Recyclable cyclic bio-based acrylic polymer via pairwise monomer enchainment by a trifunctional Lewis pair

Biobased Cyclic Polycarbonate: Synthesis and Applications via Dynamic Bis(hindered amino)disulfide Linkers

Cyclic polymers have garnered significant interest due to their unique structure; however, their synthesis remains challenging, often hindered by low yields and limited selectivity. Considering that the cyclization step during the synthesis of cyclic polymers is presumably the most challenging, using a spontaneous and selective cyclization system is ideal. Here, we present a topology transformation from linear to cyclic, which is achieved through the error-checking ability provided by the dynamic covalent bonding between bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS) and its stable radicals with a high bond exchange rate. When applying this method to biobased poly(isosorbide carbonate) (PIC), an attractive alternative to conventional petroleum-based polycarbonates, high-molecular-weight cyclic polymers were unexpectedly obtained. Since the functionalization of PICs has been traditionally limited to copolymerization techniques, we aimed to introduce dynamic covalent bonds to establish novel functionalization methods for PICs. Interestingly, the synthesis of cyclic PICs through intramolecular cyclization using dynamic covalent bonds in a heterogeneous system proceeded via a ring-expansion polymerization-like mechanism, affording high-molecular-weight cyclic polymers consisting of a PIC backbone and BiTEMPS units as dynamic units. The resulting PIC-based cyclic polymers with BiTEMPS units were applied to bond exchange reactions, providing an effective approach for the synthesis of cyclic block polymers and end-functionalized linear block polymers with a PIC skeleton. These results demonstrate the potential of dynamic covalent chemistry in polymer synthesis.

Vinyl polymers with fully degradable carbon backbones enabled by aromatization-driven C-C bond cleavage

Degradation of carbon-backbone polymers, which make up most plastics, remains a formidable challenge owing to strong and inert main-chain C-C bonds. While incorporation of comonomers that generate backbone radicals under certain conditions can induce degradation of the polymer chain, such strategies yield complex oligomer mixtures. Here we report aromatization-driven C-C bond cleavage as a viable and powerful strategy to endow the degradability into carbon backbones using acrylic polymers as a model example. The key to this new strategy is the efficient, living, alternating addition copolymerization of acrylates with simple, commercially available and biorenewable coumarin using a frustrated Lewis pair cooperative catalyst. The resulting acrylic copolymers are strong, transparent thermoplastics with key thermal, optical, mechanical properties comparable or superior to poly(methyl methacrylate). Under strong base, alternating copolymers can completely degrade at room temperature through efficient cleavage of main-chain C-C bonds utilizing aromatization as a thermodynamic driving force, to generate pure, pharmaceutically valuable molecules, thus affording durable, robust yet fully degradable carbon-backbone acrylic polymers.

Functionalized Cyclic Poly(α-Hydroxy Acids) via Controlled Ring-Opening Polymerization of O-Carboxyanhydrides

Linear poly(α-hydroxy acids) are important degradable polymers, and they can be efficiently prepared by ring-opening polymerization of O-carboxyanhydrides with pendant functional groups. However, attempts to prepare cyclic poly(α-hydroxy acids) have been plagued by side reactions, including epimerization and uncontrolled intramolecular chain transfers or termination, that prevent the synthesis of high-molecular-weight stereoregular cyclic polyesters. Herein, we report a scalable method for the synthesis of high-molecular-weight (>100 kDa) stereoregular functionalized cyclic poly(α-hydroxy acids) by means of controlled polymerization of O-carboxyanhydrides using a catalytic system consisting of a lanthanum complex with a sterically bulky ligand and a manganese silylamide. Additionally, using this system, we could readily prepare cyclic block poly(α-hydroxy acids) by means of sequential addition of O-carboxyanhydrides. The obtained cyclic polyesters and their cyclic block copolyesters exhibit distinctive physicochemical properties-including elevated phase transition temperature, improved toughness, and reduced viscosity-compared to their linear counterparts.

A Pseudo-Block Copolymerization Access to Cyclic Alternating Copolymers through Segment-Selective Transesterification

Efficient synthesis of cyclic polymers remains a frontier challenge. We report here that macromolecular transesterification during a pseudoblock copolymerization process can be utilized for such a purpose. Organobase-catalyzed ring-opening alternating copolymerization of 3,4-dihydrocoumarin and epoxide is conducted with four-armed poly(ethylene oxide) (PEO) as a macroinitiator. Intramolecular transesterification (backbiting) occurs selectively on the newly formed polyester segments. The disconnected cyclic alternating copolymers can be easily isolated by precipitation owing to their substantial solubility difference from the PEO-containing acyclic parts. The obtained cyclic alternating copolymers exhibit low dispersity (<1.2) and a molar mass of around 3 kg mol-1, irrespective of the monomer-to-initiator feed ratio, indicating thermodynamic control over the ring size. The macrocyclic structure is confirmed by both mass spectroscopy and microscopic visualization and then utilized to prepare cyclic-brush terpolymer by thiol-ene modification, followed by graft polymerization of propylene oxide.

Chemoselective Ring-Opening Polymerization of α-Methylene-δ-valerolactone Catalyzed by a Simple Organoaluminum Complex to Prepare Closed-Loop Recyclable Functional Polyester

α-Methylene-δ-valerolactone (MVL) as a bio-renewable bifunctional monomer has shown great promise to prepare closed-loop recyclable polyester with pendent functionalizable double bond. However, the chemoselective ring-opening polymerization (ROP) of MVL still faces challenges including low polymerization temperature, expensive catalyst as well as high catalyst loading. In this contribution, we present the chemoselective and controlled ROP of MVL using a simple organoaluminum complex [MeAl(BHT)2] (BHT=2,6-di-tert-butyl-4-methylphenoxy), which can be easily prepared from commercially available trimethylaluminum and 2,6-di-tert-butyl-4-methylphenol without purification. MeAl(BHT)2 exhibits much higher catalytic activity (TOF=668 h-1) than that of MeAl[Salen] (TOF=89 h-1), a commonly used organoaluminum catalyst. The high chemoselectivity and activity of MeAl(BHT)2 is proposed to originate from the cooperative activation of propagating chain-ends and monomers via the "coordination-insertion" mechanism. Remarkably, high-molecular-weight P(MVL)ROP can be prepared in bulk using MeAl(BHT)2, which is not accessible by the previous catalysts. This study may advance the development of closed-loop recyclable polymers considering the easy preparation, low cost and good catalytic performance of MeAl(BHT)2.

Binary Catalyst Manipulating the Sequences of Poly(ester-carbonate) Copolymers in Metal-Free Terpolymerization of Epoxide, Anhydride, and CO2

The one-pot terpolymerization of epoxide (EP), anhydride (AH), and CO2 to synthesize polyester-polycarbonate copolymers with precise sequences remains a significant challenge in polymer chemistry. In this study, promising progress was achieved by utilizing a cyclic trimeric phosphazene base (CTPB) and triethylboron (TEB) as a binary catalyst, enabling the synthesis of both well-defined block and truly random poly(ester-carbonate) copolymers through the one-pot terpolymerization of EP/AH/CO2. By adjusting the molar ratio of CTPB/TEB to 1/0.5, remarkable chemoselectivity for ring-opening alternating copolymerization (ROAC) of propylene oxide (PO) and phthalic anhydride (PA) was achieved, followed by the ROAC of PO/CO2. This sequential control allowed for the synthesis of well-defined block poly(ester-carbonate) copolymers, containing three possible sequences, ester-ester sequence (EE)/ester-carbonate sequence (EC)/carbonate-carbonate sequence (CC) = 59/4/37, from a mixture of PO, PA, and CO2. Moreover, the versatility of this CTPB/TEB catalyst in regulating chemoselectivity was demonstrated, with a ratio of 1/3 facilitating the simultaneous ROAC of PO/PA and PO/CO2 with compatible rates, resulting in the production of random poly(ester-carbonate) copolymers, in which three possible sequences (EE/EC/CC = 26/50/24) are very close to theoretical values. This metal-free catalytic system and its flexible chemoselectivity regulation strategy proved to be applicable to a wide range of epoxides (PO, cyclohexene oxide (CHO)) and anhydrides (PA, diglycolic anhydride (DGA), and succinic anhydride (SA)), enabling the successful synthesis of poly(ester-carbonate) copolymers with diverse sequences and compositions.

Frustrated Lewis Pair-Promoted Organocatalytic Transformation of Hydrosilanes into Silanols with Water Oxidant

Owing to their unique properties, the silanols have attracted intense attention but remain challenging to prepare from the organocatalytic oxidation of hydrosilanes using H2O as a green oxidant. Herein, we employ a frustrated Lewis pair (FLP) to successfully suppress the formation of undesired siloxanes and produce silanols in high to excellent yields in the presence of H2O. Mechanistic studies suggest that the reaction is initiated with the activation of FLP by H2O rather than by silanes and goes through a concerted SN2 mechanism. More importantly, the combination of the FLP-catalyzed oxidation of hydrosilanes with B(C6F5)3-catalyzed dehydrogenation enables us to realize the precise synthesis of sequence-controlled oligosiloxanes. This method exhibits a broad substrate scope and can be easily scaled up, thus exhibiting promising application potentials in the precision synthesis of silicon-containing polymer materials.

Highly Tolerant Living/Controlled Anionic Polymerization of Dialkyl Acrylamides Enabled by Zinc Triflate/Phosphine Lewis Pair

Living polymerizations of polar vinyl monomers have been successful for decades. However, they still suffer the following challenges: fast propagation, air-moisture tolerance, and negligible side reactions even at elevated temperatures. Here, we developed an unprecedented polymerization that overcomes these limitations using a Lewis pair catalyst. The anionic polymerization of dialkyl acrylamides proceeded in a living/controlled matter using Zn(OTf)2/PPh3 within a wide temperature range of 25-100 °C for short times (1-10 min) even under open-air conditions. The recovery and reuse of Zn(OTf)2 without loss of polymerization activity were observed to be possible. The polymerization was retarded by excess Zn(OTf)2, the additive methanol, and water, indicating equilibriums of the propagating species with them. The putative propagating zinc triflate-ate complex was tolerant to the protic additives and significantly selective for the propagation.

Chemically recyclable polyvinyl chloride-like plastics

Polyvinyl chloride (PVC) is the world's third-most widely manufactured thermoplastic, but has the lowest recycling rate. The development of PVC-like plastics that can be depolymerized back to monomer contributes to a circular plastic economy, but has not been accessed. Here, we develop a series of chemically recyclable plastics from the reversible copolymerization of cyclic anhydride with chloral. The copolymerization is highly efficient through the anionic or cationic mechanism under mild conditions, yielding polyesters with tunable structure and properties from multiple commercial monomers. Notably, these polyesters manifest mechanical properties comparable to PVC and polystyrene. Meanwhile, such polyesters are flame-retardant like PVC due to high chloride content. Of significance, these polyesters can be depolymerized back to starting monomers at high temperatures owing to the reversibility of the copolymerization, leading to a circular economy. Overall, the readily available monomers, simple synthesis, advantageous performance, and practical recyclability make the polymers promising for applications.

Proton-triggered topological transformation in superbase-mediated selective polymerization enables access to ultrahigh-molar-mass cyclic polymers

The selective synthesis of ultrahigh-molar-mass (UHMM, >2 million Da) cyclic polymers is challenging as an exceptional degree of spatiotemporal control is required to overcome the possible undesired reactions that can compete with the desired intramolecular cyclization. Here we present a counterintuitive synthetic methodology for cyclic polymers, represented here by polythioesters, which proceeds via superbase-mediated ring-opening polymerization of gem-dimethylated thiopropiolactone, followed by macromolecular cyclization triggered by protic quenching. This proton-triggered linear-to-cyclic topological transformation enables selective, linear polymer-like access to desired cyclic polythioesters, including those with UHMM surpassing 2 MDa. In addition, this method eliminates the need for stringent conditions such as high dilution to prevent or suppress linear polymer contaminants and presents the opposite scenario in which protic-free conditions are required to prevent cyclic polymer formation, which is capitalized to produce cyclic polymers on demand. Furthermore, such UHMM cyclic polythioester exhibits not only much enhanced thermostability and mechanical toughness, but it can also be quantitatively recycled back to monomer under mild conditions due to its gem-disubstitution.

Precise Distance Control and Functionality Adjustment of Frustrated Lewis Pairs in Metal-Organic Frameworks

We report the construction of frustrated Lewis pairs (FLPs) in a metal-organic framework (MOF), where both Lewis acid (LA) and Lewis base (LB) are fixed to the backbone. The anchoring of a tritopic organoboron linker as LA and a monotopic linker as LB to separate metal oxide clusters in a tetrahedron geometry allows for the precise control of distance between them. As the type of monotopic LB linker varies, pyridine, phenol, aniline, and benzyl alcohol, a series of 11 FLPs were constructed to give fixed distances of 7.1, 5.5, 5.4, and 4.8 Å, respectively, revealed by 11B-1H solid-state nuclear magnetic resonance spectroscopy. Keeping LA and LB apart by a fixed distance makes it possible to investigate the electrostatic effect by changing the functional groups in the monotopic LB linker, while the LA counterpart remains unaffected. This approach offers new chemical environments of the active site for FLP-induced catalysis.

Topology Controlled All-(Meth)acrylic Thermoplastic Elastomers by Multi-Functional Lewis Pairs-Mediated Polymerization

It remains challenging to synthesize all-(meth)acrylic triblock thermoplastic elastomers (TPEs), due to the drastically different reactivities between the acrylates and methacrylates and inevitable occurrence of side reactions during polymerization of acrylates. By taking advantage of the easy structural modulation features of N-heterocyclic olefins (NHOs), we design and synthesize strong nucleophilic tetraphenylethylene-based NHOs varying in the number (i.e. mono-, dual- and tetra-) of initiating functional groups. Its combination with bulky organoaluminum [iBuAl(BHT)2] (BHT=bis(2,6-di-tBu-4-methylphenoxy)) constructs Lewis pair (LP) to realize the living polymerization of both acrylates and methacrylates, furnishing polyacrylates with ultrahigh molecular weight (Mn up to 2174 kg ⋅ mol-1) within 4 min. Moreover, these NHO-based LPs enable us to not only realize the control over the polymers' topology (i.e. linear and star), but also achieve triblock star copolymers in one-step manner. Mechanical studies reveal that the star triblock TPEs exhibit better mechanical properties (elongation at break up to 1863 % and tensile strength up to 19.1 MPa) in comparison with the linear analogs. Moreover, the presence of tetraphenylethylene group in the NHOs entitled the triblock TPEs with excellent AIE properties in both solution and solid state.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

Cyclic and Linear Tetrablock Copolymers Synthesized at Speed and Scale by Lewis Pair Polymerization of a One-Pot (Meth)acrylic Mixture and Characterized at Multiple Levels

Cyclic block copolymers (cBCP) are fundamentally intriguing materials, but their synthetic challenges that demand precision in controlling both the monomer sequence and polymer topology limit access to AB and ABC block architectures. Here, we show that cyclic ABAB tetra-BCPs (cABAB) and their linear counterpart (lABAB) can be readily obtained at a speed and scale from one-pot (meth)acrylic monomer mixtures, through coupling the Lewis pair polymerization's unique compounded-sequence control with its precision in topology control. This approach achieves fast (<15 min) and quantitative (>99%) conversion to tetra-BCPs of predesignated linear or cyclic topology at scale (40 g) in a one-pot procedure, precluding the needs for repeated chain extensions, stoichiometric addition steps, dilute conditions, and postsynthetic modifications, and/or postsynthetic ring-closure steps. The resulting lABAB and cABAB have essentially identical molecular weights (Mn = 165-168 kg mol-1) and block degrees/symmetry, allowing for direct behavioral comparisons in solution (hydrodynamic volume, intrinsic viscosity, elution time, and refractive indices), bulk (thermal transitions), and film (thermomechanical and rheometric properties and X-ray scattering patterns) states. To further the morphological characterizations, allylic side-chain functionality is exploited via the thiol-ene click chemistry to install crystalline octadecane side chains and promote phase separation between the A and B blocks, allowing visualization of microdomain formation.

Cyclic Polyesters with Closed-Loop Recyclability from A New Chemically Reversible Alternating Copolymerization

Polyesters with both cyclic topology and chemical recyclability are attractive. Here, the alternating copolymerization of cyclic anhydride and o-phthalaldehyde to synthesize a series of cyclic and recyclable polyesters are reported for the first time. Besides readily available monomers, the copolymerization is carried out at 25 °C, uses common Lewis/Brønsted acids as catalysts, and achieves high yields within 1 h. The resulting polyesters possess well-defined alternating sequences, high-purity cyclic topology, and tunable structures using distinct two monomer sets. Of interest, the copolymerization manifests obvious chemical reversibility as revealed by kinetic and thermodynamic studies, making the unprecedented polyesters easy to recycle to their distinct two monomers in a closed loop at high temperatures. This work furnishes a facile and efficient method to synthesize cyclic polyesters with closed-loop recyclability.

A Binary Silicon-Centered Organoboron Catalyst with Superior Performance to That of Its Bifunctional Analogue

This work reported that a silicon-centered alkyl borane/ammonium salt binary (two-component) catalyst exhibits much higher activity than its bifunctional analogue (one-component) for the ring-opening polymerization of propylene oxide, showing 7.3 times the activity of its bifunctional analogue at a low catalyst loading of 0.01 mol %, and even 15.3 times the activity at an extremely low loading of 0.002 mol %. By using 19 F NMR spectroscopy, control experiments, and theoretical calculation we discovered that the central silicon atom displays appropriate electron density and a larger intramolecular cavity, which is useful to co-activate the monomer and to deliver propagating chains, thus leading to a better intramolecular synergic effect than its bifunctional analogue. A unique two-pathway initiation mode was proposed to explain the unusual high activity of the binary catalytic system. This study breaks the traditional impression of the binary Lewis acid/nucleophilic catalyst with poor activity because of the increase in entropy.

Light-Induced Polymeric Frustrated Radical Pairs as Building Blocks for Materials and Photocatalysts

Polymeric frustrated Lewis pairs, or poly(FLP)s, have served to bridge the gap between functional polymer science and main group catalysis, pairing the uniqueness of sterically frustrated Lewis acids and bases with a polymer scaffold to create self-healing gels and recyclable catalysts. However, their utilization in radical chemistry is unprecedented. In this paper, we disclose the synthesis of polymeric frustrated radical pairs, or poly(FRP)s, by in situ photoinduction of FLP moieties, where their Lewis acidic and basic centers are tuned to promote single electron transfer (SET). Through systematic manipulation of the chemical structure, we demonstrate that inclusion of ortho-methyl groups on phosphine monomers is crucial to enable SET. The generation of radicals is evidenced by monitoring the stable polymeric phosphine radical cations via UV/vis and EPR spectroscopy. These new poly(FRP)s enable both catalytic hydrogenation and radical-mediated photocatalytic perfluoroalkylations. These polymeric radical systems open new avenues to design novel functional polymers for catalysis and photoelectrical chemistry.

Ancillary Ligand Lability Improves Control in Cyclic Ruthenium Benzylidene Initiated Ring-Expansion Metathesis Polymerizations

The synthesis of well-defined cyclic polymers is crucial to exploring applications spanning engineering, energy, and biomedicine. These materials lack chain-ends and are therefore imbued with unique bulk properties. Despite recent advancements, the general methodology for controlled cyclic polymer synthesis via ring-expansion metathesis polymerization (REMP) remains challenging. Low initiator activity leads to high molar mass polymers at short reaction times that subsequently "evolve" to smaller polymeric products. In this work, we demonstrate that in situ addition of pyridine to the tethered ruthenium-benzylidene REMP initiator CB6 increases ancillary ligand lability to synthesize controlled and low dispersity cyclic poly(norbornene) on a short time scale without relying on molar mass evolution events.

Modular Alcohol Click Chemistry Enables Facile Synthesis of Recyclable Polymers with Tunable Structure

The facile synthesis of chemically recyclable polymers derived from sustainable feedstocks presents enormous challenges. Here, we develop a novel, modular, and efficient click reaction for connecting primary, secondary, or tertiary alcohols with activated alkenes via a bridge molecule of carbonyl sulfide (COS). The click reaction is successfully applied to synthesize a series of recyclable polymers by the step polyaddition of diols, diacrylates, and COS. Diols and diacrylates are common chemicals and can be produced from biorenewable sources, and COS is released as the industrial waste. In addition to sustainable monomers, the approach is atom-economical, wide in scope, metal-free, and performed under mild conditions, affording unprecedented polymers with nearly quantitative yields. The produced polymers also possess predesigned and widely tunable structure owing to the versatility of our method and the broad variety of monomers. The in-chain thiocarbonate and ester polar groups can play as breakpoints, allowing these polymers to be easily recycled. Overall, the polymers have broad prospects for green materials given their facile synthesis, readily available feedstocks, desirable performance, and chemical recyclability.

A recyclable polyester library from reversible alternating copolymerization of aldehyde and cyclic anhydride

Our society is pursuing chemically recyclable polymers to accelerate the green revolution in plastics. Here, we develop a recyclable polyester library from the alternating copolymerization of aldehyde and cyclic anhydride. Although these two monomer sets have little or no thermodynamic driving force for homopolymerization, their copolymerization demonstrates the unexpected alternating characteristics. In addition to readily available monomers, the method is performed under mild conditions, uses common Lewis/Brønsted acids as catalysts, achieves the facile tuning of polyester structure using two distinct monomer sets, and yields 60 polyesters. Interestingly, the copolymerization exhibits the chemical reversibility attributed to its relatively low enthalpy, which makes the resulting polyesters perform closed-loop recycling to monomers at high temperatures. This study provides a modular, efficient, and facile synthesis of recyclable polyesters using sustainable monomers.

The B(C6F5)3·H2O promoted synthesis of fluoroalkylated 3,3',3''-trisindolylmethanes from fluorocarboxylic acids and indoles

Trisindolylmethanes (TIMs) exist in many bioactive natural products and are frequently applied in medicinal chemistry and materials science. Herein, a simple and efficient protocol promoted by B(C₆F₅)₃·H₂O for the synthesis of their fluoroalkylated analogues, fluoroalkylated 3,3',3''-TIMs, is reported for the first time. Easily accessible fluorocarboxylic acids are utilized as the fluoroalkyl sources, exhibiting an obvious fluorine effect. This convenient and green process features mild and metal-free conditions, easy scale-up, and an environmentally friendly byproduct.

An Arbitrarily Regulated Monomer Sequence in Multi-Block Copolymer Synthesis by Frustrated Lewis Pairs

Rapid access to sequence-controlled multi-block copolymers (multi-BCPs) remains as a challenging task in the polymer synthesis. Here we employ a Lewis pair (LP) composed of organophosphorus superbase and bulky organoaluminum to effectively copolymerize the mixture of methacrylate, cyclic acrylate, and two acrylates, into well-defined di-, tri-, tetra- and even a hepta-BCP in one-pot one-step manner. The combined livingness, dual-initiation and CSC feature of Lewis pair polymerization enable us to achieve not only a trihexaconta-BCP with the highest record in 8 steps by using four-component monomer mixture as building blocks, but also the arbitrarily-regulated monomer sequence in multi-BCP, simply by changing the composition and adding order of the monomer mixtures, thus demonstrating the powerful capability of our strategy in improving the efficiency and enriching the composition of multi-BCP synthesis.