Catalytic Ring-opening Polymerization Research Articles

Facile catalytic ring opening polymerization of lactic acid: Comparing the performance of Fe and Zn metal species

As a bio‐compatible/degradable polymer, poly(lactic acid) (PLA) has been widely noticed in tissue engineering and medical applications. Proper catalytic ring‐opening polymerization of lactic acid is pertinent to development of active metal species capable of producing high‐molecular‐weight polymers. In this work, ligand catalyst (2,4‐di‐tertio‐butyl‐6‐{[(2‐di‐methyl‐amino ethyl) methyl amino] methyl} phenol) was synthesized and then attached to Fe‐based and Zn‐based metal substitutes, and performance of metal species toward ring opening polymerization of lactic acid has been investigated. Regulation of reaction condition at 180°C for 36 h with Zn‐based catalyst allowed for late‐stage production of a high‐molecular‐weight PLA, while Fe‐based facilitated polymerization at early stages leading to PLAs with comparatively twofold higher molecular weight at lower time, monomer‐to‐initiator ratio, and temperature (at 170°C). A ratio of monomer to initiator of 5000 with Fe catalyst resulted in the highest molecular weight. Thus, polymerization has been facilitated by the use of Fe‐based catalyst. For this class of catalyst with Fe substitute, the product was characterized by means of 1H‐NMR, Fourier transform infrared spectroscopy, and differential scanning calorimetry analyses. J. VINYL ADDIT. TECHNOL., 25:215–224, 2019. © 2018 Society of Plastics Engineers

Mechanistic Insights of BHT-Mg-Catalyzed Ethylene Phosphate's Coordination Ring-Opening Polymerization: DFT Modeling and Experimental Data.

Poly(ethylene phosphates) are promising polymers for use in biomedical applications. Catalytic ring-opening polymerization (ROP) of cyclic ethylene phosphate monomers (CEPMs) is the most effective approach for obtaining these polymers. The mechanism of coordination ROP of CEPMs remains unclear. We report, for the first time, the results of DFT modeling of CEPM ROP. In these calculations by Gaussian-09 program package with the B3PW91/DGTZVP basis set, we explored methyl ethylene phosphate (MeOEP) ROP catalyzed by dimeric and monomeric catalytic species derived from heteroleptic complex [(BHT)Mg(μ-OBn)(THF)]2 (Mg1, BHT = 2,6-di-tert-butyl-4-methylphenolate). Analysis of the reaction profiles for the binuclear and mononuclear reaction mechanisms allowed us to conclude that the ROP of MeOEP is preferentially catalyzed by mononuclear Mg complexes. This estimation was confirmed by comparative polymerization experiments using MeOEP and traditional monomers ε-caprolactone (εCL), racemic lactide (rac-LA), and l-lactide (l-LA) initiated by Mg1. ROP of MeOEP proceeds at an extremely high rate due to the substantially lower activation barrier calculated for mononuclear mechanism in comparison with that of cyclic esters that polymerize without the dissociation of BHT-Mg binuclear species. We also demonstrated the use of MeOEP as a "monomerization" agent in the synthesis of MeOEP-lactide block copolymers. Comparison of the multiple acceleration of l-LA ROP after MeOEP prepolymerization and formation of atactic PLA blocks in rac-LA polymerization with the heterotactic PLA formation during Mg1-catalyzed homopolymerization also confirmed the mononuclear nature of the polyphosphate-containing catalytic particles.

Enzymatic degradation of synthetic poly(3-hydroxybutyrates) as a tool for combinatorial microstructure determination

Microstructure of synthetic poly(3-hydroxybutyrate) (PHB) prepared by catalytic ring opening polymerization of β-butyrolactone determines the merit of the polymer for applications. Depending on the catalyst used, synthetic PHB can possess a complex microstructure, elucidation of which is not always straightforward. One of such PHBs is analyzed in this work using fractionation of the polymer, differential scanning calorimetry, X-ray powder diffraction, 1H and 13C NMR spectroscopy and, particularly, enzymatic hydrolysis. Only by correlation of the data obtained from different analytic methods, the detailed structural elucidation of the synthesized polymer becomes possible. That is, the bulk polymer features isotactic and syndiotactic enrichment at the same time. Based on the results of the analyses, a multi-stereoblock microstructure of the studied synthetic PHB is ruled out, suggesting a negligible rate of chain transfer/shuttling/transesterification reactions compared to polymerization rate under the applied conditions. An enantiomorphic site control of stereoregularity both for isotactic and for syndiotactic enchainment is found.

Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis, Characterization, and Ring-Opening Polymerization Catalysis

The unsymmetrical salphen ligand (3,5-tBu-1-OH-C6H2)CH═N-C6H4-N═CH(3-Ph-1-OH-C6H3) (tBu-PhLH2) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5-tBu-1-OH-C6H2)CH═N-C6H4-N═CH(3,5-tBu-1-OH-C6H2) (tBuLH2) and (3-Ph-1-OH-C6H3)CH═N-C6H4-N═CH(3-Ph-1-OH-C6H3) (PhLH2), were also explored and compared. The methane elimination reactions between ligands and AlMe3 resulted in formation of tBu-PhLAlMe (1), tBuLAlMe (2), and PhLAlMe (3) in high yields, which were characterized by elemental analysis, 1H and 13C NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a five-coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1–3 reacted with BnOH at 70 °C to give tBu-PhLAlOBn (4), tBuLAlOBn (5), and PhLAlOBn (6), respectively, which existed as monometallic species in solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reacti...

Toward Polyethylene–Polyester Block and Graft Copolymers with Tunable Polarity

The synthesis and characterization of polyethylene–polyester block and graft copolymers and their potential as compatibilizers in polyethylene-based polymer blends are being described. The various routes to functionalized polyethylenes and the corresponding block/graft copolymers have been compared and evaluated for their scalability to industrial scale production. Hydroxyl chain-end and randomly OH-functionalized HDPE as well as randomly OH-functionalized LLDPE were employed as macroinitiators for producing the corresponding block and graft copolymers. These materials were prepared using two different strategies. The grafting from approach entails catalytic ring-opening polymerization of lactones, i.e., e-caprolactone and ω-pentadecalactone and hydroxyl-functionalized polyethylenes as macroinitiator. The alternative grafting onto approach involves the preparation of block and graft copolymers via simple and convenient transesterification of polycaprolactone or polypentadecalactone with OH-functionalized ...

Tailor-Made Stereoblock Copolymers of Poly(lactic acid) by a Truly Living Polymerization Catalyst.

Poly(lactic acid) (PLA) is a biodegradable polymer prepared by the catalyzed ring opening polymerization of lactide. An ideal catalyst should enable a sequential polymerization of the lactide enantiomers to afford stereoblock copolymers with predetermined number and lengths of blocks. We describe a magnesium based catalyst that combines very high activity with a true-living nature, which gives access to PLA materials of unprecedented microstructures. Full consumption of thousands of equivalents of L-LA within minutes gave PLLA of expected molecular weights and narrow molecular weight distributions. Precise PLLA-b-PDLA diblock copolymers having block lengths of up to 500 repeat units were readily prepared within 30 min, and their thermal characterization revealed a stereocomplex phase only with very high melting transitions and melting enthalpies. The one pot sequential polymerization was extended up to precise hexablocks having "dialed-in" block lengths.

Group 13 metal (Al, Ga, In) alkyls supported by N-heterocyclic carbenes for use in lactide ring-opening polymerization catalysis

The synthesis of several N-heterocyclic carbene (NHC) group 13 metal adducts of the type (NHC)MR3 [NHC=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp); M=Al, Ga, In; R=Me, Et] is described. In the case of the In adduct (IMes)InMe3, characterization data include its X-ray crystallographic determined molecular structure. The prepared compounds act as efficient ring-opening polymerization (ROP) initiators of rac-lactide, allowing access to either linear poly(lactic acid) (PLA) or cyclic PLA depending on the nature of the initiator and the reaction conditions. For these NHC-MMe3 systems, the addition of benzyl alcohol (acting as a chain transfer agent) was found to be beneficial both to ROP activity and control. In particular, the Al and In systems (IMes)MMe3 (M=Al, In) in combination with BnOH are highly effective ROP initiators of rac-lactide at room temperature (TOF=600h−1).

Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts for ROMP of Norbornene

This report details the synthesis and characterization of a series of Nb complexes and one tantalum complex supported by the [CF3–ONO]3– trianionic pincer-type ligand. Access to the trianionic Nb dichloride complex, [CF3–ONO]NbCl2(OEt2) (3-Et2O), allows for the synthesis of Nb-dialkyl complexes, [CF3–ONO]NbR2 (where R = benzyl (5), neopentylsilyl (6), neophyl (7)). The sterically encumbered Nb-neophyl complex (7) is thermally stable and fails to convert to the alkylidene even in the presence of donor ligands. Complex 7, however, promotes catalytic ring-opening metathesis polymerization (ROMP) of norbornene, suggesting the plausible intermediacy of a Nb-alkylidene. The corresponding Ta analogue, [CF3–ONO]Ta(CH2C(CH3)2(C6H5))2 (9), requires dramatically higher temperatures to initiate ROMP and provides poor yields of polymer. Complexes 5 and 6 also promote ROMP of norbornene. Characterization of all new complexes includes multinuclear NMR spectroscopy and combustion analysis. For complex 7, characterization also includes solid-state structure elucidation via a single crystal X-ray diffraction experiment.

Catalytic Ring-Opening (Co)polymerization of Semiaromatic and Aliphatic (Macro)lactones

The ring-opening polymerization (ROP) of cyclic butylene terephthalate oligomers (cBT) using an aluminum salen catalyst was investigated. The kinetic analysis of the ROP of cBT in tetrachloroethane (TCE) revealed a first-order dependence of the rate constant in catalyst as well as monomer concentration. Comparison with the ROP in other solvents showed that TCE significantly retards the reaction by short-term reversible catalyst deactivation and by long-term permanent deactivation. The apparent activation energy for the ROP of cBT and e-caprolactone (CL) were determined independently, and the difference in reactivity was used to investigate the possibility to synthesize blocky copolymers. Sequence distribution analysis of the copolymers obtained over a period of time from a single feed copolymerization revealed that over the full reaction range the degree of randomness was high. A sequential feed approach, in which CL was added after the polymerization of cBT was completed, showed that the propagation reac...

Thiourea binding with carboxylic acid promoted cationic ring-opening polymerization

H-bonding organocatalysis using (thio)urea/amine achieved massive success. H-bonding binding with Brønsted acid catalysis, especially in polymerizations, was scarcely explored. Here we disclosed 1,3-bis-(3,5-bis(trifluoromethyl)phenyl)thiourea (TU) as hydrogen bond donor (HBD), combined with trifluoroacetic acid (TFA) as a Brønsted acid (BA), promoted efficient controlled/living ring-opening polymerization (ROP) of δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL) at room temperature. The ROPs of δ-VL and ε-CL with benzyl alcohol (BnOH) as the initiator yielded poly(δ-valerolactone) (PVL, Mn, NMR = 1900–9900 g mol−1, Mw/Mn = 1.28–1.34) and poly(ε-caprolactone) (PCL, Mn, NMR = 2400–11,600 g mol−1, Mw/Mn = 1.19–1.35) with predicted molecular weights and narrow dispersities. A plausible mechanism of the ROP promoted by associations of TU with TFA through double hydrogen bonding was suggested, in which stabilizing of the carboxylate anion and enhancing of the Brønsted acidity made the catalytic ROPs by TU–TFA viable. The quantitative incorporation of the initiator into the polymer chains was demonstrated by 1H NMR and MALDI-ToF MS measurements. The controlled/living fashion of the polymerization was supported by the kinetics and chain extension experiments. Synthesis of well defined block copolymers of PVL-b-PCL and PCL-b-PVL verified again the ROPs were in controlled/living manner, and suggested the hydrogen bond donor binding with Brønsted acid catalysis to be a generally applicable method.

Well-defined (co)polypeptides bearing pendant alkyne groups

A novel metal-free strategy, using hydrogen-bonding catalytic ring opening polymerization of alkyne-functionalized N-carboxy anhydrites of α-amino acids, was developed for the synthesis of well-defined polypeptides bearing pendant alkyne groups.

Aluminum complexes containing biphenolate phosphine ligands: synthesis and living ring-opening polymerization catalysis.

This report describes the synthesis, structure, and reactivity of aluminum complexes containing tridentate biphenolate phosphine ligands of the type [RP(2-O-3,5-C6H2tBu2)2]2- (R = tBu (2a), Ph (2b)). Alkane elimination of AlMe3 with one equiv. of H2[2a] or H2[2b] in THF at 0 °C cleanly affords colorless crystalline [2a]AlMe(THF) (3a) and [2b]AlMe(THF) (3b), respectively. An X-ray diffraction study of 3a showed it to be a five-coordinate THF-bound species, whose coordination geometry is best described as trigonal bipyramidal, having the phosphorus donor and THF at axial positions. Treatment of either in situ prepared or isolated methyl complexes 3a,b with one equiv. of benzyl alcohol in toluene or THF generated their corresponding benzyloxides {[2a,b]Al(μ2-OCH2Ph)}2 (4a,b). An X-ray diffraction study of 4a revealed a dimeric structure, in which the coordination geometry of aluminum is also distorted trigonal bipyramidal with the tridentate 2a ligand being facially bound. In the presence of one equiv. of benzyl alcohol, complex 3a is a competent catalyst precursor for the living ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), producing poly(ε-caprolactone) and poly(rac-lactide), respectively, in a controlled manner. As such, well-defined block copolymers of ε-CL with rac-LA can also be prepared by catalytic 3a. Kinetic studies revealed that 3a catalyzes the polymerization of rac-lactide at a rate 2-fold faster than that of 3b, indicating the significance of the P-substituent effect on this catalysis. Interestingly, the polymerization rate of rac-lactide by 3a is 16.5 times faster than that of l-lactide under otherwise identical conditions.

Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides

A review. Many discrete cationic complexes have been designed and exhibited increasingly impressive catalytic performances for the ROP of epoxides and cyclic esters. The current state-of-the-art suggests that major breakthroughs cannot be foreseen in the homopolymn. of epoxides, due to the nature of the assocd. mechanisms, and as a consequence, interest in the use of metal cations to polymerize epoxides has dwindled. It seems to the authors that in view of the activated monomer mechanism that is often assocd. with discrete metal cations, this is clearly an area where the use of rationally devised cationic complexes could bring original breakthroughs.

Multiarm star poly(ɛ-caprolactone) with hyperbranched polyamidoamine as core capable of selective accommodating cationic or anionic guests

Hyperbranched polyamidoamines (HPAs) were directly employed as macroinitiators to initiate the Sn(Oct)2 catalyzed ring-opening polymerization of ɛ-caprolactone (CL), resulting in multiarm star copolymers with poly(ɛ-caprolactone) (PCL) as shells and HPA as core (HPA-b-PCL). From 1H-NMR characterization it was deduced that both the primary amines and the secondary amide groups of HPAs could initiate the CL polymerization, and the initiation efficiency increased when more CL monomers were fed. The average arm-numbers of the obtained stars were in the range of 115–353. Differential scanning calorimetry measurements demonstrated that the melting and crystallization temperatures, fusion and crystallization enthalpy and the degree of crystallinity of the star polymers increased as the PCL arm length increased. HPA-b-PCL stars could be used as nanocarriers to efficiently accommodate anionic dyes at acidic condition, while load cationic dyes at basic condition. Compared with the dye-loading behavior of multiarm star PCL with the neutral hyperbranched polyglycerol as core, it was deduced that HPA-b-PCL nanocarriers accommodated anionic dyes using the HPA core, while loaded cationic dyes using both the HPA core and the PCL shell. Dynamic light scattering analyses also supported such deduction. Furthermore, HPA-b-PCL nanocarriers could selectively load the anionic Eosin Y or the cationic methylene blue from their mixture at pH = 6 or 9, respectively, realizing their separation.

Beyond stereoselectivity, switchable catalysis: some of the last frontier challenges in ring-opening polymerization of cyclic esters.

Metal-based catalysts and initiators have played a pivotal role in the ring-opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis--stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal-based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts--primarily metal-based catalysts, but also organocatalysts--that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.

Poly(ω-pentadecalactone)-b-poly(l-lactide) Block Copolymers via Organic-Catalyzed Ring Opening Polymerization and Potential Applications

Poly(pentadecalactone)-b-poly(l-lactide) (PPDL-b-PLLA) diblock copolymers were prepared via the organic catalyzed ring-opening polymerization (ROP) of l-lactide (l-LA) from PPDL macroinitiators using either 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Synthesis of PLLA blocks targeting degrees of polymerization (DP) up to 500 were found to yield diblock copolymers with crystalline PPDL and PLLA segments when TBD was used as the catalyst. The synthesis was further improved in a one-pot, two-step process using the same TBD catalyst for the synthesis of both segments. The application of these diblock copolymers as a compatibilizing agents resulted in homogenization of a biobased PLLA/poly(ω-hydroxytetradecanoate) (90:10) blend upon a melt-process, yielding enhanced material properties.

Rapid and controlled polymerization of rac-lactide using copper(II) complexes of methyl-naphthalenylmethyl-(R,R)-1,2-diaminocyclohexanes

Novel enantiopure Cu(II) complexes of (R,R)-N1,N2-dimethyl-bis(naphthalen-1-ylmethyl)cyclohexane-1,2-diamine (1-NMCD) and (R,R)-N1,N2-dimethyl-bis(naphthalen-2-ylmethyl)cyclohexane-1,2-diamine (2-NMCD) ligands were synthesized and characterized by X-ray diffraction. X-ray diffraction analysis revealed that the molecular structures of the synthesized complexes were mononuclear with Cu(II) atoms in distorted square planar geometries. The diisopropoxide derivatives generated in situ catalyzed ring opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled fashion and displayed unusually high activities (10s or 25s for complete conversion). Heterotactic enriched PLA (with Pr up to 0.90) was obtained with initiator bearing 2-NMCD in CH2Cl2.

Actinide Amidinate Complexes with a Dimethylamine Side Arm: Synthesis, Structural Characterization, and Reactivity

The reactivity of monoanionic amidinate ligands containing a dimethylamine side arm with variable lengths of the linker chain and aromatic substituents of the ipso carbon atom was investigated for the early actinides thorium and uranium. The bis(amidinate) actinide complexes obtained were structurally characterized, displaying a coordination of both dimethylamine nitrogen atoms to the respective metal center, allowing for a fine tuning of the reactivity of the complex by manipulation of the coordination environment around the metal center. The reactivity of the actinide amidinate complexes was studied in the catalytic ring-opening polymerization of e-caprolactone.

Ring-opening homo- and co-polymerization of lactides and ε-caprolactone by salalen aluminum complexes.

Aluminum complexes of non-chiral-salalen ligands were investigated in the catalysis of ring-opening polymerization (ROP) of lactide and ε-caprolactone and in their copolymerization. The aluminum-salalen complexes were found to polymerize all varieties of lactide, namely: l-, d-, rac- and meso-lactide, and showed moderate productivities. rac-LA gave rise to isotactic polylactides (with Pm up to 72%), while meso-LA gave rise to heterotactic polylactides (with a Pm of 79%). An experiment was designed for distinguishing between chain-end control and enantiomorphic-site control combined with polymeryl exchange for the isotactic stereoblock microstructure observed for the PLA produced from rac-LA; it gave strong evidence for polymeryl exchange between propagating species. Finally, this class of catalysts promoted the copolymerization of ε-caprolactone and lactides. In particular, compound allowed controlled random copolymerization of ε-caprolactone and l-lactide.

Mono-, di- and tetra-zinc complexes derived from an amino-benzotriazole phenolate ligand containing a bulkier N-alkyl pendant arm: synthesis, structure and catalysis for ring-opening polymerization of cyclic esters.

Zinc complexes constructed from the amino-modified benzotriazole phenol pro-ligand, 2-(2H-benzotriazol-2-yl)-6-((diisopropylamino)methyl)-4-(2,4,4-trimethylpentan-2-yl)phenol ((C8DIA)BTP-H, 1), were synthesized stepwise and structurally characterized. The reaction of (C8DIA)BTP-H (1) with one equivalent of diethyl zinc (ZnEt2) generates a dimeric and four-coordinated zinc complex, [(μ-(C8DIA)BTP)ZnEt]2 (2), which is doubly bridged by two phenolate groups of C8DIABTP ligands. Further reaction of 2 with benzyl alcohol (BnOH) in stoichiometric proportions affords a tetranuclear zinc benzylalkoxide complex [(μ-OBn)((C8DIA)BTP)Zn]4 (3) that possesses a saddle-shaped core with four μ2-bridging benzylalkoxy groups upon four Zn centres. Interestingly, the di-nuclear Zn alkoxide [(μ-OBn)((C8DIA)BTP)Zn(DMAP)]2 (4) could be prepared by treatment of 3 with a stoichiometric amount of 4-(dimethylamino)pyridine (DMAP). ZnEt2 reacts with two equivalents of 1 in the presence of DMAP (1.0 mol equiv.) to yield a five-coordinated mononuclear zinc complex, [((C8DIA)BTP)2Zn(DMAP)] (5). All complexes adopt an N,O-bidentate coordination mode from the phenoxy oxygen atom and benzotriazole nitrogen atom, in which the nitrogen atom of the pendent arm substituent is not coordinated to the zinc centre. Ring-opening polymerization of ε-caprolactone and β-butyrolactone catalysed by 2 and 3 was investigated.