We report the preparation, structural characterization, and detailed lactide polymerization behavior of a new Zn(II) alkoxide complex, (L(1)ZnOEt)(2) (L(1) = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate). While an X-ray crystal structure revealed the complex to be dimeric in the solid state, nuclear magnetic resonance and mass spectrometric analyses showed that the monomeric form L(1)ZnOEt predominates in solution. The polymerization of lactide using this complex proceeded with good molecular weight control and gave relatively narrow molecular weight distribution polylactide, even at catalyst loadings of <0.1% that yielded M(n) as high as 130 kg mol(-)(1). The effect of impurities on the molecular weight of the product polymers was accounted for using a simple model. Detailed kinetic studies of the polymerization reaction enabled integral and nonintegral orders in L(1)ZnOEt to be distinguished and the empirical rate law to be elucidated, -d[LA]/dt = k(p)[L(1)ZnOEt][LA]. These studies also showed that L(1)ZnOEt polymerizes lactide at a rate faster than any other Zn-containing system reported previously. This work provides important mechanistic information pertaining to the polymerization of lactide and other cyclic esters by discrete metal alkoxide complexes.

Highly active zinc catalyst for the controlled polymerization of lactide.

The majority of materials used for short-term and disposable packaging application are non-biodegradable which are not satisfying the demands in environmental safety and sustainability. Biodegradable polymers are an alternative for these non-biodegradable materials. The biodegradable polymeric materials can degrade in a reasonable time period without causing environmental problems. However, biodegradable polymers possess some limitations such as comparatively high cost, insufficient mechanical performances, and inferior thermal stability to extend their widespread application in packaging industry. To overcome these limitations, one of the most commonly used strategies is melt blending of dissimilar biodegradable polymers. Unfortunately, most of the biodegradable polymer blends exhibit insufficient performance because they are thermodynamically immiscible as well as exhibit poor compatibility between the blended components. It has been established that the compatibilization is a well-known strategy to improve the performances of the immiscible biodegradable polymer blends by enhancing the adhesion between the phases. As a result, recent studies focus on various compatibilizers to enhance the performances of the resulting biodegradable polymer blends. This review summarizes the recent developments on a variety of biodegradable polymer blends compatibilized by melt processing with a main focus of ex situ and in situ compatibilization strategies. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 135, 45726.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/app.45726

Biodegradable compatibilized polymer blends for packaging applications: A literature review

Aliphatic polyesters derived from lactides of various configurations (LL, DD, and DL) are promising as materials not only for packaging and for production of many other commonly used polymer products but also for unique medical applications. This paper describes current situation in synthesis and major applications of polylactides. Results of basic studies of “classical” polymerization of lactides prepared with stannous carboxylates or alkoxides, allowing to obtain polymers with molar masses (Mn) up to 106 g/mol and with the low content of tin are also presented. There are discussed advantages and disadvantages of catalyzing/initiating systems that contain zinc, magnesium, calcium, and other metal carboxylates and alkoxides as well as strong organic base initiators. Syntheses of polylactides with well defined microstructure are described. Characteristic features of bulk, solution, and dispersion polymerizations are compared. An outlook for development of polylactide production and applications is presented. Copyright © 2014 John Wiley & Sons, Ltd.

Polylactides—an overview

Poly(lactic acid) or poly(lactide) (PLA) is a renewable, bio-based, and biodegradable aliphatic thermoplastic polyester that is considered a promising alternative to petrochemical-derived polymers in a wide range of commodity and engineering applications. However, PLA is inherently brittle, with less than 10% elongation at break and a relatively poor impact strength, which limit its use in some specific areas. Therefore, enhancing the toughness of PLA has been widely explored in academic and industrial fields over the last two decades. This work aims to summarize and organize the current development in super tough PLA fabricated via polymer blending. The miscibility and compatibility of PLA-based blends, and the methods and approaches for compatibilized PLA blends are briefly discussed. Recent advances in PLA modified with various polymers for improving the toughness of PLA are also summarized and elucidated systematically in this review. Various polymers used in toughening PLA are discussed and organized: elastomers, such as petroleum-based traditional polyurethanes (PUs), bio-based elastomers, and biodegradable polyester elastomers; glycidyl ester compatibilizers and their copolymers/elastomers, such as poly(ethylene-co-glycidyl methacrylate) (EGMA), poly(ethylene-n-butylene-acrylate-co-glycidyl methacrylate) (EBA-GMA); rubber; petroleum-based traditional plastics, such as PE and PP; and various biodegradable polymers, such as poly(butylene adipate-co-terephthalate) (PBAT), polycaprolactone (PCL), poly(butylene succinate) (PBS), and natural macromolecules, especially starch. The high tensile toughness and high impact strength of PLA-based blends are briefly outlined, while the super tough PLA-based blends with impact strength exceeding 50 kJ m−2 are elucidated in detail. The toughening strategies and approaches of PLA based super tough blends are summarized and analyzed. The relationship of the properties of PLA-based blends and their morphological parameters, including particle size, interparticle distance, and phase morphologies, are presented.

https://pubs.rsc.org/en/content/articlepdf/2020/RA/D0RA01801E

Super tough poly(lactic acid) blends: a comprehensive review

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a Pi value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (Ps=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ange.201403643

Metal-size influence in iso-selective lactide polymerization.

Bimetallic salen–aluminum complexes: synthesis, characterization and their reactivity with rac-lactide and ε-caprolactone

Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Aluminum Hemi-Salen Complexes

Polylactide (PLA), the popular bio-based plastics, has attracted more interest as a benign alternative for oil-based plastics Lowering the manufacture cost of PLA and improving the performance of PLA products are the essential challenges in PLA study PLA stereocomplex is a promising choice to improve the thermal and mechanical properties Efficient preparation of the PLA stereocomplex by the direct ring-opening polymerization (ROP) of inexpensive racemic lactide (rac-LA) requires the ROP catalysts with high activity and stereoselectivity For this purpose, the novel trinuclear Salen–aluminum complexes were prepared in this work and exerted extraordinary improvement of catalytic activity and stereoselectivity at the same time These complexes showed the excellent isoselectivity (typically Pm = 098), high activity (typically kp = 154 L mol–1 min–1), and low catalyst loading amount (001 mol %) The Tm of PLA was 2199 °C, which is the highest value among the reported PLA derived directly from rac-LA unt

Breaking the Paradox between Catalytic Activity and Stereoselectivity: rac-Lactide Polymerization by Trinuclear Salen–Al Complexes

A series of iron(III) chloride complexes based upon Schiff base framework have been synthesized and characterized by mass spectra, elemental analysis, and X-ray crystallography. These bench-stable complexes were for the first time capable as highly efficient catalysts for lactide and e-caprolactone polymerization in the presence of propylene oxide (PO), greatly surpassing conventional aluminum analogies. Electron-withdrawing substituents as well as elevated temperature boosted the activity while a bulky group on salicylaldehyde moieties abnormally produces the same effect, whereas rigid backbone retarded the reactivity. Polylactide tactics ranging from isotactic to hererotactic enchainment were obtained by tuning the ligand backbone and substituents. The stereoselectivity was confirmed to proceed via a chain-end control mechanism by kinetic studies using different isomers of lactide, and the overall polymerization process was also investigated in detail by the oligomer mass spectrum as well as end group (...

Air-Stable Salen–Iron Complexes: Stereoselective Catalysts for Lactide and ε-Caprolactone Polymerization through in Situ Initiation

A series of zinc complexes based on asymmetrical N,N,O-tridentate ligands were prepared via binaphthyl diamine derivatives. These complexes were characterized and employed as catalysts in lactide polymerization. The X-ray diffraction analyses revealed that molecular structures of 1b and 2b were mononuclear complexes with zinc atoms in distorted octahedral geometries. Upon co-catalysis with isopropanol, complex 2a showed the highest activity among these zinc complexes for the ring-opening polymerization of L-lactide, and complex 3a exhibited the highest stereoselectivity for the ring-opening polymerization of rac-lactide affording substantially isotactic polylactide (PLA) with a Pm of 0.62. The polymerization kinetics using 2a as a catalyst was studied in detail. The kinetics of the polymerization results revealed that the rates of polymerization were first-order both in the monomer and the catalyst, and there was a linear relationship between the L-LA conversion and the number-average molecular weight of PLA with a narrow molecular distribution (1.07–1.17). The activation energy (31.49 kJ mol−1) was deduced according to the Arrhenius equation.

Ranlong Duan

Papers

Bimetallic salen–aluminum complexes: synthesis, characterization and their reactivity with rac-lactide and ε-caprolactone

Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Aluminum Hemi-Salen Complexes

Breaking the Paradox between Catalytic Activity and Stereoselectivity: rac-Lactide Polymerization by Trinuclear Salen–Al Complexes

Air-Stable Salen–Iron Complexes: Stereoselective Catalysts for Lactide and ε-Caprolactone Polymerization through in Situ Initiation

Zinc complexes containing asymmetrical N,N,O-tridentate ligands and their application in lactide polymerization