**1. Introduction**

The emission of volatile organic compounds (VOC) is harmful to the environment and human health, causing many countries and regions have formulated strict restrictions on it. This has driven the development of waterborne polymer materials [1]. In addition to the characteristics of traditional polyester (including high gloss, good weather resistance, and biocompatibility), waterborne polyester (WPE) also has the advantages of low VOC, low toxicity, lack of odor, nonflammability, ease of disposal, and so on [2,3]. These characteristics make waterborne polyester attract lots of attention in coatings, adhesives, biomedical applications, and other fields [4–6].

According to whether an emulsifier is used in the preparation process, WPEs can be divided into two groups: external-emulsifying polyester and self-emulsifying polyester. The emulsifiers are required in the preparation of external-emulsifying WPE. There are often residual emulsifiers in the prepared emulsion which would migrate to the surface of the emulsion in the film-forming process. This event would affect the performance of emulsions, as well as reduce the surface evenness and the water resistance of cross-linked films [7]. Selfemulsifying WPE can be roughly divided into ionic and nonionic types according to their hydrophilic components. The ionic WPE usually contains carboxyl groups or sulfonic acid groups. When Ma et al. [8] prepared WPE with trimellitic anhydride as terminal groups, the polyester dispersed well in water after neutralization, but the molecular weight was below 2000 g mol−1, which would limit its application scope. Zhang et al. [9] introduced 5-sulfoisophthalic acid monosodium salt into the molecular backbone and obtained a WPE with a solid content of 10%. Although the ionic polyesters are facile in synthesis and dispersing procedures, there are still defects to be resolved. The anionic polyester is usually difficult to apply in acidic environments and has poor compatibility with certain additives and inert pigments, [10] and the sulfonate-based polyesters are corrosive to metal

**Citation:** Fu, H.; Gong, L.; Gong, S. A New Approach Utilizing Aza-Michael Addition for Hydrolysis-Resistance Non-Ionic Waterborne Polyester. *Polymers* **2022**, *14*, 2655. https://doi.org/10.3390/ polym14132655

Academic Editor: Edina Rusen

Received: 28 May 2022 Accepted: 26 June 2022 Published: 29 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

substrates [11]. Waterborne polyester prepolymer with a molecular weight of around 6000 g mol−<sup>1</sup> usually exhibits better comprehensive properties and is suitable for the classical hardening formulations, [12] but it was difficult for ionic WPE to achieve such a molecular weight. For the carboxyl-terminated polyester, decarboxylation is unavoidable at high temperatures; moreover, only terminal carboxyl groups cannot provide sufficient hydrophilic capacity for high molecular weight polyester [13]. For the multi-carboxyl containing polyester, the uncontrolled branching reaction between carboxyl groups and polyols would limit the growth of molecular weight [14]. For the sulfonate-based polyester, the molecular weight of polymer decreases when a sulfonic acid monomer is involved in copolymerization [15]. Moreover, ionic WPEs are usually poor in hydrolysis resistance. Non-ionic WPEs, which rely on polyethylene glycol (PEG) segments on the main chain to achieve water-dispersibility, would sacrifice the hydrolysis resistance and mechanical properties, as well as the hydrophilicity of the obtained polymer is usually weak. [16,17].

Therefore, researchers have made some attempts to explore other methods for the preparation of water solubility polymers [18]. It was found that by grafting the hydrophilic segments as side chains to the molecular backbone, the molecular weight of the WPE was improved, as was the mechanical property, etc. [19,20] Li et al. [20] used dimethylolpropionic acid modified by PEG to synthesize a polyester prepolymer. After conducting a reaction with isocyanate, a series of non-ionic waterborne comb-like polyurethane were obtained. With the increase in the comb-like polyester segment, the elongation at break (ε) of the product increased by 30%, and the solid content increased from 20% to 30%. However, the modification process has to undergo tedious protection and deprotection procedures. Taniguchi et al. [21] prepared a water-soluble, comb-like polyester through a chemoselective reaction between PEO side chains and ketone-bearing polyester; however, the water-solubility and mechanical properties of these polymers were not discussed in detail, and the synthetic pathways were complex, involving multiple steps with multiple catalysts. Furthermore, when side chains with a certain steric hindrance were grafted to the molecular skeleton to form a comb-like structure, the ester bonds of the polyester were protected according to the "rule of six" theory, and the hydrolysis resistance of the WPE would be improved [22]. However, the synthesis of polyesters with side chains or side chain functionalized groups usually require complex multistep reactions under harsh conditions including multiple expensive catalysts and long reaction times, which usually leads to the degradation of polyester chains in deprotection reactions [21,23]. In recent years, the aza-Michael addition, which is considered as a very efficient method to create new C-N bond and β-amino carbonyl derivatives, has attracted the attention of researchers in the post-polymerization modification of polymers [24–26]. The aza-Michael addition proceeds between a nucleophilic amine and an electron-deficient alkene. Interestingly, dimethyl maleate, a common structural unit in polyester, has been confirmed to be a class of Michael receptors with high reactivity and selectivity [27]. Bosica et al. [28] demonstrated that dimethyl maleate could undergo an efficient Michael addition reaction with various aliphatic amines at room temperature without any catalyst. Yu et al. [29] also confirmed that the primary amines with different chain lengths react efficiently with *cis*methyl maleate and that the obtained sample was highly functionalized, with the addition reaction being almost irreversible. These works inspire us for preparing NWCPE via green aza-Michael addition.

This work aims to develop a facile and green methodology for preparing self-dispersible NWCPE. By step-growth polycondensation and aza-Michael addition, the NWCPE with hydrophilic polyether-amines Jeffamine M-1000 as side chains was prepared. In this way, the properties of waterborne polyester, such as molecular weight, storage stability, hydrolysis resistance, and mechanical properties have been improved. The results exhibited that the NWCPE has excellent application prospects in eco-friendly waterborne coating.

### **2. Materials and Methods**
