In Situ Generated ABA Block Copolymers from CO2, Cyclohexene Oxide, and Poly(dimethylsiloxane)s

Directed evolution of enzymatic silicon-carbon bond cleavage in siloxanes

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Controlled synthesis of polycarbonate diols and their polylactide block copolymers using amino-bis(phenolate) chromium hydroxide complexes

A diamine-bis(phenolate) chromium(III) complex, CrOH[L] ([L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate)), 2, in the presence of tetrabutylammonium hydroxide effectively copolymerizes CO2 and cyclohexene oxide (CHO) into a polycarbonate diol. The resultant low molar mass (6.3 kg mol-1) diol is used to initiate ring-opening polymerization of rac-lactide with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) giving ABA-type block copolymers with good molar mass control through varying rac-LA-to-diol loadings and with narrow dispersities. As the degree of rac-LA incorporation increases, the glass transition temperatures (Tg) are found to decrease, whereas decomposition temperatures (Td) increase. (Diphenylphosphonimido)triphenylphosphorane (Ph2P(O)NPPh3) was used as a neutral nucleophilic cocatalyst with 2, giving phosphorus-containing polycarbonates with an Mn value of 28.5 kg mol-1, a dispersity of 1.13, a Tg value of 110 °C and a Td value of over 300 °C. A related Cr(III) complex (4) having a methoxyethyl pendent group rather than a dimethylaminoethyl group was structurally characterized as a hydroxide-bridged dimer.

Switchable Polymerization: A Practicable Strategy to Produce Biodegradable Block Copolymers with Diverse Properties

With the global demand for sustainable development, there has been an increasing interest in using natural biomass as raw resources to produce sustainable polymers as an alternative to petroleum-based polymers. Because monocomponent biodegradable polymers are often insufficient in performance, copolymers with well-engineered block structures are synthesized to reach wide tunability. Switchable polymerization is such a practical strategy to produce biodegradable block copolymers with diverse performance. This review focus on the performance of block copolymers bearing biodegradable polymer segments produced by diverse switchable polymerization. We highlight two main segments that are critical for biodegradable block copolymers, i. e., polyester and polycarbonate, summarize the multiple characters of materials from switchable polymerization such as antibacterial, shape memory, adhesives, etc. The state-of-the-art research on biodegradable block copolymers, as well as an outlook on the preparation and application of novel materials, are presented.

Two-in-One: Photothermal Ring-Opening Copolymerization of CO2 and Epoxides

A two-in-one strategy for the photothermal ring-opening copolymerization (PROCOP) of carbon dioxide (CO₂) and epoxides was developed by using visible light as an external stimulus. This strategy bridges two processes involving light-to-heat conversion and the alternating copolymerization of CO₂ and epoxides. As a proof-of-concept, aluminum porphyrin complexes were explored as photothermal catalysts to afford the copolymerization of CO₂/epoxides under a 635 nm laser irradiation. We demonstrated photothermally enhanced polymerization activity, in which the polymerization initiated by the photothermal effect showed a much higher turnover frequency than in the thermal system. Moreover, the PROCOP demonstrated a spatial-temporal control by a light on/off switch. This study provides a fascinating photothermal strategy not only for the CO₂/epoxides copolymerization but also for the ring-opening (co)polymerization of other cyclic monomers.

Switchable Polymerization Organocatalysis: From Monomer Mixtures to Block Copolymers

One-pot production of sequence-controlled block copolymer from mixed monomers is a crucial but rarely reached goal. Using a switchable Lewis-pair organocatalyst, we have accomplished sequence-selective polymerization from a mixture of O-carboxyanhydride (OCA) and epoxide. Polymerization of the OCA monomer occurs first and exclusively because of its exceedingly high polymerizability. When OCA is fully consumed, alternating copolymerization of epoxide and CO₂ liberated in OCA polymerization is triggered from the termini of the first block. The two polymerizations thus occur in tandem, both in chemoselective fashion, so that a sequence-controlled block polymer with up to 99 % CO₂ conversion is furnished in this one-pot protocol. Calculations and experimental results demonstrate a chemoselective and cooperative mechanism, where the high polymerizability of the OCA monomers guarantees exquisite sequence selectivity and the cooperative decarboxylation partly arose from the stabilization effect by triethylborane, which facilitates the smooth transformation of the chain end from carbonate to alkoxide.

Orthogonally grown polycarbonate and polyvinyl block copolymers from mechanistically distinct (co)polymerizations

Mechanistically distinct polymerization systems can afford unique block copolymers that would not be accessible by mere sequential polymerization.

Switchable Catalysis Improves the Properties of CO2-Derived Polymers: Poly(cyclohexene carbonate-b-ε-decalactone-b-cyclohexene carbonate) Adhesives, Elastomers, and Toughened Plastics

Carbon dioxide/epoxide copolymerization is an efficient way to add value to waste CO2 and to reduce pollution in polymer manufacturing. Using this process to make low molar mass polycarbonate polyols is a commercially relevant route to new thermosets and polyurethanes. In contrast, high molar mass polycarbonates, produced from CO2, generally under-deliver in terms of properties, and one of the most widely investigated, poly(cyclohexene carbonate), is limited by its low elongation at break and high brittleness. Here, a new catalytic polymerization process is reported that selectively and efficiently yields degradable ABA-block polymers, incorporating 6-23 wt % CO2. The polymers are synthesized using a new, highly active organometallic heterodinuclear Zn(II)/Mg(II) catalyst applied in a one-pot procedure together with biobased ε-decalactone, cyclohexene oxide, and carbon dioxide to make a series of poly(cyclohexene carbonate-b-decalactone-b-cyclohexene carbonate) [PCHC-PDL-PCHC]. The process is highly selective (CO2 selectivity >99% of theoretical value), allows for high monomer conversions (>90%), and yields polymers with predictable compositions, molar mass (from 38-71 kg mol-1), and forms dihydroxyl telechelic chains. These new materials improve upon the properties of poly(cyclohexene carbonate) and, specifically, they show good thermal stability (Td,5 ∼ 280 °C), high toughness (112 MJ m-3), and very high elongation at break (>900%). Materials properties are improved by precisely controlling both the quantity and location of carbon dioxide in the polymer chain. Preliminary studies show that polymers are stable in aqueous environments at room temperature over months, but they are rapidly degraded upon gentle heating in an acidic environment (60 °C, toluene, p-toluene sulfonic acid). The process is likely generally applicable to many other lactones, lactides, anhydrides, epoxides, and heterocumulenes and sets the scene for a host of new applications for CO2-derived polymers.

Update and Challenges in Carbon Dioxide-Based Polycarbonate Synthesis

The utilization of carbon dioxide as a comonomer to produce polycarbonates has attracted a great deal of attention from both industrial and academic communities because it promises to replace petroleum-derived plastics and supports a sustainable environment. Significant progress in the copolymerization of cyclic ethers (e.g., epoxide, oxetane) and carbon dioxide has been made in recent decades, owing to the rapid development of catalysts. In this Review, the focus is to summarize and discuss recent advances in the development of homogeneous catalysts, including metal- and organo-based complexes, as well as the preparation of carbon dioxide-based block copolymer and functional polycarbonates.

Catalytic Chain Transfer Copolymerization of Propylene Oxide and CO2 using Zinc Glutarate Catalyst

Oligo and poly(propylene ether carbonate)-polyols with molecular weights from 0.8 to over 50 kg/mol and with 60-92 mol % carbonate linkages were synthesized by chain transfer copolymerization of carbon dioxide (CO2) and propylene oxide (PO) mediated by zinc glutarate. Online-monitoring of the polymerization revealed that the CTA controlled copolymerization has an induction time which is resulting from reversible catalyst deactivation by the CTA. Latter is neutralized after the first monomer additions. The outcome of the chain transfer reaction is a function of the carbonate content, i. e. CO2 pressure, most likely on account of differences in mobility (diffusion) of the various polymers. Melt viscosities of poly(ether carbonate)diols with a carbonate content between 60 and 92 mol % are reported as function of the molecular weight, showing that the mobility is higher when the ether content is higher. The procedure of PO/CO2 catalytic chain copolymerization allows tailoring the glass temperature and viscosity.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Highly Efficient One-Pot Synthesis of COS-Based Block Copolymers by Using Organic Lewis Pairs

A one-pot synthesis of block copolymer with regioregular poly(monothiocarbonate) block is described via metal-free catalysis. Lewis bases such as guanidine, quaternary onium salts, and Lewis acid triethyl borane (TEB) were equivalently combined and used as the catalysts. By using polyethylene glycol (PEG) as the macromolecular chain transfer agent (CTA), narrow polydispersity block copolymers were obtained from the copolymerization of carbonyl sulfide (COS) and propylene oxide (PO). The block copolymers had a poly(monothiocarbonate) block with perfect alternating degree and regioregularity. Unexpectedly, the addition of CTA to COS/PO copolymerization system could dramatically improve the turnover frequency (TOF) of PO (up to 240 h-1), higher than that of the copolymerization without CTA. In addition, the conversion of CTA could be up to 100% in most cases, as revealed by ¹H NMR spectra. Of consequence, the number-average molecular weights (Mns) of the resultant block copolymers could be regulated by varying the feed ratio of CTA to PO. Oxygen-sulfur exchange reaction (O/S ER), which can generate randomly distributed thiocarbonate and carbonate units, was effectively suppressed in all of the cases in the presence of CTA, even at 80 °C. This work presents a versatile method for synthesizing sulfur-containing block copolymers through a metal-free route, providing an array of new block copolymers.

Copolymerization of CO2 and epoxides mediated by zinc organyls

Herein we report the copolymerization of CHO with CO2 in the presence of various zinc compounds R2Zn (R = Et, Bu, iPr, Cy and Ph). Several zinc organyls proved to be efficient catalysts for this reaction in the absence of water and co-catalyst. Notably, readily available Bu2Zn reached a TON up to 269 and an initial TOF up to 91 h-1. The effect of various parameters on the reaction outcome has been investigated. Poly(ether)carbonates with molecular weights up to 79.3 kg mol-1 and a CO2 content of up to 97% were obtained. Under standard reaction conditions (100 °C, 2.0 MPa, 16 h) the influence of commonly employed co-catalysts such as PPNCl and TBAB has been investigated in the presence of Et2Zn (0.5 mol%). The reaction of other epoxides (e.g. propylene and styrene oxide) under these conditions led to no significant conversion or to the formation of the respective cyclic carbonate as the main product.

Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols

Hydroxy-telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity.

A Lewis acid β-diiminato-zinc-complex as all-rounder for co- and terpolymerisation of various epoxides with carbon dioxide

A β-diiminato-zinc-N(SiMe3)2 complex (1) was synthesised and fully characterised, including an X-ray diffraction study. The activity of catalyst 1 towards the coupling reaction of CO2 and various epoxides, including propylene oxide (PO), cyclohexene oxide (CHO), styrene oxide (SO), limonene oxide (LO), octene oxide (OO) and epichlorohydrin (ECH), was investigated. Terpolymerisation of CO2, PO and LO, as well as CO2, CHO and PO, was successfully realised, resulting in polymers with adjustable glass transition temperatures and transparencies. Reaction conditions such as temperature, pressure and catalyst concentration were varied to find the optimal reaction values, especially regarding LO/CO2. In situ IR experiments hinted that at 60 °C and a critical LO concentration, polymerisation and depolymerisation are in an equilibrium (ceiling effect). Pressurising catalyst 1 with carbon dioxide resulted in a dimeric catalyst (2) with a OSiMe3 group as a new initiator. Homopolymerisation of different epoxides was carried out in order to explain the reactivity concerning copolymerisation reaction of CO2 and epoxides.

Zinc glutarate-mediated copolymerization of CO2 and PO – parameter studies using design of experiments

Parameter studies of the PO/CO₂-copolymerization revealed the importance of the surface coverage of a nanoscopic ZnGA catalyst.