenable to the synthesis of alternating diblock polymers that can be degraded to small-molecule fragments under aqueous acidic conditions. This work furthers the potential of Fischer-type ruthenium alkylidenes in polymerization strategies and presents new avenues for the generation of functional metathesis materials.

Alternating Ring-Opening Metathesis Polymerization Provides Easy Access to Functional and Fully Degradable Polymers

Polymers with hydrolyzable groups in their backbones have numerous potential applications in biomedicine, lithography, energy storage and electronics. In this study, acetal and ester functionalities were incorporated into the backbones of copolymers by means of alternating ring-opening metathesis polymerization catalyzed by third-generation Grubbs ruthenium catalyst. Specifically, combining large-ring (7-10 atoms) cyclic acetal or lactone monomers with bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide monomers provided perfectly alternating copolymers with acetal or ester functionality in the backbones and low to moderate molecular weight distribution (Đ M = 1.2-1.6). Copolymers containing ester and acetal backbones hydrolyzed to significant extent under basic condition (pH 13) and acidic conditions (pH ≤ 5) respectively to yield the expected by-products within 30 hours at moderate temperature. Unlike the copolymer with all-carbon backbone, copolymers with heteroatom-containing backbone exhibited viscoelastic behavior with crossover frequency which decreases as the size of the R group on the acetal increases. In contrast, the glass transition temperature (T g) decreases as the size of the R group decreases. The rate of hydrolysis of the acetal copolymers was also dependent on the R group. Thus, ruthenium-catalyzed alternating ring-opening metathesis copolymerization provides heterofunctional copolymers whose degradation rates, glass transition temperatures, and viscoelastic moduli can be controlled.

Incorporation of Large Cycloalkene Rings into Alternating Copolymers Allows Control of Glass Transition and Hydrophobicity

We previously reported that cyclohexene and bicyclo[4.2.0]oct-1(8)-ene-8-carboxamides undergo efficient ruthenium-catalyzed alternating ring-opening metathesis polymerization (AROMP). Here, we demonstrate that cyclodecene and cyclododecene also function as cycloalkene monomers in the bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide AROMP system, thus enabling the synthesis of linear alternating copolymers with spacers of different lengths, as demonstrated by means of NMR spectroscopy and gel permeation chromatography. The glass transition temperature and hydrophilicity of the alternating copolymers decrease as the length of the spacers increases, as determined by differential scanning calorimetry and water contact angle measurements.

Alternating Ring-Opening Metathesis Polymerization (AROMP) of Hydrophobic and Hydrophilic Monomers Provides Oligomers with Side-Chain Sequence Control

We report the formation of oligomers with side-chain sequence control using ruthenium-catalyzed alternating ring-opening metathesis polymerization (AROMP). These oligomers are prepared through sequential, stoichiometric addition of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide (monomer A) at 85 °C and cyclohexene (monomer B) at 45 °C to generate sequences up to 24 monomeric units composed of (A-alt- B) n and (A'-alt-B) n microblocks, where n ranges from 1 to 6. Herein, monomer A has an alkyl side chain, and monomer A' has a glycine methyl ester side chain. Increasing microblock size from one to six results in an increasing water contact angle on spin-coated thin films, despite the constant ratio of hydrophilic and hydrophobic moieties. However, a disproportionately high contact angle was observed when n equals 2. Thus, the unique all-carbon backbone formed in the AROMP of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamides and cyclohexene provides a platform for the nontemplated preparation of materials with specific sequences of side chains.