Synthesis of zinc complexes bearing pyridine derivatives and their application of ε-caprolactone and L-Lactide polymerization

Highlights

• The polypyridne-zinc complexes were synthesized.

• 1,10-Phenanthroline derivatives enhanced the catalytic activity of Zn complexes.

• Hydrogens of polypyridine co-activate the carbonyl group of monomers.

Abstract

A series of pyridine and polypyridine-zinc complexes were synthesized, and their application in ε-caprolactone and l-lactide polymerization was studied. In ε-caprolactone polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a higher polymerization rate than did diethyl zinc (2–10 times). In l-lactide polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a considerably higher polymerization rate than did diethyl zinc (2–220 times). According to density functional theory results, the ortho-hydrogens of polypyridine ligand can co-activate the carbonyl group of monomers with zinc metal center to increase the catalytic activity of zinc catalysts.

Introduction

Biodegradable polymers, such as polylactide (PLA) and poly-ε-caprolactone (PCL), are used as replacements for petrochemical plastics to mitigate the plastic pollution caused after discarding non-degradable petrochemicals. In addition, PLA [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]] and PCL [7,9,[11], [12], [13], [14], [15], [16], [17], [18], [19]] and are also regarded as biomaterials with various applications because of their biodegradability, biocompatibility, and permeability [20,21]. Ring-opening polymerization (ROP) is useful method in which metal complexes [[22], [23], [24], [25], [26], [27], [28], [29]] are used as catalysts. This method exhibits higher controllability for producing polymer than using traditional polycondensation [30,31].

Zinc, the 24th most abundant element in Earth's crust, has normal oxidation state of +2 which yield to an excellent Lewis acid and is suitable to be used as a catalyst [[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]]. In Zn catalyst researches, the use of binary catalyst systems for ROP is quite common in the literature [[45], [46], [47], [48]]. Specifically, combining a metal alkyl complex with an alcohol (iso-propanol or benzyl alcohol) in an alcoholysis reaction is a common approach for raising initiating ligand nucleophilicity and as a result ROP activity. However, others types of Zn catalysts were also reported. Herres-Pawlis [[33], [34], [35], [36], [37], [38], [39], [40]] have reported several Zn complexes with neutral nitrogen-donor ligands and two monoanionic ligands for ε-caprolactone (CL) and l-lactide (LA) polymerization. However, these Zn complexes with two monoanionic ligands revealed very low reactivity in ROP at condition of 130–150 °C in bulk and reaction for 20–48 h as shown in Fig. 1.

The low reactivity of these Zn complexes are most likely due to halides or acetates are not good nucleophiles, and, therefore, lower their initiating polymerization ability. The ROP process is a two-step process (Fig. 2). The first step is the initiation (Fig. 2, (a)–(b)), and the second step is propagation (Fig. 2, (c)–(d)) of the second monomer attacked by the metal-alkoxy group formed in the initiation step. Although the monomer can be coordinated to a metal center, a low nucleophilic initiator cannot open the ring easily (Fig. 2, (b)). In addition, if the initiation rate is lower than the propagation rate, the molar mass of polymers cannot be controlled using the ratio of monomer/initiator, and their dispersity (Đ) values are broad as observed by Herres-Pawlis (Fig. 1).

To solve this problem, Jeong [[41], [42], [43], [44]] introduced methyl lithium as an alkyl-transferred reagent in the ROP process and assumed that the dimethyl Zn complex was produced in situ. The methyl groups functioned as initiators and were stronger nucleophiles than halides or acetates. The polymerization results in Fig. 3 revealed that the LA polymerization can be conducted at 25 °C, and the polymerization time is 12 h with 97%–99% conversion. In addition, the narrow polydispersity index (Đ) values (1.23–1.27) confirmed the high controllability of these Zn complex and LiMe. Though the addition of LiMe greatly increases the ROP performance, LiMe is such a strong base that it may deprotonate the ligands and/or attack the imino group to produce unexpected byproducts. To reduce the damages caused by LiMe, LiOCHMe₂ [[49], [50], [51]] was incorporated instead of LiMe. As shown in Fig. 3b, the polymerization time has been significantly reduced.

Based on aforementioned reports, we believe dialkyl coordinated zinc complexes will outperform those in produced analogies, and, therefore, designed and synthesized a series of diethyl Zn complexes bearing various pyridine, bipyridine, and 1,10-phenanthroline derivatives as depicted in Fig. 4. These Zn complexes were used as catalysts with benzyl alcohol as an initiator to study the effect of ligands in LA and CL polymerization. In addition to the polymerization results, density functional theory-based calculations enable us to have deeper insight to the mechanism of the ROP reactions.