Selective Ring‐Opening Polymerization of Non‐Strained γ‐Butyrolactone Catalyzed by A Cyclic Trimeric Phosphazene Base

A new superbase, the cyclic trimeric phosphazene base (CTPB), was prepared with high yield and purity. In the presence of alcohol, the CTPB serves as a highly efficient organocatalyst for ring‐opening polymerization of the “non‐polymerizable” γ‐butyrolactone to offer well‐defined poly(γ‐butyrolactone) with high conversions (up to 98 %) at −60 °C. The produced polymers have high molecular weights (up to 22.9 kg mol⁻¹) and low polydispersity distributions (1.27–1.50). NMR analysis of initiation process and the structural analysis of resulting polymers by MALDI‐TOF suggest a mechanism involving an activating initiator which leads only to linear polymers with BnO/H chain ends.     

Influence of Microwave Irradiation on the Lipase‐Catalyzed Ring‐Opening Polymerization of ε‐Caprolactone

Summary: The microwave (MW)‐assisted lipase‐catalyzed ring‐opening polymerization of ε‐caprolactone in boiling solvents was investigated for the first time. In case of boiling toluene or benzene the MW‐assisted reaction proceeded significantly slower compared to oil bath heating. On the other hand, using boiling diethyl ether as solvent, an increase of the polymerization rate due to MW irradiation was found. Yield, molecular weight measurements, and MALDI‐TOF analysis supported the results.

Reactivity of the MW‐assisted ring‐opening polymerization of ε‐caprolactone compared with conventional thermal heating in different solvents.     

Enzyme‐Catalyzed Ring‐Opening Polymerization of Unsubstituted β‐Lactam

The synthesis of poly(β‐alanine) by Candida antarctica lipase B immobilized as novozyme 435 catalyzed ring‐opening of 2‐azetidinone is reported. After removal of cyclic side products and low molecular weight species pure linear poly(β‐alanine) is obtained. The formation of the polymer is confirmed with ¹H NMR spectroscopy and MALDI‐TOF mass spectrometry. The average degree of polymerization of the obtained polymer is limited to = 8 by its solubility in the reaction medium. Control experiments with β‐alanine as a substrate confirmed that the ring structure of the 2‐azetidinone is necessary to obtain the polymer.

     

Lipase‐Catalyzed Transformation of Unnatural‐Type Poly(3‐hydroxybutanoate) into Reactive Cyclic Oligomer

Unnatural‐type syndiotactic and atactic poly[(R,S)‐3‐hydroxybutanoate]s [P(3HB)s] were enzymatically transformed into a reactive cyclic 3HB oligomer of molecular weight ca. 500 in an organic solvent, such as toluene, using immobilized lipase from Candida antarctica at 40°C for 24 h. It was confirmed that similar results were obtained for both syndiotactic and atactic P(3HB)s. On the other hand, the acidic degradation of these polymers using a protonic acid, such as p‐toluenesulfonic acid, exclusively produced the linear 3HB oligomer instead of the cyclic oligomer. The formation of the cyclic oligomer was regarded as the characteristic feature of the lipase‐catalyzed degradation in organic media. The cyclic oligomer obtained readily reacted with alcohol as a nucleophile, and using lipase, to produce the alkyl ester of the 3HB oligomer.     

Lipase‐Catalyzed Ring‐Opening Polymerization of 3(S)‐sec‐Butylmorpholine‐2,5‐dione

Lipase‐catalyzed ring‐opening bulk polymerizations of 3(S)‐sec‐butylmorpholine‐2,5‐dione (BMD) were investigated. Selected commercial lipases were screened as catalysts for BMD polymerization at 110°C. Polymerizations catalyzed with 10 wt.‐% of lipase PPL and PC result in BMD conversions of about 70% and in molecular weights of the products ranging from 5 500 to 10 700. Lipases MJ, CR and ES showed lower catalytic activities for the polymerization of BMD. Poly(3‐sec‐butylmorpholine‐2,5‐dione) has a carboxylic acid group at one end and a hydroxy group at the other end. During the polymerization racemization of the isoleucine residue takes place. Lipase PPL was selected for a more detailed study. The apparent rate of polymerization increases with increasing PPL concentration when the polymerization temperature is 110°C. When the PPL concentration is 5 and 10 wt.‐% with respect to the monomer, a conversion of about 70% is reached after 5 d and 3 d, respectively, while for a PPL concentration of 1 wt.‐% the conversion is less than 7% even after 6  d. High concentrations of PPL (10 wt.‐%) result in high M_n values (< 4  d). The highest molecular weight poly(BMD), M_n = 19 900, resulted from a polymerization conducted at 120°C with 5 wt.‐% PPL for 6 d. The general trend observed by varying the polymerization temperature is as follows: (i) monomer conversion and M_n increase with increasing reaction temperature from 110 to 125°C, (ii) monomer conversion and M_n decrease with an increase in reaction temperature from 125 to 130°C. Water content was found to be an important factor that controls both the conversion and the molecular weight. With increasing water content, enhanced polymerization rates are achieved while the molecular weight of poly(BMD) decreases.     

Mechanistic Investigations of the Stereoselective Rare Earth Metal‐Mediated Ring‐Opening Polymerization of β‐Butyrolactone

Poly(3‐hydroxybutyrate) (PHB) is produced by numerous bacteria as carbon and energy reserve storage material. Whereas nature only produces PHB in its strictly isotactic (R) form, homogeneous catalysis, when starting from racemic (rac) β‐butyrolactone (BL) as monomer, can in fact produce a wide variety of tacticities. The variation of the metal center and the surrounding ligand structure enable activity as well as tacticity tuning. However, no homogeneous catalyst exists to date that is easy to modify, highly active, and able to produce PHB with high isotacticities from rac‐β‐BL. Therefore, in this work, the reaction kinetics of various 2‐methoxyethylamino‐bis(phenolate) lanthanide (Ln=Sm, Tb, Y, Lu) catalysts are examined in detail. The order in monomer and catalyst are determined to elucidate the reaction mechanism and the results are correlated with DFT calculations of the catalytic cycle. Furthermore, the enthalpies and entropies of the rate‐determining steps are determined through temperature‐dependent in situ IR measurements. Experimental and computational results converge in one specific mechanism for the ring‐opening polymerization of BL and even allow us to rationalize the preference for syndiotactic PHB.     

Recent Developments and Optimization of Lipase‐Catalyzed Lactone Formation and Ring‐Opening Polymerization

To obtain materials useful for the biomedical field, toxic catalysts should be removed from the synthetic route of polymerization reactions and of their precursors. Lipase‐catalyzed ring‐opening polymerization and the synthesis of cyclic precursors can be performed with the same catalyst under different conditions. Here, we highlight the use of lipases as catalysts and optimization of their performance for both ring‐closing and ring‐opening polymerization, via varying parameters such as ring size, concentration, substrate molar ratio, temperature, and solvent. While the conditions for ring‐closing reactions and ring‐opening polymerizations of small molecules, such as ε‐caprolactone, have been extensively explored using Candida antarctica lipase B (CALB), the optimization of macrocyclization, especially for more bulky substrates is surveyed here. Finally, recent methods and polymer architectures are summarized with an emphasis on new procedures for more sustainable chemistry, such as the use of ionic liquids as solvents and recycling of polyesters by enzymatic pathways.

     

Ring‐Opening Polymerization of ε‐Caprolactone by Poly(ethylene glycol) by an Activated Monomer Mechanism

Summary: The polymerization of ε‐caprolactone (CL) in the presence of HCl · Et₂O by an activated monomer mechanism was performed to synthesize diblock or triblock copolymers composed of poly(ethylene glycol) (PEG) and poly(ε‐caprolactone) (PCL). The obtained PCLs had molecular weights close to the theoretical values calculated from the CL to PEG molar ratios and exibited monomodal GPC curves. We successfully prepared PEG and PCL block copolymers by a metal‐free method.

The non‐metal catalyzed living ring‐opening polymerisation of ε‐caprolactone by PEG.     

Controlled Ring‐Opening Polymerization of O‐Carboxyanhydrides Using a β‐Diiminate Zinc Catalyst

Recently O‐carboxyanhydrides (OCAs) have emerged as a class of viable monomers which can undergo ring‐opening polymerization (ROP) to prepare poly(α‐hydroxyalkanoic acid) with functional groups that are typically difficult to achieve by ROP of lactones. Organocatalysts for the ROP of OCAs, such as dimethylaminopyridine (DMAP), may induce undesired epimerization of the α‐carbon atom in polyesters resulting in the loss of isotacticity. Herein, we report the use of (BDI‐IE)Zn(OCH(CH₃)COOCH₃) ((BDI)Zn‐1, (BDI‐IE)=2‐((2,6‐diethylphenyl)amino)‐4‐((2,6‐diisopropylphenyl)imino)‐2‐pentene), for the controlled ROP of various OCAs without epimerization. Both homopolymers and block copolymers with controlled molecular weights, narrow molecular weight distributions, and isotactic backbones can be readily synthesized. (BDI)Zn‐1 also enables controlled copolymerization of OCAs and lactide, facilitating the synthesis of block copolymers potentially useful for various biomedical applications. Preliminary mechanistic studies suggest that the monomer/dimer equilibrium of the zinc catalyst influences the ROP of OCAs, with the monomeric (BDI)Zn‐1 possessing superior catalytic activity for the initiation of ROP in comparison to the dimeric (BDI)Zn complex.     

Preparation of Poly(ε‐caprolactone)/Clay Nanocomposites by Microwave‐Assisted In Situ Ring‐Opening Polymerization

PCL/clay nanocomposites were prepared by microwave‐assisted in situ ROP of ε‐caprolactone in the presence of either unmodified clay (Cloisite® Na⁺) or clay modified by quaternary ammonium cations containing hydroxyl groups (Cloisite 30B). This PCL showed significantly improved monomer conversion and molecular weight compared with that produced by conventional heating. An intercalated structure was observed for the PCL/Cloisite Na⁺ nanocomposites, while a predominantly exfoliated structure was observed for the PCL/Cloisite 30B nanocomposites. Microwave irradiation proved to be an effective and efficient method for the preparation of PCL/clay nanocomposites.

     

Discrete Metal Catalysts for Stereoselective Ring‐Opening Polymerization of Chiral Racemic β‐Lactones

Poly(β‐hydroxyalkanoate)s (PHAs) are a class of aliphatic polyesters that can be efficiently synthesized by ring‐opening polymerization (ROP) of β‐lactones. The case of chiral racemic β‐substituted β‐lactones is particularly appealing since these monomers open the way to original tacticities and materials different from those biotechnologically produced. In this overview, after briefly surveying general considerations associated to the ROP of β‐lactones and metal‐based catalysts used in stereoselective ROP of racemic β‐butyrolactone, special emphasis is given to discrete rare earth catalysts that have allowed the preparation of highly syndiotactic poly(3‐hydroxybutyrate)s. Recent developments – such as preparation of stereocontrolled PHAs with pendant structural groups via (co)polymerization of functional β‐substituted β‐lactones, and highly alternating copolymers obtained by ROP of mixtures of enantiomerically pure but different monomers – are also discussed.

     

Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand‐Controlled Yttrium‐Catalyzed Stereoselective Ring‐Opening Polymerization of Functional Racemic β‐Lactones

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β‐lactones, namely 4‐alkoxymethylene‐β‐propiolactones (BPL^ORs). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOO^R′2}²⁻, which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3‐hydroxyalkanoate)s. A simple modification of the R′ substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (P_s up to 0.91 with R′=cumyl) to very high isoselectivity (P_i up to 0.93 with R′=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral β‐lactone.     

Antimicrobial Hydantoin‐grafted Poly(ε‐caprolactone) by Ring‐opening Polymerization and Click Chemistry

Novel degradable and antibacterial polycaprolactone‐based polymers are reported in this work. The polyesters with pendent propargyl groups are successfully prepared by ring‐opening polymerization and subsequently used to graft antibacterial hydantoin moieties via click chemistry by a copper(I)‐catalyzed azide‐alkyne cycloaddition reaction. The well‐controlled chemical structures of the grafted copolymers and its precursors are verified by FT‐IR spectroscopy, NMR spectroscopy, and GPC characterizations. According to the DSC and XRD results, the polymorphisms of these grafted copolymers are mostly changed from semicrystalline to amorphous depending on the amount of grafted hydantoin. Antibacterial assays are carried out with Bacillus subtilis and two strains of Escherichia coli and show fast antibacterial action.

     

Synthesis of Novel μ‐Star Copolymers with Poly(N‐Octyl Benzamide) and Poly(ε‐Caprolactone) Miktoarms through Chain‐Growth Condensation Polymerization,Styrenics‐Assisted Atom Transfer Radical Coupling,and Ring‐Opening Polymerization

Star copolymers are known to phase separate on the nanoscale, providing useful self‐assembled morphologies. In this study, the authors investigate synthesis and assembly behavior of miktoarm star (μ‐star) copolymers. The authors employ a new strategy for the synthesis of unprecedented μ‐star copolymers presenting poly(N‐octyl benzamide) (PBA) and poly(ε‐caprolactone) (PCL) arms: a combination of chain‐growth condensation polymerization, styrenics‐assisted atom transfer radical coupling, and ring‐opening polymerization. Gel permeation chromatography, mass‐analyzed laser desorption/ionization mass spectrometry, and ¹H NMR spectroscopy reveal the successful synthesis of a well‐defined (PBA₁₁)₂‐(PCL₁₅)₄ μ‐star copolymer (M _n,NMR ≈ 12 620; Đ = 1.22). Preliminary examination of the PBA₂PCL₄ μ‐star copolymer reveals assembled nanofibers having a uniform diameter of ≈20 nm.

     

Synthesis and Characterization of β‐Diketiminate Aluminum Compounds and Their Use in the Ring‐Opening Polymerization of ε‐Caprolactone

Four aluminum alkyl compounds, [CH{(CH₃)CN‐2,4,6‐MeC₆H₂}₂AlMe₂] ( 1 ), [CH{(CH₃)CN‐2,4,6‐MeC₆H₂}₂AlEt₂] ( 2 ), [CH{(CH₃)CN‐2‐iPrC₆H₄}₂AlMe₂] ( 3 ), and [CH{(CH₃)CN‐2‐iPrC₆H₄}₂AlEt₂] ( 4 ), bearing β‐diketiminate ligands [CH{(Me)CN‐2,4,6‐MeC₆H₂}]₂ (L¹H) and [CH{(Me)CN‐2‐ⁱPrC₆H₄}]₂ (L²H) were obtained from the reactions of trimethylaluminum, triethylaluminum with the corresponding β‐diketiminate, respectively. All compounds were characterized by ¹H NMR and ¹³C NMR spectroscopy, single‐crystal X‐ray structural analysis, and elemental analysis. Compounds 1 – 4 were found to catalyze the ring‐opening polymerization (ROP) of ε‐caprolactone (ε‐CL) with good activity.     

Microwave‐Assisted Ring‐Opening Polymerization of ε‐Caprolactone in the Presence of Ionic Liquid

Summary: Microwave‐assisted ring‐opening polymerization of ε‐caprolactone in the presence of 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid using zinc oxide as a catalyst is investigated. By adding 30 wt.‐% ionic liquid, poly(ε‐caprolactone) with a weight‐average molar mass of 28 500 g · mol⁻¹ is obtained at 85 W for 30 min. The results indicate that the polymerization could be efficiently enhanced in the presence of ionic liquids under microwave irradiation because ionic liquids can effectively absorb microwave energy.

     

Surface‐Initiated Ring‐Opening Polymerization of ϵ‐Caprolactone from a Patterned Poly(hydroxymethyl‐ p‐xylylene)

A new strategy toward patterned polymer brushes combining the spatially controlled deposition of poly[(hydroxymethyl‐p‐xylylene)‐co‐(p‐xylylene)] ( 1 ) by chemical vapor deposition (CVD) polymerization of 4‐(hydroxymethyl)[2.2]paracyclophane and surface‐initiated ring‐opening polymerization was developed. Patterns of polymer brushes with thicknesses between 53 and 538 Å were created. The approach does not require photolithographic tools and has potential applicability to a wide range of different substrates, such as glasses, polymers, metals or composites.     

Poly(arylenevinylene)s through Ring‐Opening Metathesis Polymerization of an Unsymmetrical Donor‐Acceptor Cyclophane

Reported are well‐defined donor‐acceptor alternating copolymers prepared using ring‐opening metathesis polymerization (ROMP). Unsymmetrical cyclophanedienes comprising electron‐donating (4‐methoxy‐1‐(2‐ethylhexyl)oxy)benzene (MEH) and electron‐accepting benzothiadiazole (BT) rings were synthesized from the corresponding [3.3]dithiaparacyclophanes. ROMP of the strained unsymmetrical and “electronically‐ambiguous” cyclophanedienes proceeded in a controlled manner in the presence of either Hoveyda–Grubbs II or Grubbs II initiator in wake of both steric and electronic encumbrance. The resulting polymers, comprising alternating BT and MEH‐PPV units, are achieved in molecular weights exceeding 20k with ? values ranging from 1.1–1.4. The living nature of the polymerization is verified through the formation of rod‐coil and rod‐rod block copolymers. Our strategy to develop previously unrealized polymers from functional building blocks featuring a locked‐in D‐A unit is significant in a field striving to achieve well‐defined and sequence‐specific materials.