Zwitterionic Ring Opening Polymerization with Isothioureas

Bicyclic isothioureas <b>1</b> and <b>2</b> mediate controlled ring opening polymerizations (ROP) of lactides in the absence of protic initiators to afford high molecular weight polylactides (PLA) with narrow polydispersities. The cyclic structure of the resulting PLA was determined by dilute solution viscosity measurement and MALDI-TOF mass spectrometry. Compared to DBU initiator, isothioureas are more selective for producing cyclic PLA without appreciable linear contaminants. Mechanistic studies involving acyl amidinium support our hypothesis that DBU-initiated ZROP generates linear chains from a ketene aminal intermediate

Organic Ring-Opening Polymerization Catalysts: Reactivity Control by Balancing Acidity

Organocatalysts derived from thioureas and amines exhibit high functional group tolerance and extraordinary selectivities for ring-opening relative to chain transesterification. The modest activities of the thiourea/amine catalysts prompted a detailed investigation of ureas and thiourea with organic bases for the ring-opening polymerization of lactones. An array of ureas or thioureas and organic bases were evaluated to assess the effect of the acidity of the urea (thiourea) and the basicity of the base cocatalyst on the activity for ring-opening polymerization. These studies reveal that for a given urea or thiourea stronger bases lead to faster rates. For a given base, the observed catalytic activity is highest when the acidity of the (thio)­urea is closely matched with that of the B–H⁺. For ureas and thioureas of comparable acidity, the urea/base catalyst systems are considerably more active than the corresponding thiourea/base systems. These results are consistent with two mechanisms: one mediated by deprotonated (thio)­urea anions when (thio)­ureas are combined with bases of sufficient basicity and one mediated by neutral (thio)­ureas when the base is incapable of deprotonating the (thio)­urea. Opposing trends in reactivity for (thio)­urea anions and neutral (thio)­ureas as a function of (thio)­urea acidity lead to the maximal activity when the acidities of the (thio)­ureas are closely matched with that of the protonated base (B–H⁺). These findings provide the basis for understanding the reactivity of ring-opening polymerization cocatalysts as well as guidelines for the rational design of other acid/base catalyst pairs

Organic Ring-Opening Polymerization Catalysts: Reactivity Control by Balancing Acidity

Organocatalysts derived from thioureas and amines exhibit high functional group tolerance and extraordinary selectivities for ring-opening relative to chain transesterification. The modest activities of the thiourea/amine catalysts prompted a detailed investigation of ureas and thiourea with organic bases for the ring-opening polymerization of lactones. An array of ureas or thioureas and organic bases were evaluated to assess the effect of the acidity of the urea (thiourea) and the basicity of the base cocatalyst on the activity for ring-opening polymerization. These studies reveal that for a given urea or thiourea stronger bases lead to faster rates. For a given base, the observed catalytic activity is highest when the acidity of the (thio)­urea is closely matched with that of the B–H⁺. For ureas and thioureas of comparable acidity, the urea/base catalyst systems are considerably more active than the corresponding thiourea/base systems. These results are consistent with two mechanisms: one mediated by deprotonated (thio)­urea anions when (thio)­ureas are combined with bases of sufficient basicity and one mediated by neutral (thio)­ureas when the base is incapable of deprotonating the (thio)­urea. Opposing trends in reactivity for (thio)­urea anions and neutral (thio)­ureas as a function of (thio)­urea acidity lead to the maximal activity when the acidities of the (thio)­ureas are closely matched with that of the protonated base (B–H⁺). These findings provide the basis for understanding the reactivity of ring-opening polymerization cocatalysts as well as guidelines for the rational design of other acid/base catalyst pairs

1,2-Dithiolane-Derived Dynamic, Covalent Materials: Cooperative Self-Assembly and Reversible Cross-Linking

The use of dithiolane-containing polymers to construct responsive and dynamic networks is an attractive strategy in material design. Here, we provide a detailed mechanistic study on the self-assembly and gelation behavior of a class of ABA triblock copolymers containing a central poly­(ethylene oxide) block and terminal polycarbonate blocks with pendant 1,2-dithiolane functionalities. In aqueous solution, these amphiphilic block copolymers self-assemble into bridged flower micelles at high concentrations. The addition of a thiol initiates the reversible ring-opening polymerizations of dithiolanes in the micellar cores to induce the cross-linking and gelation of the micellar network. The properties of the resulting hydrogels depend sensitively on the structures of 1,2-dithiolanes. While the methyl asparagusic acid-derived hydrogels are highly dynamic, adaptable, and self-healing, those derived from lipoic acid are rigid, resilient, and brittle. The thermodynamics and kinetics of ring-opening polymerization of the two dithiolanes were investigated to provide important insights on the dramatically different properties of the hydrogels derived from the two different dithiolanes. The incorporation of both dithiolane monomers into the block copolymers provides a facile way to tailor the properties of these hydrogels

Urea Anions: Simple, Fast, and Selective Catalysts for Ring-Opening Polymerizations

Aliphatic polyesters and polycarbonates are a class of biorenewable, biocompatible, and biodegradable materials. One of the most powerful methods for accessing these materials is the ring-opening polymerization (ROP) of cyclic monomers. Here we report that the deprotonation of ureas generates a class of versatile catalysts that are simultaneously fast and selective for the living ring-opening polymerization of several common monomers, including lactide, δ-valerolactone, ε-caprolactone, a cyclic carbonate, and a cyclic phosphoester. Spanning several orders of magnitude, the reactivities of several diaryl urea anions correlated to the electron-withdrawing substituents on the aryl rings. With the appropriate urea anions, the polymerizations reached high conversions (∼90%) at room temperature within seconds (1–12 s), yielding polymers with narrow molecular weight distributions (<i>Đ</i> = 1.06 to 1.14). These versatile catalysts are simple to prepare, easy to use, and exhibit a range of activities that can be tuned for the optimal performance of a broad range of monomers

Organocatalytic Ring-Opening Polymerization of Morpholinones: New Strategies to Functionalized Polyesters

The oxidative lactonization of N-substituted diethanolamines with the Pd catalyst [LPd­(OAc)]₂²⁺[OTf^–]₂ generates N-substituted morpholin-2-ones. The organocatalytic ring-opening polymerization of <i>N</i>-acyl morpholin-2-ones occurs readily to generate functionalized poly­(aminoesters) with <i>N</i>-acylated amines in the polyester backbone. The thermodynamics of the ring-opening polymerization depends sensitively on the hybridization of the nitrogen of the heterocyclic lactone. <i>N</i>-Acyl morpholin-2-ones polymerize readily to generate polymorpholinones, but the <i>N</i>-aryl or <i>N</i>-alkyl substituted morpholin-2-ones do not polymerize. Experimental and theoretical studies reveal that the thermodynamics of ring opening correlates to the degree of pyramidalization of the endocyclic N-atom. Deprotection of the poly­(<i>N</i>-Boc-morpholin-2-one) yields a water-soluble, cationic polymorpholinone

Octahedral Group IV Bis(phenolate) Catalysts for 1‑Hexene Homopolymerization and Ethylene/1-Hexene Copolymerization

Octahedral group IV bis­(phenolate) catalysts are highly active catalysts for the isospecific polymerization of 1-hexene and the copolymerization of ethylene with 1-hexene. These catalysts are active for the production of high molecular weight copolymers even at 130 °C. The copolymerization parameters for these complexes were determined; all of the bis­(phenolate) complexes tested incorporate 1-hexene with high efficiency to give random copolymers. The complexes prepared from the more sterically demanding ligands showed higher molecular weights but similar comonomer incorporations to those prepared from the less sterically demanding ligands

Structurally Dynamic Hydrogels Derived from 1,2-Dithiolanes

The design and generation of adaptable materials derived from structurally dynamic polymers provides a strategy for generating smart materials that can respond to environmental stimuli or exhibit self-healing behavior. Herein we report an expedient organocatalytic ring-opening polymerization of cyclic carbonates containing pendant dithiolanes (trimethylene carbonate/dithiolane, TMCDT) from poly­(ethylene oxide) diols to generate water-soluble triblock (ABA) copolymers containing a central poly­(ethylene oxide) block and terminal dithiolane blocks. Hydrogels generated from the triblock copolymers and a cross-linking dithiol exhibited dynamic behavior as a result of the reversible ring opening of the pendant 1,2-dithiolanes. These materials exhibit self-healing behavior, can be injected through a syringe, and rapidly recover their mechanical properties after a severe strain deformation. The dynamic properties of these gels can be modulated with the number of dithiolane units, pH, and temperature

Synthesis and Topological Trapping of Cyclic Poly(alkylene phosphates)

The zwitterionic ring-opening polymerization of 2-isopropoxy-2-oxo-1,3,2-dioxaphospholane (iPP) with <i>N</i>-heterocyclic carbenes (NHC) generates poly­(alkylene phosphate)­s with molecular weights of <i>M</i>_n = 55000–202000 Da. MALDI-TOF mass spectrometry provided clear evidence for cyclic poly­(alkylene phosphate)­s (poly­(iPP)) for lower molecular weight fractions (<i>m</i>/<i>z</i> ≤ 3000). The cyclic topology of the higher molecular weight fractions was inferred by trapping of poly­(iPP) in cross-linked 2-hydroxyethyl methacrylate (HEMA) hydrogels. Cross-linked HEMA hydrogels were generated in the presence of a high molecular weight (<i>M</i>_n = 202000 Da) poly­(iPP) generated from the zwitterionic ring-opening polymerization of iPP with the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene <b>2</b>. Extraction of the resulting gel with methanol for 11 days revealed that 36% of the poly­(iPP) was retained in the gel, whereas a linear poly­(iPP) was completely extracted under similar conditions. The retention of the poly­(iPP)­s in the gels is attributed to topological trapping of the cyclic poly­(iPP) in the cross-linked network

Zwitterionic Polymerization to Generate High Molecular Weight Cyclic Poly(Carbosiloxane)s

The zwitterionic ring-opening of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane (TMOSC) with <i>N</i>-heterocyclic carbenes generates high molecular weight cyclic p­(TMOSC). The NHC-mediated polymerization of TMOSC with 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene (IMes, <b>1</b>) generates the poly­(carbosiloxane) p­(TMOSC) with molecular weights from 27 000 < <i>M</i>_n < 80 000 Da (1.4 < <i>M</i>_w/<i>M</i>_n < 2.2) within 30 min at room temp. With the more nucleophilic carbene 1,3,4,5-tetramethyl-imidazol-2-ylidene (<b>4</b>), the ring-opening polymerization occurs within minutes at room temperature to generate cyclic p­(TMOSC) with molecular weights up to <i>M</i>_n = 940 000 Da (<i>M</i>_w/<i>M</i>_n = 3.2). The resulting p­(TMOSC)­s are predominantly cyclic as evidenced by dilute solution viscosity studies and MALDI-TOF MS. DFT calculations provide support for both zwitterionic and neutral, cyclic intermediates