Haixin Wang

A preorganized dual H-bond donor promotes benzoic acid active in the polymerization of δ-valerolactone

Ring-opening polymerization (ROP) of lactones catalyzed by a (super)strong Bronsted acid offers a valuable approach to important biodegradable aliphatic polyesters. However, the need for a mild acidic catalyst in ROP has been long sought after but unmet. Here, we describe a truly weak Bronsted acid, benzoic acid, in combination with a dual H-bond donor (dHBD), which catalyzes the ROP of δ-valerolactone (VL) in solution at room temperature. A unique preorganized sulfonyl guanidine type of dHBD, 3-amino-1,2,4-benzothiadiazine-1,1-dioxide (ABTD), proves optimal to work with benzoic acid as a cocatalyst, promoting benzoic acid activity in the ROP of VL. Poly-δ-valerolactones of predictable molecular weights (from 2.13 to 9.33 kg mol−1) and narrow dispersities (Đ ≤ 1.16) are synthesized. The controlled character of the ROP is verified by NMR, SEC, and MALDI-ToF MS measurements. NMR titration experiments imply that ABTD preferentially binds with benzoic acid, and the benzoic acid/ABTD pair protonates the VL monomer. Weak benzoic acid is not weak for the first time in the efficient cationic ROP of VL by the protonation mechanism.

Squaramide and amine binary H-bond organocatalysis in polymerizations of cyclic carbonates, lactones, and lactides

Multiple combinations of six squaramides (Sq) and eight amines as the co-catalysts were tried in ring-opening polymerizations (ROPs) of cyclic carbonates, lactones, and lactides. Sq and sparteine co-catalysts enabled ROPs of trimethylene carbonate (TMC) to poly(trimethylene carbonates) (PTMCs) with benzyl alcohol (BnOH) as the initiator. The polymerization proceeded in 3 to 16 h without decarboxylation to afford polycarbonates with precise molecular weights (Mn,NMR = 1.95 to 10.1 kg mol−1) and narrow polydispersity indices (Đ = 1.12–1.17). 1H NMR, SEC and MALDI-ToF MS measurements of the obtained PTMCs clearly indicated the quantitative incorporation of the initiator at the chain end. Kinetics and chain extension experiments demonstrated the controlled/living nature for the ROP of TMC using Sq and sparteine. NMR titration experiments confirmed that the polymerization proceeded in a H-bonding dual activation mechanism. In addition, 1,3-propanediol, pentaerythritol, propargyl alcohol, furfuryl alcohol and N-(2-hydroxyethyl)maleimide were used as functional initiators leading to production of α,ω-dihydroxy telechelic, star-shaped, and clickable end-functionalized polycarbonates. Homopolymers of valerolactone (VL), caprolactone (CL), and lactide (LA) and diblock copolymers PVL-b-PTMC and PCL-b-PTMC were successfully synthesized by using Sq and 1,8-diazabicyclo[5.4.0]undec-7-ene binary co-catalysts with BnOH as the initiator in dichloromethane at room temperature. Block copolymers PTMC-b-PVL, PTMC-b-PCL, and PTMC-b-PLA were successfully obtained by using the binary catalysts. Squaramides combined with amine co-catalysts are a generally applicable polymerization tool.

Tunable intramolecular H-bonding promotes benzoic acid activity in polymerization: inspiration from nature

Ring-opening polymerization (ROP) of lactones and cyclic carbonates catalyzed by (super)strong Bronsted acids offers a valuable approach to generate biodegradable aliphatic polyesters. However, these strong acids usually lead to backbiting and decarboxylation; thus a mild and effective acidic catalysis for these ROPs becomes necessary. Inspired by weak Bronsted acidic catalysis in squalene–hopene cyclases, we propose that ortho-amido group(s) on benzoic acids would increase the acidity of the carboxylic moiety by intramolecular H-bonding, and make carboxylic acid active in promoting the ROPs. A series of o-amido- and o,o′-bis(amido)-benzoic acids are evaluated as typical intramolecular H-bonding enhanced Bronsted acidic catalysts in the ROPs. Both o-amido- and o,o′-bis(amido)-benzoic acids exhibited good to excellent performances in the rate and control of ROPs of δ-valerolactone (VL), e-caprolactone (CL), and trimethylene carbonate (TMC) at room temperature in solutions. An exceptional carboxylic acid, o,o′-bis(pivalamido)benzoic acid, showed efficient activation in solution and precise control with high conversions (91–96%), predicted molecular weights from 3.09 to 10.31 kg mol−1, and narrow dispersities (Đ 1.03–1.12) in ROPs of CL and TMC. Well-defined diblock copolymers consisting of PTMC, PVL and PCL segments were synthesized. The controlled/living characteristics of the ROPs were verified by chain extension experiments. 1H NMR, SEC, and MALDI-TOF MS analyses strongly indicated that the obtained polymers were exactly the designated ones. A cationic monomer activation mechanism was proposed and was supported by NMR titrations. The experimental results indicated that mild and tunable ortho-amido benzoic acid with intramolecular H-bonding is a competent organocatalyst in living polymerization.

Metallic organophosphate catalyzed bulk ring-opening polymerization

Aliphatic polyesters are broadly used in biomedical materials, food packaging and drug delivery. The use of catalytic ring-opening polymerization (ROP) by dual catalysis, combining organocatalysis and metal complex catalysis, is a powerful strategy toward these valuable polyesters. In an endeavor to combine metal catalysis and organocatalysis in polymerization, we suggested the use of metal salts of an organophosphoric acid as bifunctional catalysts in the ROP of cyclic esters and of cyclic carbonates. Four metal organophosphates, viz. lithium, sodium, magnesium, and calcium diphenyl phosphates, were evaluated as catalysts for the ROPs of trimethylene carbonate (TMC), δ-valerolactone and lactide with 3-phenyl-1-propanol (PPA) as an initiator for obtaining aliphatic polyesters. Magnesium diphenyl phosphate (MgDP) showed relatively high catalytic activity and ideal control. The molecular weights of the PTMCs measured using 1H NMR matched well with the theoretical ones. 1H NMR, 13C NMR, and MALDI-ToF MS measurements demonstrated that the initiator and monomer polymerized quantitatively. Bulk polymerization with various [TMC]0/[PPA]0/[MgDP]0 ratios afforded PTMCs with expected molecular weights (Mn, NMR = 3.21–11.7 kg mol−1) and relatively narrow dispersities (Mw/Mn = 1.16–1.19). Kinetic studies, chain extension experiments and diblock copolymer synthesis were carried out to identify that the MgDP-catalyzed ROP proceeded in a controlled/living manner. A bifunctional catalytic mechanism for the MgDP catalyzed ROP was suggested from 1H NMR, 13C NMR and 31P NMR titration experiments. For evaluating the biosafety of the PTMC produced in the bulk ROP under MgDP catalysis, untreated PTMC samples were tested by an MTT assay using the L929 cell line. A result of more than 90% relative cell viabilities demonstrates the excellent cell compatibility of the PTMCs for potential biomedical applications.

Brønsted base mediated one-pot synthesis of catechol-ended amphiphilic polysarcosine-b-poly(N-butyl glycine) diblock copolypeptoids

Abstract Catechol moiety offers a versatile platform in the preparation of functionalized polymers, but it is not usually compatible with catalysis in polymerizations. To address these challenges, we suggest employment of one Brønsted base in masking the activity of catechol moiety and to modulate the polymerization. Based on this strategy, the ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides (Sar-NCA) was carried out using dopamine hydrochloride as an initiator and triethylamine as a Brønsted base. PSar with predicted molecular weights (Mn,NMR=3.7 kg mol−1) and narrow dispersities (Đ<1.13) was prepared. Catechol initiator was successfully linked to PSar end as confirmed by MALDI-ToF MS. Subsequently, copolymerization of N-butyl glycine N-carboxyanhydrides (Bu-Gly-NCA) from the PSar in one-pot produced catechol end-functionalized amphiphilic polysarcosine-block-poly(N-butyl glycine) diblock copolypeptoids (cat-PSar-b-PGlyBu). Further, cat-PSar-b-PGlyBu enabled the aqueous dispersion of manganese oxide nanoparticles which was attributable to the anchor of the diblock copolymers onto the surface of the nanoparticles. The strategy for catechol masking and polymerization mediating by one Brønsted base offered a new avenue into the synthesis of catechol-ended block copolymers.

Opposite-charge repulsive cation and anion pair cooperative organocatalysis in ring-opening polymerization

Cations and anions attract each other by electrostatic force to form an ion pair. However, repulsion between cations and anions does exist. There are few examples of repulsive “ion pair strain” where catalysis by a strained ion pair is absent. Here, we describe substituted cyclopropenium, the minimal Huckel aromatic ring, which when mounted with PhNH groups on the positive cyclopropenium core C3, behaves as an electron-rich cation to repel its counter negative anion. The formal positive charge on C3 turns the NH moiety into a strong H-bond donor (HBD) and the weakly coordinating chloride exhibits strong H-bond acceptor (HBA) character. The strained ion pair composed of a HBD and a HBA displays cooperative organocatalysis in ring-opening polymerizations of δ-valerolactone initiated with benzyl alcohol. A cooperative catalysis mechanism of tris(phenylamino)cyclopropenium as a HBD and chloride as a HBA was elucidated.