A “Catalyst Switch” Strategy for the Sequential Metal-Free Polymerization of Epoxides and Cyclic Esters/Carbonate

Catalyst switch strategy enabled a single polymer with five different crystalline phases

Well-defined multicrystalline multiblock polymers are essential model polymers for advancing crystallization physics, phase separation, self-assembly, and improving the mechanical properties of materials. However, due to different chain properties and incompatible synthetic methodologies, multicrystalline multiblock polymers with more than two crystallites are rarely reported. Herein, by combining polyhomologation, ring-opening polymerization, and catalyst switch strategy, we synthesized a pentacrystalline pentablock quintopolymer, polyethylene-b-poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(L-lactide)-b-polyglycolide (PE-b-PEO-b-PCL-b-PLLA-b-PGA). The fluoroalcohol-assisted catalyst switch enables the successful incorporation of a high melting point polyglycolide block into the complex multiblock structure. Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and differential scanning calorimetry revealed the existence of five different crystalline phases.

Copolymerization of Carbon Dioxide with 1,2-Butylene Oxide and Terpolymerization with Various Epoxides for Tailorable Properties

The copolymerization of carbon dioxide (CO₂) with epoxides demonstrates promise as a new synthetic method for low-carbon polymer materials, such as aliphatic polycarbonate materials. In this study, a binary Schiff base cobalt system was successfully used to catalyze the copolymerization of 1,2-butylene oxide (BO) and CO₂ and its terpolymerization with other epoxides such as propylene oxide (PO) and cyclohexene oxide (CHO). ¹H nuclear magnetic resonance (¹H NMR), diffusion-ordered spectroscopy (DOSY), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC) confirmed the successful synthesis of the alternating terpolymer. In addition, the effects of the polymerization reaction conditions and copolymerization monomer composition on the polymer structure and properties were examined systematically. By regulating the epoxide feed ratio, polycarbonates with an adjustable glass transition temperature (T_g) (11.2-67.8 °C) and hydrophilicity (water contact angle: 85.2-95.2°) were prepared. Thus, this ternary polymerization method provides an effective method of modulating the surface hydrophobicity of CO₂-based polymers and their biodegradation properties.

Diblock dialternating terpolymers by one-step/one-pot highly selective organocatalytic multimonomer polymerization

The synthesis of well-defined block copolymers from a mixture of monomers without additional actions ("one-pot/one-step") is an ideal and industrially valuable method. In addition, the presence of controlled alternating sequences in one or both blocks increases the structural diversity of polymeric materials, but, at the same time, the synthetic difficulty. Here we show that the "one-pot/one-step" ring-opening terpolymerization of a mixture of three monomers (N-sulfonyl aziridines; cyclic anhydrides and epoxides), with tert-butylimino-tris(dimethylamino)phosphorene (t-BuP1) as a catalyst, results in perfect diblock dialternating terpolymers having a sharp junction between the two blocks, with highly-controllable molecular weights and narrow molecular weight distributions (Ð < 1.08). The organocatalyst switches between two distinct polymerization cycles without any external stimulus, showing high monomer selectivity and kinetic control. The proposed mechanism is based on NMR, in-situ FTIR, SEC, MALDI-ToF, reactivity ratios, and kinetics studies.

Hybrid Copolymerization of Ethylene Oxide and tert-Butyl Methacrylate with Organocatalyst

Hybrid copolymerization of structurally different, reactivity and mechanism distinct monomers (e.g., cyclic and vinyl type monomers) is of great interest and challenge for both academic research and practical application. Herein, ethylene oxide-co-tert-butyl methacrylate-co-poly(ethylene glycol) benzyl methacrylate (EO-co-BMA-co-bPEO), a statistical copolymer was synthesized via hybrid copolymerization of EO and BMA using an uncharged, non-nucleophilic organobase t-BuP₄ as the catalyst. Detailed characterizations indicate that hybrid copolymerization of ethylene oxide and vinyl monomer forms a statistical copolymer concurrently with the transesterification of tert-butyl group and oligomer PEO anions. The application of the copolymer as all solid lithium-ion battery polymer electrolyte was investigated by detecting the ionic conductivity (σ) with electrical impedance spectrum measurement.

Amphiphilic diblock copolymers of poly(glycidol) with biodegradable polyester/polycarbonate. organocatalytic one-pot ROP and self-assembling property

Poly(glycidol)-based block copolymers with excellent micelle formation properties were prepared via organocatalytic one-pot ROP.

Strategies for the synthesis of block copolymers with biodegradable polyester segments

Oxygenated block copolymers with biodegradable polyester segments can be prepared in one-pot through sequential or simultaneous addition of monomers. This review highlights the state of the art in this area.

Organocatalytic sequential ring-opening polymerization of cyclic ester/epoxide and N-sulfonyl aziridine: metal-free and easy access to block copolymers

Sequential ring-opening polymerization of ε-caprolactone (ε-CL)/propylene oxide (PO) and N-sulfonyl aziridine switched by tosyl isocyanate (TSI) allows the metal-free synthesis of polysulfonamide-based copolymers.

Poly(trimethylene carbonate-co-valerolactone) copolymers are materials with tailorable properties: from soft to thermoplastic elastomers

Aliphatic poly(ester-carbonates) are receiving extensive research attention as tailorable materials suitable for multiple applications from tissue engineering and 3D scaffold printing to drug delivery. Thus, simple reliable procedures for producing easily tailorable poly(ester-carbonates) without metal residues are continuously sought after. In this work, we report on one-pot synthesis of random copolymers of TMC and δ-VL using metal-free biocompatible 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a catalyst and benzyl alcohol and poly(ethylene oxide) as initiators. Random poly(ester-carbonates) with TMC : VL unit ratios ranging from 80 : 20 to 20 : 80 were synthesized via ring-opening polymerization while displaying excellent agreement of comonomers' ratios in the feed and copolymer chains. The copolymers' supramolecular structure, thermal and mechanical properties were thoroughly analyzed by various methods. The obtained results clearly indicated that the physicochemical properties can be controlled simply by varying the ratio of comonomers and the length of segments in the copolymer chain. Several copolymers exhibited behavior of thermoplastic elastomers with the most promising one exhibiting a 2200% increase in elongation at break compared to the poly(valerolactone) homopolymer while retaining tensile strength and Young's modulus suitable for biomedical applications. Overall, our work contributed to widening the portfolio of tailorable copolymers for specialized bioapplications and possibly paving a way for the use of more sustainable polymers in the biomedical field.

Organocatalytic sequential ring-opening polymerization of a cyclic ester and anionic polymerization of a vinyl monomer

Organocatalysis has provided new tools for making block copolymers, in particular active species able to polymerize monomers of different chemical nature such as cyclic esters, cyclic carbonates and epoxides. We report herein the first example of an organocatalytic active species able to polymerize sequentially a cyclic ester, ε-decalactone, and a vinyl monomer, methyl methacrylate. The resulting block copolymer shows the properties of thermoplastic elastomers.

Synthesis of poly(disulfide)s with narrow molecular weight distributions via lactone ring-opening polymerization

We report the first example of controlled polymerization of poly(disulfide)s with narrow molecular weight distributions. 1,4,5-oxadithiepan-2-one (OTP), a disulfide-containing 7-membered ring lactone, was polymerized by using the diphenylphosphate (DPP) catalyzed lactone ring-opening polymerization method. The polymerization proceeded in a living manner, and the resulting polymers displayed very narrow polydispersity index (PDI) values below 1.1 and excellent backbone degradability responding to reducing conditions and UV irradiation.

We report the first example of controlled polymerization of poly(disulfide)s with narrow molecular weight distributions.

Turning natural δ-lactones to thermodynamically stable polymers with triggered recyclability

Extending the use of natural δ-lactones in circular materials via a synthetic strategy yielding thermodynamically stable polyesters with triggered recyclability.

Tetracrystalline Tetrablock Quarterpolymers: Four Different Crystallites under the Same Roof

Multicrystalline block polymers having three or more crystalline segments are essential materials for the advancement of physics in the field of crystallinity. The challenging synthesis of multicrystalline polymers has resulted in only a limited number of tricrystalline terpolymers having been reported to date. We report, for the first time, the synthesis of polyethylene-b-poly(ethylene oxide)-b-poly(ϵ-caprolactone)-b-poly(l-lactide) (PE-b-PEO-b-PCL-b-PLLA), a tetracrystalline tetrablock quarterpolymer, by combining polyhomologation, ring-opening polymerization, and an organic/metal "catalyst switch" strategy. ¹ H NMR spectroscopy and gel-permeation chromatography confirmed the formation of the tetrablock quarterpolymer, while differential scanning calorimetry, X-ray diffraction, and wide-line separation solid-state NMR spectroscopy revealed the existence of four different crystalline domains.

One-Pot Synthesis of Star Copolymers by the Combination of Metal-Free ATRP and ROP Processes

A completely metal-free strategy is demonstrated for the preparation of star copolymers by combining atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP) for the syntheses of block copolymers. These two different metal-free controlled/living polymerizations are simultaneously realized in one reaction medium in an orthogonal manner. For this purpose, a specific core with functional groups capable of initiating both polymerization types is synthesized. Next, vinyl and lactone monomers are simultaneously polymerized under visible light irradiation using specific catalysts. Spectral and chromatographic evidence demonstrates the success of the strategy as star copolymers are synthesized with controlled molecular weights and narrow distributions.

Biased Lewis Pairs: A General Catalytic Approach to Ether-Ester Block Copolymers with Unlimited Ordering of Sequences

Polymerizing epoxides after cyclic esters remains a major challenge, though their block copolymers have been extensively studied and used for decades. Reported here is a simple catalytic approach based on a metal-free Lewis pair that addresses the challenge. When the Lewis acid is used in excess of a base, selective (transesterification-free) polymerization of epoxides occurs in the presence of esters, while selectivity toward cyclic esters is achieved by an oppositely biased catalyst. Hence, one-pot block copolymerization can be performed in both ester-first and ether-first orders with selectivity being switchable at any stage, yielding ether-ester-type block copolymers with unlimited ordering of sequences as well as widely variable compositions and architectures. The selectivity can also be switched back and forth several times to generate a multiblock copolymer. Experimental and calculational results indicate that the selectivity originates mainly from the state of catalyst-activated hydroxy species.

Easy access to oxygenated block polymers via switchable catalysis

Oxygenated block polyols are versatile, potentially bio-based and/or degradable materials widely applied in the manufacture of coatings, resins, polyurethanes and other products. Typical preparations involve multistep syntheses and/or macroinitiator approaches. Here, a straightforward and well-controlled one-pot synthesis of ABA triblocks, namely poly(ether-b-ester-b-ether), and ABCBA pentablocks, of the form poly(ester-b-ether-b-ester’-b-ether-b-ester), using a commercial chromium catalyst system is described. The polymerization catalysis exploits mechanistic switches between anhydride/epoxide ring-opening copolymerization, epoxide ring-opening polymerization and lactone ring-opening polymerization without requiring any external stimuli. Testing a range of anhydrides, epoxides and chain-transfer agents reveals some of the requirements and guidelines for successful catalysis. Following these rules of switch catalysis with multiple monomer additions allows the preparation of multiblock polymers of the form (ABA)_n up to 15 blocks. Overall, this switchable catalysis delivers polyols in a straightforward and highly controlled manner. As proof of potential for the materials, methods to post-functionalize and/or couple the polyols to make higher polymers are demonstrated.

Multiblock oxygenated polyols often show better properties than the constituent polyols, but their synthesis can be complex and difficult. Here a switchable catalysis concept is described which allows for the efficient preparation of multiblock poly(ether-b-ester) materials starting from mixtures of common monomers.

N-Heterocyclic olefins and thioureas as an efficient cooperative catalyst system for ring-opening polymerization of δ-valerolactone

An organocatalytic ring-opening polymerization of δ-valerolactone has been developed.

Sequence-Selective Terpolymerization from Monomer Mixtures Using a Simple Organocatalyst

One-step synthesis of block copolymer from mixed monomers is of great interest and challenge. Using a simple non-nucleophilic organobase as the catalyst, we have achieved sequence-selective terpolymerization from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA). Alcohol-initiated alternating copolymerization of PA and epoxide occurs first and exclusively because PA is substantially more active than LA for reacting with base-activated hydroxyl. When PA is fully consumed, LA polymerizes from the termini of the first block while excess epoxide stays intact because of the mild basicity of the catalyst. The two polymerizations thus occur tandemly, both in chemoselective manners, so that an aromatic-aliphatic block copolyester is generated in this one-step synthesis. The effectiveness and versatility of this approach is demonstrated by the use of ethylene oxide and several monosubstituted epoxides as well as mono-, di-, or tetrahydroxy initiators.

Betulin-Constituted Multiblock Amphiphiles for Broad-Spectrum Protein Resistance

Multiblock-like amphiphilic polyurethanes constituted by poly(ethylene oxide) and biosourced betulin are designed for antifouling and synthesized by a convenient organocatalytic route comprising tandem chain-growth and step-growth polymerizations. The doping density of betulin (D_B) in the polymer chain structure is readily varied by a mixed-initiator strategy. The spin-coated polymer films exhibit unique nanophase separation and protein resistance behaviors. Higher D_B leads to enhanced surface hydrophobicity and, unexpectedly, improved protein resistance. It is found that the surface holds molecular-level heterogeneity when D_B is substantially high due to restricted phase separation; therefore, broad-spectrum protein resistance is achieved despite considerable surface hydrophobicity. As D_B decreases, the distance between adjacent betulin units increases so that hydrophobic nanodomains are formed, which provide enough landing areas for relatively small-sized proteins to adsorb on the surface.

A switch from anionic to bifunctional H-bonding catalyzed ring-opening polymerizations towards polyether–polyester diblock copolymers

A switch of an anionic ROP of epoxides into a bifunctional H-bonding ROP of cyclic esters paved a new avenue to one-pot, sequential, and block copolymerizations to previously rare polyether-block-polyester copolymers.

Synthesis of a ROS-responsive analogue of poly(ε-caprolactone) by the living ring-opening polymerization of 1,4-oxathiepan-7-one

A well-defined ROS-responsive block amphiphilic diblock copolymer PEO-b-POTO was synthesized to elucidate the oxidative degradation mechanism in assemblies.

Hybrid block copolymers of polyesters/polycarbonates and polypeptides synthesized via one-pot sequential ring-opening polymerization

Polyester/polycarbonate-b-polypeptide hybrid block copolymers were synthesized by a sequential ring-opening polymerization in a one-pot manner.

Controlled preparation of amphiphilic triblock-copolyether in a metal- and solvent-free approach for tailored structure-directing agents

Organocatalysis is employed as a precise tool for the preparation of triblock-copolyethers for a systematic access to structure-direction.

A sustainable photocontrolled ATRP strategy: facile separation and recycling of a visible-light-mediated catalyst fac-[Ir(ppy)3]

A visible-light-mediated catalyst (fac-[Ir(ppy)₃]) in situ separation and recycling ATRP system for PEG-based water-soluble monomers was constructed.

Anionic Polymerization of Styrene and 1,3-Butadiene in the Presence of Phosphazene Superbases

The anionic polymerization of styrene and 1,3-butadiene in the presence of phosphazene bases (t-BuP₄, t-BuP₂ and t-BuP₁), in benzene at room temperature, was studied. When t-BuP₁ was used, the polymerization proceeded in a controlled manner, whereas the obtained homopolymers exhibited the desired molecular weights and narrow polydispersity (Ð < 1.05). In the case of t-BuP₂, homopolymers with higher than the theoretical molecular weights and relatively low polydispersity were obtained. On the other hand, in the presence of t-BuP₄, the polymerization of styrene was uncontrolled due to the high reactivity of the formed carbanion. The kinetic studies from the polymerization of both monomers showed that the reaction rate follows the order of [t-BuP₄]/[sec-BuLi] >>> [t-BuP₂]/[sec-BuLi] >> [t-BuP₁]/[sec-BuLi] > sec-BuLi. Furthermore, the addition of t-BuP₂ and t-BuP₁ prior the polymerization of 1,3-butadiene allowed the synthesis of polybutadiene with a high 1,2-microstructure (~45 wt %), due to the delocalization of the negative charge. Finally, the one pot synthesis of well-defined polyester-based copolymers [PS-b-PCL and PS-b-PLLA, PS: Polystyrene, PCL: Poly(ε-caprolactone) and PLLA: Poly(L-lactide)], with predictable molecular weights and a narrow molecular weight distribution (Ð < 1.2), was achieved by sequential copolymerization in the presence of t-BuP₂ and t-BuP₁.