Reversible-deactivation anionic alternating ring-opening copolymerization of epoxides and cyclic anhydrides: access to orthogonally functionalizable multiblock aliphatic polyesters

Al(III)/K(I) Heterodinuclear Polymerization Catalysts Showing Fast Rates and High Selectivity for Polyester Polyols

Low molar mass, hydroxyl end-capped polymers, often termed "polyols," are widely used to make polyurethanes, resins, and coatings and as surfactants in liquid formulations. Epoxide/anhydride ring-opening copolymerization (ROCOP) is a controlled polymerization route to make them, and its viability depends upon catalyst selection. In the catalysis, the polyester polyol molar masses and end-groups are controlled by adding specific but excess quantities of diols (vs catalyst), known as the chain transfer agent (CTA), to the polymerizations, but many of the best current catalysts are inhibited or even deactivated by alcohols. Herein, a series of air-stable Al(III)/K(I) heterodinuclear polymerization catalysts show rates and selectivity at the upper end of the field. They also show remarkable increases in activity, with good selectivity and control, as quantities of diol are increased from 10-400 equiv. The reactions are accelerated by alcohols, and simultaneously, their use allows for the production of hydroxy telechelic poly/oligoesters (400 < Mn (g mol-1) < 20,400, Đ < 1.19). For example, cyclohexene oxide (CHO)/phthalic anhydride (PA) ROCOP, using the best Al(III)/K(I) catalyst with 200 equiv of diol, shows a turnover frequency (TOF) of 1890 h-1, which is 4.4× higher than equivalent reactions without any diol (Catalyst/Diol/PA/CHO = 1:10-400:400:2000, 100 °C). In all cases, the catalysis is well controlled and highly ester linkage selective (ester linkages >99%) and operates effectively using bicyclic and/or biobased anhydrides with bicyclic or flexible alkylene epoxides. These catalysts are recommended for future production and application development using polyester polyols.

A Supported Catalyst that Enables the Synthesis of Colorless CO2 -Polyols with Ultra-Low Molecular Weight

Ultra-low molecular weight (ULMW) CO2 -polyols with well-defined hydroxyl end groups represent useful soft segments for the preparation of high-performance polyurethane foams. However, owing to the poor proton tolerance of catalysts towards CO2 /epoxide telomerization, it remains challenging to synthesize ULMW yet colorless CO2 -polyols. Herein, we propose an immobilization strategy of constructing supported catalysts by chemical anchoring of aluminum porphyrin on Merrifield resin. The resulting supported catalyst displays both extremely high proton tolerance (≈8000 times the equivalents of metal centers) and independence of cocatalyst, affording CO2 -polyols with ULMW (580 g mol-1 ) and high polymer selectivity (>99 %). Moreover, the ULMW CO2 -polyols with various architectures (tri-, quadra-, and hexa-arm) can be obtained, suggesting the wide proton universality of supported catalysts. Notably, benefiting from the heterogeneous nature of the supported catalyst, colorless products can be facilely achieved by simple filtration. The present strategy provides a platform for the synthesis of colorless ULMW polyols derived from not only CO2 /epoxides, but also lactone, anhydrides etc. or their combinations.

Block Poly(carbonate-ester) Ionomers as High-Performance and Recyclable Thermoplastic Elastomers

Thermoplastic elastomers based on polyesters/carbonates have the potential to maximize recyclability, degradability and renewable resource use. However, they often underperform and suffer from the familiar trade-off between strength and extensibility. Herein, we report well-defined reprocessable poly(ester-b-carbonate-b-ester) elastomers with impressive tensile strengths (60 MPa), elasticity (>800 %) and recovery (95 %). Plus, the ester/carbonate linkages are fully degradable and enable chemical recycling. The superior performances are attributed to three features: (1) Highly entangled soft segments; (2) Fully reversible strain-induced crystallization and (3) Precisely placed Zn^II -carboxylates dynamically crosslinking the hard domains. The one-pot synthesis couples controlled cyclic monomer ring-opening polymerization and alternating epoxide/anhydride ring-opening copolymerization. Efficient convresion to ionomers is achieved by reacting vinyl-epoxides to install Zn^II -carboxylates.

Switchable Polymerization: A Practicable Strategy to Produce Biodegradable Block Copolymers with Diverse Properties

With the global demand for sustainable development, there has been an increasing interest in using natural biomass as raw resources to produce sustainable polymers as an alternative to petroleum-based polymers. Because monocomponent biodegradable polymers are often insufficient in performance, copolymers with well-engineered block structures are synthesized to reach wide tunability. Switchable polymerization is such a practical strategy to produce biodegradable block copolymers with diverse performance. This review focus on the performance of block copolymers bearing biodegradable polymer segments produced by diverse switchable polymerization. We highlight two main segments that are critical for biodegradable block copolymers, i. e., polyester and polycarbonate, summarize the multiple characters of materials from switchable polymerization such as antibacterial, shape memory, adhesives, etc. The state-of-the-art research on biodegradable block copolymers, as well as an outlook on the preparation and application of novel materials, are presented.

Stereocomplexation of Stereoregular Aliphatic Polyesters: Change from Amorphous to Semicrystalline Polymers with Single Stereocenter Inversion

Stereocomplexation is a useful strategy for the enhancement of polymer properties by the co-crystallization of polymer strands with opposed chirality. Yet, with the exception of PLA, stereocomplexes of biodegradable polyesters are relatively underexplored and the relationship between polymer microstructure and stereocomplexation remains to be delineated, especially for copolymers comprising two different chiral monomers. In this work, we resolved the two enantiomers of a non-symmetric chiral anhydride (CPCA) and prepared a series of polyesters from different combinations of racemic and enantiopure epoxides and anhydrides, via metal-catalyzed ring-opening copolymerization (ROCOP). Intriguingly, we found that only specific chiral combinations between the epoxide and anhydride building blocks result in the formation of semicrystalline polymers, with a single stereocenter inversion inducing a change from amorphous to semicrystalline copolymers. Stereocomplexes of the latter were prepared by mixing an equimolar amount of the two enantiomeric copolymers, yielding materials with increased melting temperatures (ca. 20 °C higher) compared to their enantiopure constituents. Following polymer structure optimization, the stereocomplex of one specific copolymer combination exhibits a particularly high melting temperature (T_m = 238 °C).

Synthesis and Characterization of Alkyne-Functionalized Photo-Cross-Linkable Polyesters

An alkyne-functionalized elastomer derived from sebacic acid, 1,3-propanediol, and alkyne-functionalized serinol is synthesized via melt condensation. A low-power UV lamp triggers the cross-linking rapidly via thiol-yne click chemistry. The cross-linking behavior is studied by photorheology and NMR spectroscopy. The resultant elastomer possesses mechanical properties similar to those of human soft tissues and exhibits in vitro degradability and good cytocompatibility.

Switchable Polymerization Organocatalysis: From Monomer Mixtures to Block Copolymers

One-pot production of sequence-controlled block copolymer from mixed monomers is a crucial but rarely reached goal. Using a switchable Lewis-pair organocatalyst, we have accomplished sequence-selective polymerization from a mixture of O-carboxyanhydride (OCA) and epoxide. Polymerization of the OCA monomer occurs first and exclusively because of its exceedingly high polymerizability. When OCA is fully consumed, alternating copolymerization of epoxide and CO₂ liberated in OCA polymerization is triggered from the termini of the first block. The two polymerizations thus occur in tandem, both in chemoselective fashion, so that a sequence-controlled block polymer with up to 99 % CO₂ conversion is furnished in this one-pot protocol. Calculations and experimental results demonstrate a chemoselective and cooperative mechanism, where the high polymerizability of the OCA monomers guarantees exquisite sequence selectivity and the cooperative decarboxylation partly arose from the stabilization effect by triethylborane, which facilitates the smooth transformation of the chain end from carbonate to alkoxide.

Catalytic Ring-Opening Copolymerization of Fatty Acid Epoxides: Access to Functional Biopolyesters

Fatty acid epoxies serve as valuable starting materials for the development of bio-based polyesters. Here we present a new and efficient catalytic process that allows for the copolymerization of fatty acid-based epoxides and various cyclic anhydrides under attractive process conditions affording functional polyesters. The degree of functionalization and the nature of the polymer backbone can be modulated via monomer design. Postpolymerization cross-linking processes were examined to create rigid macromolecular networks that build on orthogonal polyester functionality, creating possible entries for materials with switchable thermal and mechanical properties.

Exploiting Sodium Coordination in Alternating Monomer Sequences to Toughen Degradable Block Polyester Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) that are closed-loop recyclable are needed in a circular material economy, but many current materials degrade during recycling, and almost all are pervasive hydrocarbons. Here, well-controlled block polyester TPEs featuring regularly placed sodium/lithium carboxylate side chains are described. They show significantly higher tensile strengths than unfunctionalized analogues, with high elasticity and elastic recovery. The materials are prepared using controlled polymerizations, exploiting a single catalyst that switches between different polymerization cycles. ABA block polyesters of high molar mass (60–100 kg mol^–1; 21 wt % A-block) are constructed using the ring-opening polymerization of ε-decalactone (derived from castor oil; B-block), followed by the alternating ring-opening copolymerization of phthalic anhydride with 4-vinyl-cyclohexene oxide (A-blocks). The polyesters undergo efficient functionalization to install regularly placed carboxylic acids onto the A blocks. Reacting the polymers with sodium or lithium hydroxide controls the extent of ionization (0–100%); ionized polymers show a higher tensile strength (20 MPa), elasticity (>2000%), and elastic recovery (>80%). In one case, sodium functionalization results in 35× higher stress at break than the carboxylic acid polymer; in all cases, changing the quantity of sodium tunes the properties. A leading sample, 2-COONa75 (M_n 100 kg mol^–1, 75% sodium), shows a wide operating temperature range (−52 to 129 °C) and is recycled (×3) by hot-pressing at 200 °C, without the loss of mechanical properties. Both the efficient synthesis of ABA block polymers and precision ionization in perfectly alternating monomer sequences are concepts that can be generalized to many other monomers, functional groups, and metals. These materials are partly bioderived and have degradable ester backbone chemistries, deliver useful properties, and allow for thermal reprocessing; these features are attractive as future sustainable TPEs.

Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Temperature triggered alternating copolymerization of epoxides and lactones via pre-sequenced spiroorthoester intermediates

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer. The reaction is catalyzed by the temperature triggered, bifunctional cationic indium complex (±)-[(NN_iO_tBu)In(CH₂SiMe₃)][B(C₆F₅)₄] (1). 1 can catalyze the coupling of epoxide and lactone to form spiroorthoester at 60 °C and its double ring-opening polymerization at 110 °C to form poly(ether-alt-ester). The post-polymerization modification and degradation of the poly(ether-alt-ester) are further investigated.

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer.

Metallocene catalysts for the ring-opening co-polymerisation of epoxides and cyclic anhydrides

The ring-opening co-polymerization (ROCOP) of epoxides and cyclic anhydrides is a versatile route to new polyesters. The vast number of monomers that are readily available means that an effectively limitless...

Iodine and alkali metal alkoxides: a simple and versatile catalytic system for fully alternating polyester synthesis from phthalic anhydride and epoxides

Regioselective ROCOP of various epoxides with phthalic anhydride using readily available and economical catalysts i.e. I2 in combination with alkali metal alkoxides was used.

Mg(ii) heterodinuclear catalysts delivering carbon dioxide derived multi-block polymers

Carbon dioxide derived polymers are emerging as useful materials for applications spanning packaging, construction, house-hold goods and automotive components. To accelerate and broaden their uptake requires both more active and selective catalysts and greater structural diversity for the carbon dioxide derived polymers. Here, highly active catalysts show controllable selectivity for the enchainment of mixtures of epoxide, anhydride, carbon dioxide and lactone. Firstly, metal dependent selectivity differences are uncovered using a series of dinuclear catalysts, Mg(ii)Mg(ii), Zn(ii)Zn(ii), Mg(ii)Zn(ii), and Mg(ii)Co(ii), each exposed to mixtures of bio-derived tricyclic anhydride, cyclohexene oxide and carbon dioxide (1 bar). Depending upon the metal combinations, different block structures are possible with Zn(ii)Zn(ii) yielding poly(ester-b-carbonate); Mg(ii)Mg(ii) or Mg(ii)Co(ii) catalysts delivering poly(carbonate-b-ester); and Mg(ii)Zn(ii) furnishing a random copolymer. These results indicate that carbon dioxide insertion reactions follow the order Co(ii) > Mg(ii) > Zn(ii). Using the most active and selective catalyst, Mg(ii)Co(ii), and exploiting reversible on/off switches between carbon dioxide/nitrogen at 1 bar delivers precision triblock (ABA), pentablock (BABAB) and heptablock (ABABABA) polymers (where A = poly(cyclohexylene oxide-alt-tricyclic anhydride), PE; B = poly(cyclohexene carbonate), PCHC). The Mg(ii)Co(ii) catalyst also selectively polymerizes a mixture of anhydride, carbon dioxide, cyclohexene oxide and ε-caprolactone to deliver a CBABC pentablock copolymer (A = PE, B = PCHC C = poly(caprolactone), PCL). The catalysts combine high activity and selectivity to deliver new polymers featuring regularly placed carbon dioxide and biomass derived linkages.

Sequence Control from Mixtures: Switchable Polymerization Catalysis and Future Materials Applications

There is an ever-increasing demand for higher-performing polymeric materials counterbalanced by the need for sustainability throughout the life cycle. Copolymers comprising ester, carbonate, or ether linkages could fulfill some of this demand as their monomer-polymer chemistry is closer to equilibrium, facilitating (bio)degradation and recycling; many monomers are or could be sourced from renewables or waste. Here, an efficient and broadly applicable route to make such copolymers is discussed, a form of switchable polymerization catalysis which exploits a single catalyst, switched between different catalytic cycles, to prepare block sequence selective copolymers from monomer mixtures. This perspective presents the principles of this catalysis, catalyst design criteria, the selectivity and structural copolymer characterization tools, and the properties of the resulting copolymers. Uses as thermoplastic elastomers, toughened plastics, adhesives, and self-assembled nanostructures, and for programmed degradation, among others, are discussed. The state-of-the-art research into both catalysis and products, as well as future challenges and directions, are presented.

Facile Synthesis of Unimodal Polymethacrylates with Narrow Dispersity via NIR LED Light-Controlled Bromine-Iodine Transformation Reversible-Deactivation Radical Polymerization

A facile and clean strategy for synthesizing unimodal polymethacrylates with narrow dispersity (Đ < 1.10) is successfully developed by a near-infrared (NIR) light-emitting diode (LED) light (λ_max = 740 nm)-controlled in situ bromine-iodine transformation reversible-deactivation radical polymerization system without the use of NIR dyes and expensive catalysts. In this system, alkyl iodide ethyl α-iodophenylacetate (EIPA) initiator is generated in situ by the nucleophilic substitution reaction between an alkyl bromide compound ethyl α-bromophenylacetate and sodium iodide (NaI). At the same time, excessive NaI is also acted as a highly active catalyst by forming halogen bonds with terminal iodine of the polymer chains in this system to make it capable of precisely synthesizing polymethacrylates with narrow dispersities (Đ = 1.03-1.10). In addition, the strong penetration ability of NIR LED light is illustrated by the successful polymerization even through 11 pieces of A4 paper.

Recent Developments in Ring-Opening Copolymerization of Epoxides With CO2 and Cyclic Anhydrides for Biomedical Applications

The biomedical applications of polyesters and polycarbonates are of interest due to their potential biocompatibility and biodegradability. Confined by the narrow scope of monomers and the lack of controlled polymerization routes, the biomedical-related applications of polyesters and polycarbonates remain challenging. To address this challenge, ring-opening copolymerization (ROCOP) has been exploited to prepare new alternating polyesters and polycarbonates, which would be hard to synthesize using other controlled polymerization methods. This review highlights recent advances in catalyst development, including the emerging dinuclear organometallic complexes and metal-free Lewis pair systems. The post-polymerization modification methods involved in tailoring the biomedical functions of resultant polyesters and polycarbonates are summarized. Pioneering attempts for the biomedical applications of ROCOP polyesters and polycarbonates are presented, and the future opportunities and challenges are also highlighted.

Organo-catalyzed/initiated ring opening co-polymerization of cyclic anhydrides and epoxides: an emerging story

This review deals with recent organo-catalyzed/initiated developments of co-polymerization of cyclic anhydrides and epoxides to access polyesters.

Bio-based and Degradable Block Polyester Pressure-Sensitive Adhesives

A new class of bio-based fully degradable block polyesters are pressure-sensitive adhesives. Bio-derived monomers are efficiently polymerized to make block polyesters with controlled compositions. They show moderate to high peel adhesions (4-13 N cm-1 ) and controllable storage and loss moduli, and they are removed by adhesive failure. Their properties compare favorably with commercial adhesives or bio-based polyester formulations but without the need for tackifier or additives.

Bifunctional Catalysis Prevents Inhibition in Reversible-Deactivation Ring-Opening Copolymerizations of Epoxides and Cyclic Anhydrides

Reversible-deactivation chain transfer is a viable strategy to increase the catalytic efficiency of ring-opening polymerizations, such as the alternating copolymerization of epoxides and cyclic anhydrides. In conjunction with the catalyst, protic chain transfer agents (CTAs) initiate polymerization and facilitate rapid proton transfer between active and dormant chains. Functional-group-tolerant Lewis acid catalysts are therefore required to successfully apply protic CTAs in reversible-deactivation ring-opening copolymerizations (RD-ROCOP), yet the predominant binary Lewis acid catalyst/nucleophilic cocatalyst systems suffer lower polymerization rates when used with protic CTAs. New mechanistic insight into the inhibition pathways reveals that the alcohol chain ends compete with epoxide binding to the Lewis acid and hydrogen-bond with anionic chain ends to impede epoxide ring opening. We report that a bifunctional aminocyclopropenium aluminum salen complex maintains excellent activity in the presence of protic functionality, exhibiting resilience against these inhibition pathways, even at high CTA concentrations. We apply reversible-deactivation chain transfer in the bifunctional ROCOP system to demonstrate precise molecular-weight control, CTA functional group scope, and accessible polymer architectures.

Mechanically robust and reprocessable imine exchange networks from modular polyester pre-polymers

Covalent adaptable networks (CANs) containing dynamic imine cross-links impart recyclability to thermoset materials, and the distribution of these cross-links greatly affects their observed thermomechanical properties.

Polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids: an efficient access to functionalized alternating polyesters

We report an efficient and mild approach to functionalized alternating polyesters on the basis of O–H insertion polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids.

Orthogonal functionalization of alternating polyesters: selective patterning of (AB) n sequences

Precision functionalized polyesters, with defined monomer sequences, are prepared using an orthogonal post-polymerization strategy. These polyesters can be synthesized from bio-derived monomers and are targeted to degrade, by hydrolysis processes, to biocompatible diols and diacids; the new structures enabled by this methodology would be very difficult to synthesize by alternative strategies. A series of 9 well-defined highly alternating AB-type copolyesters, containing terminal and internal alkene functionalities, are synthesized in high conversions by the ring-opening copolymerization of epoxides and cyclic anhydrides. Firstly, the polyesters are functionalized by a selective hydroboration-oxidation reaction to exclusively and quantitatively hydroxylate the terminal alkenes, leaving the alternating internal alkenes unreacted. Subsequently, the internal alkenes are quantitatively transformed into carboxylic acid, amine, alkyl and oligo-ether groups, by thiol-ene reactions, to afford AB polyesters with alternating functional substituents. Three polyesters showing alternating hydrophilic/hydrophobic side-chain sequences self-assemble in solution to form nanostructures that are characterized using transmission electron microscopy and dynamic light scattering methods (R h = 100-300 nm). The selective patterning methodology provides facile, efficient and orthogonal functionalization of alternating polyesters with near-quantitative (AB) n repeat sequences. The method is expected to be generalizable to other polymers and provides access to completely new AB alternating structures with the potential to exploit ligand multi-valency and adjacency to enhance properties.

Mechanism-Inspired Design of Bifunctional Catalysts for the Alternating Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides

Advances in catalysis have enabled the ring-opening copolymerization of epoxides and cyclic anhydrides to afford structurally and functionally diverse polyesters with controlled molecular weights and dispersities. However, the most common systems employ binary catalyst/cocatalyst pairs which suffer from slow polymerization rates at low loadings. Inspired by new mechanistic insight into the function of binary metal salen/nucleophilic cocatalyst systems at low concentrations, we report a bifunctional complex in which the salen catalyst and an aminocyclopropenium cocatalyst are covalently tethered. A modular ligand design circumvents the extended linear syntheses typical of bifunctional catalysts, enabling systematic variation to understand and enhance catalytic activity. The optimized bifunctional aluminum salen catalyst maintains excellent activity for the ring-opening copolymerization of epoxides and cyclic anhydrides at low concentrations (≥0.025 mol %), and the aminocyclopropenium cocatalyst suppresses undesirable transesterification and epimerization side reactions, preserving the integrity of the polymer backbone.

Tetracoordinate aluminum complexes bearing phenoxy-based ligands as catalysts for epoxide/anhydride copolymerization: some mechanistic insights

Phenoxy-imine aluminum complexes, in combination with DMAP, produce efficient catalysts for the alternating copolymerization of epoxides and anhydrides. A zwitterionic species is formed in the initiation step.

Sequence-Selective Terpolymerization from Monomer Mixtures Using a Simple Organocatalyst

One-step synthesis of block copolymer from mixed monomers is of great interest and challenge. Using a simple non-nucleophilic organobase as the catalyst, we have achieved sequence-selective terpolymerization from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA). Alcohol-initiated alternating copolymerization of PA and epoxide occurs first and exclusively because PA is substantially more active than LA for reacting with base-activated hydroxyl. When PA is fully consumed, LA polymerizes from the termini of the first block while excess epoxide stays intact because of the mild basicity of the catalyst. The two polymerizations thus occur tandemly, both in chemoselective manners, so that an aromatic-aliphatic block copolyester is generated in this one-step synthesis. The effectiveness and versatility of this approach is demonstrated by the use of ethylene oxide and several monosubstituted epoxides as well as mono-, di-, or tetrahydroxy initiators.

Copolymerization of epoxides with cyclic anhydrides catalyzed by dinuclear cobalt complexes

The alternating copolymerization of epoxides with cyclic anhydrides (CAs) is a highly diverse synthetic method for polyesters as the polymers’ architectures and properties can be easily controlled depending on the combination of two monomers. Thus, a variety of catalyst designs has been reported to prepare the desired copolymers efficiently. We herein report dinuclear cobalt–salen complexes with a benzene ring as a linker and their activities in copolymerization reactions. The dinuclear cobalt complexes showed a higher catalytic activity for the copolymerization of propylene oxide with phthalic anhydride than the corresponding mononuclear cobalt–salen complex and achieved one of the highest turnover frequencies ever reported. A variety of epoxides and CAs were also found to be copolymerized successfully by the dinuclear cobalt complex with a high catalytic activity.

Across the Board: Arjan Kleij

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Arjan W. Kleij, who discusses the use of terpenes as raw materials for the synthesis of biobased polyesters and polycarbonates, and the opportunities and challenges that lie ahead for these renewable polymers in the area of material science, trying to meet the requirements of a circular economy.

Fully alternating and regioselective ring-opening copolymerization of phthalic anhydride with epoxides using highly active metal-free Lewis pairs as a catalyst

Cooperative metal-free Lewis pairs effectively catalysed controlled ring-opening copolymerization of phthalic anhydride (PA) with epoxides.