Synthesis of macrocyclic poly(epsilon-caprolactone) by intramolecular cross-linking of unsaturated end groups of chains precyclic by the initiation

Insights into substitution strategy towards thermodynamic and property regulation of chemically recyclable polymers

The development of chemically recyclable polymers serves as an attractive approach to address the global plastic pollution crisis. Monomer design principle is the key to achieving chemical recycling to monomer. Herein, we provide a systematic investigation to evaluate a range of substitution effects and structure-property relationships in the ɛ-caprolactone (CL) system. Thermodynamic and recyclability studies reveal that the substituent size and position could regulate their ceiling temperatures (T_c). Impressively, M4 equipped with a tert-butyl group displays a T_c of 241 °C. A series of spirocyclic acetal-functionalized CLs prepared by a facile two-step reaction undergo efficient ring-opening polymerization and subsequent depolymerization. The resulting polymers demonstrate various thermal properties and a transformation of the mechanical performance from brittleness to ductility. Notably, the toughness and ductility of P(M13) is comparable to the commodity plastic isotactic polypropylene. This comprehensive study is aimed to provide a guideline to the future monomer design towards chemically recyclable polymers.

Syntheses of polylactides by means of tin catalysts

Reaction mechanisms and synthetic methods used for the preparation of homo- and copolylactides based on tin(ii) and tin(iv) catalysts are reviewed.

Rhodanine-based Knoevenagel reaction and ring-opening polymerization for efficiently constructing multicyclic polymers

Cyclic polymers have a number of unique physical properties compared with those of their linear counterparts. However, the methods for the synthesis of cyclic polymers are very limited, and some multicyclic polymers are still not accessible now. Here, we found that the five-membered cyclic structure and electron withdrawing groups make methylene in rhodanine highly active to aldehyde via highly efficient Knoevenagel reaction. Also, rhodanine can act as an initiator for anionic ring-opening polymerization of thiirane to produce cyclic polythioethers. Therefore, rhodanine can serve as both an initiator for ring-opening polymerization and a monomer in Knoevenagel polymerization. Via rhodanine-based Knoevenagel reaction, we can easily incorporate rhodanine moieties in the backbone, side chain, branched chain, etc, and correspondingly could produce cyclic structures in the backbone, side chain, branched chain, etc, via rhodanine-based anionic ring-opening polymerization. This rhodanine chemistry would provide easy access to a wide variety of complex multicyclic polymers.

Cyclic topology effects on the morphology of biocompatible and environment-friendly poly(ε-caprolactone) under nanoscale film confinement

Quantitative grazing incidence X-ray scattering analysis combined with X-ray reflectivity using synchrotron radiation sources was explored for the first time cyclic topology effects on the nanoscale film morphology of poly(ε-caprolactone).

Ligand-modulated ring-expansion

An unstrained palladium-containing macrocycle was expanded by a ring-opening metathesis strategy. This reaction was driven by coordination of a bulky ligand, 2,6-lutidine, to the palladium center, enabling a monometallic macrocycle to transform into a bimetallic dimeric macrocycle.

Blocking-cyclization technique for precise synthesis of cyclic polymers with regulated topology

Ring-closure and ring-expansion techniques are the two routes for extensive synthesis of cyclic polymers. Here, we report an alternative blocking-cyclization technique referred to as the third route to prepare cyclic polymers with regulated ring size and ring number by ring-opening metathesis polymerization of di- and monofunctional monomers in a one-pot process, where the polymer intermediates bearing two single-stranded blocks are efficiently cyclized by the cyclizing unit of propagated ladderphane to generate corresponding mono-, bis-, and tricyclic polymers, and the well-defined ladderphane structure plays a crucial role in forming the cyclic topology. Monocyclic polymer is further modified via Alder-ene reaction and the cyclic molecular topology is clearly demonstrated. The diversity features of cyclic polymers are comprehensively revealed. This strategy has broken through the limitations of previous two cyclizing routes, and indeed opens a facile and popular way to various cyclic polymers by commercial Grubbs catalyst and conventional metathesis polymerization.

Bimetallic Aluminum 5,6-Dihydro-7,7-dimethyl quinolin-8-olates as Pro-Initiators for the ROP of ε-CL; Probing the Nuclearity of the Active Initiator

Six examples of aluminum 5,6-dihydro-7,7-dimethylquinolin-8-olates, [{2-R¹-7,7-Me₂-8-R²C₉H₆N-8-O}AlR³₂]₂ (R¹ = R² = H, R³ = Me C1; R¹ = R² = H, R³ = Et C2; R¹ = R² = H, R³ = i-Bu C3; R¹ = Cl, R² = H, R³ = Me C4; R¹ = H, R² = R³ = Me C5; R¹ = Cl, R² = R³ = Me C6), have been prepared by treating the corresponding pro-ligand (L1⁻L4) with either AlMe₃, AlEt₃ or Al(i-Bu)₃. All complexes have been characterized by ¹H and 13C NMR spectroscopy and in the case of C1 and C4 by single crystal X-ray diffraction; dimeric species are a feature of their molecular structures. In the presence of PhCH₂OH (BnOH), C1⁻C6 displayed good control and efficiency for the ROP of ε-CL with almost 100% conversion achievable in 10 min at 90 °C; the chloro-substituted C4 and C6 notably exhibited the lowest activity of the series. However, in the absence of BnOH, C1 showed only low activity with 15% conversion achieved in 30 min forming a linear polymer capped with either a methyl or a L1 group. By contrast, when one or more equivalents of BnOH was employed in combination with C1, the resulting catalyst was not only more active but gave linear polymers capped with BnO end-groups. By using ¹H and 27Al NMR spectroscopy to monitor solutions of C1, C1/BnOH and C1/BnOH/10 ε-CL over a range of temperatures, some support for a monomeric species being the active initiator at the operational temperature is presented.

Precise Synthesis, Properties, and Structures of Cyclic Poly(ε-caprolactone)s

Cyclic PCL (c-PCL) has drawn great attention from academia and industry because of its unique, unusual structure and property characteristics due to the absence of end groups in addition to the biocompatibility and biodegradability of its linear analogue. As a result of much research effort, several synthetic methods have been developed to produce c-PCLs so far. Their chain, morphology and property characteristics were investigated even though carried out on a very limited basis. This feature article reviews the research progress made in the synthesis, morphology, and properties of c-PCL; all results and their pros and cons are discussed in terms of purity and molecular weight distribution in addition to the cyclic topology effect. In addition, we attempted to synthesize a series of c-PCL products of high purity by using intramolecular azido-alkynyl click cyclization chemistry and subsequent precise and controlled separation and purification; and their thermal degradation and phase transitions were investigated in terms of the cyclic topology effect.

Dinuclear zirconium complex bearing a 1,5-bridged-calix[8]arene ligand as an effective catalyst for the synthesis of macrolactones

“Macrocyclization of Cycles with a Macrocycle.” The intramolecular cooperation of metal centres in a novel dinuclear complex has been postulated as responsible for the observed high rate of formation of macrocyclic polyesters.

Advanced Developments in Cyclic Polymers: Synthesis, Applications, and Perspectives

Due to the topological effect, cyclic polymers demonstrate different and unique physical and biological properties in comparison with linear counterparts having the same molecular-weight range. With advanced synthetic and analytic technologies, cyclic polymers with different topologies, e.g. multicyclic polymers, have been reported and well characterized. For example, various cyclic DNA and related structures, such as cyclic duplexes, have been prepared conveniently by click chemistry. These types of DNA have increased resistance to enzymatic degradation and have high thermodynamic stability, and thus, have potential therapeutic applications. In addition, cyclic polymers have also been used to prepare organic-inorganic hybrids for applications in catalysis, e.g. catalyst supports. Due to developments in synthetic technology, highly pure cyclic polymers could now be produced in large scale. Therefore, we anticipate discovering more applications in the near future. Despite their promise, cyclic polymers are still less explored than linear polymers like polyolefins and polycarbonates, which are widely used in daily life. Some critical issues, including controlling the molecular weight and finding suitable applications, remain big challenges in the cyclic-polymer field. This review briefly summarizes the commonly used synthetic methodologies and focuses more on the attractive functional materials and their biological properties and potential applications.

Continuous cyclo-polymerisation of l-lactide by reactive extrusion using atoxic metal-based catalysts: easy access to well-defined polylactide macrocycles

Macrocyclic polylactide was synthesized by reactive extrusion polymerisation of l-lactide using lanthanide trisborohydrides as the catalysts. This is the first example of a cyclic polyester obtained directly via a continuous process.

Design and fabrication of functional polycaprolactone

Functional polycaprolactone (PCL) has great potential for applications in biomedical areas. This review summarizes the recent progress in the chemical synthesis of functional PCL. The functional PCL can be synthesized by (a) homopolymerization or copolymerization of functional ε-caprolactone (ε-CL), (b) copolymerization of 2-methylene-1-3-dioxepane with functional vinyl monomers, or (c) copolymerization of ε-CL with functional carbonate monomers. This review presents the recent trends in the synthesis of functional PCL and its biomedical applications.

Synthesis of cyclopolyolefins via ruthenium catalyzed ring-expansion metathesis polymerization

Polymers exhibiting cyclic topology have attracted great interest over the past 30 years. Macrocycles or cyclopolymers with more than 20 repeating monomer units are considered exceptional candidates for thermoplastic engineering application due to unique properties in comparison to their linear analogues including large hydrodynamic radii and functional group density, heat resistance, good insulating ability and low intrinsic viscosity. Cyclic polymers are thus expected to exhibit improved physical and mechanical properties for certain applications due to the absence of end groups. Although synthetic challenges have historically limited research on cyclic polymers, recent developments in ruthenium catalyzed ring-expansion metathesis polymerization (REMP) have enabled the synthesis and the preliminary investigation of structure-property relationships of high molecular weight macrocycles. In REMP reactions the polymer formation is proposed to proceed through a transient macrocyclic complex in which both ends of the growing polymer chain remain attached to the Ru center. This article summarizes the recent discoveries on the field of REMP assisted cyclopolymer based material development.

Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers

Cyclic polymers are an intriguing class of macromolecules. Because of the constraints of the cyclic topology and the absence of chain ends, the properties of these molecules differ from those of linear polymers in ways that remain poorly understood. Cyclic polymers present formidable synthetic challenges because the entropic penalty of coupling the chain ends grows exponentially with increasing molecular weight. In this Account, we describe recent progress in the application of zwitterionic ring-opening polymerization (ZROP) as a strategy for the synthesis of high molecular weight, cyclic polymers. Zwitterionic ring-opening polymerization involves the addition of neutral organic nucleophiles to strained heterocyclic monomers; under appropriate conditions, cyclization of the resultant macrozwitterions generates cyclic macromolecules. We discuss the mechanistic and kinetic features of these zwitterionic ring-opening reactions and the conditions that influence the efficiency of the initiation, propagation, and cyclization to generate high molecular weight cyclic polymers. N-Heterocyclic carbenes (NHC) are potent nucleophiles and relatively poor leaving groups, two features that are important for the generation of high molecular weight polymers. Investigations of the nature of the monomer and nucleophile have helped researchers understand the factors that govern the reactivity of these systems and their impact on the molecular weight and molecular weight distributions of the resulting cyclic polymers. We focus primarily on ZROP mediated by N-heterocyclic carbene nucleophiles but also discuss zwitterionic polymerizations with amidine, pyridine, and imidazole nucleophiles. The ZROP of N-carboxyanhydrides with N-hetereocyclic carbenes generates a family of functionalized cyclic polypeptoids. We can synthesize gradient lactone copolymers by exploiting differences in relative reactivity present in ZROP that differ from those of traditional metal-mediated polymerizations. These new synthetic methods have allowed us to investigate the influence of topology on the crystallization behavior, stereocomplexation, and solution properties of cyclic macromolecules.

Ring-Expansion Living Cationic Polymerization via Reversible Activation of a Hemiacetal Ester Bond

In this paper, we provide an effective route to cyclopolymers via the Lewis acid-assisted "ring-expansion" living cationic polymerization of vinyl ethers, directly from a simple "cyclic initiator" designed with a hemiacetal ester for dynamic and reversible initiation and propagation. The built-in hemiacetal ester, or a carboxylic acid-vinyl ether adduct, is a key to control the polymerization: as the leaving group, the activated carboxylate is well-suited for designing the ring structure, differing from monovalent halogens often employed in carbocationic initiation. The choice of a Lewis acid catalyst (SnBr₄) is equally crucial to retain the cyclic structure via the reversibly dissociable but relatively strong ester bond not only during propagation but also even after quenching. The formation of cyclic polymers was proved by irreversibly cleaving the hemiacetal ester linkage of the product via acidic hydrolysis into an open-chain structure, i.e., an increase in size exclusion chromatography (SEC) molecular weight (hydrodynamic radius), along with the clean transformation of the endocyclic hemiacetal ester into an α-carboxylic acid and ω-aldehyde terminals (by NMR). The polymerization was really "living" polymerization via ring-expansion, as demonstrated by successful monomer-addition experiments and a linear increase in molecular weight with conversion. This ring-expansion living polymerization would open a door to well-defined cyclic polymers free from terminus (end groups) and to hybrid macromolecules with combinations of cyclic and linear architectures.