Facile and solvent-free synthesis of a novel bio-based hyperbranched polyester with excellent low-temperature flexibility and thermal stability

Highlights

• A novel biobased hyperbranched polyester was facilely prepared without solvents.

• Lower viscosity and better solubility in polar solvents than its linear analogue.

• Excellent low-temperature flexibility with the lowest T_g up to ―59.2 °C.

• Good thermal stability with the highest T_onset up to 341.2 °C.

Abstract

In this work, a novel biobased hyperbranched polyester (HBPE) with excellent low-temperature flexibility (low T_g) and thermal stability, was prepared from the self-polycondensation of AB₂ monomer based on methyl ricinoleate (MR), a commercially available derivative from castor oil. The AB₂ monomer was facilely synthesized by UV-initiated thiol-ene addition between MR and 2-mercaptoethanol in the absence of solvents. The reaction parameters were systematically optimized, and the highest yield up to 98 % was achieved with almost complete conversion of double bond within 2 h. A series of HBPEs were then obtained via bulk polymerization of the AB₂ monomer under different conditions. Among the four transesterification catalysts investigated, Ti(OBu)₄ was demonstrated to be the most efficient one due to the higher polymerization efficiency and molecular weight (M_n = 13,000∼17,000 g/mol) of as-obtained HBPEs. And the degree of branching (DB) was estimated to be in the range of 0.30∼0.42. Notably, the HBPEs with dangling chains exhibited a low glass transition temperature (T_g) of ―42.8∼―59.2 °C, indicative of excellent low-temperature flexibility. TGA results indicated that the onset thermal degradation temperature under nitrogen exceeded 310 °C for all the HBPEs. In comparison to the polyrincinoleate as a linear polyester analogue, the HBPE behaved like Newtonian fluid behaviors with lower complex viscosities.

Introduction

Hyperbranched polymers (HBPs) first coined by Kim and Webster (Kim and Webster, 1988, 1990), are highly branched macromolecules with a three-dimensional dendritic globular architecture (Yan et al., 2011). The unique structure endows HBPs many distinct features from linear polymers, such as lower viscosity, better solubility, and a large number of terminal functional groups for further modification, which make them become versatile for a variety of advanced applications, such as coatings, biological fields, storage devices, energy convertors, catalysis, flame-retardant polymers, etc. (Shahidul et al., 2018). Besides, as compared to the dendrimers with the well-ordered treelike structure, HBPs have the randomly distributed and less perfect branches throughout their structure, and moreover are more easily to be produced in industrial quantities at the lower cost. At present, the HBPs have been prepared from either the single monomer or double monomers via various methodologies. Among these emergent synthetic approaches, the self-polymerization of AB₂ monomers via intermolecular coupling reactions between A and B groups, is one of the most commonly used ones to synthesize HBPs. However, because of short or rigid sub-chains, conventional HBPs suffer from poor processability, inferior mechanical properties (brittleness) as well as high T_g, which are evidently undesired in some occasions requiring excellent chain flexibility (Mu et al., 2018). Furthermore, the currently reported monomers used for the synthesis of HBPs, are still predominantly derived from nonrenewable petrochemical feedstocks.

In the recent decade, with ever-increasing concerns about environmental and sustainable issues with respect to petroleum-based polymers, the utilization of annually renewable resources in the preparation of various polymeric materials (e.g. polyesters) has gained considerable attentions. Therefore, there is an urgent need to develop biobased HBPs using renewable resources as an alternative to conventional fossil-based ones. Vegetable oils (VO) and derivatives obtained from naturally occurring plants, have become one of the most important renewable platform chemicals for the synthesis of polymeric materials due to the relatively low cost, low toxicity, flexible chain structure, worldwide availability, inherent biodegradability/biocompatibility, and built-in functionalities (Biermann et al., 2011; Meier et al., 2007). Nevertheless, there have been few efforts in the utilization of them to prepare biobased hyperbranched polyesters (HBPEs) to date (Petrović et al., 2012; Sun et al., 2017; Testud et al., 2017; Türünç and Meier, 2010).

Türünnç and Meirt (Türünç and Meier, 2010) reported the thiol-ene addition between methyl 10-undecenoate and 1-thioglycerol as a convenient route for the synthesis of an AB₂ monomer bearing bifunctional hydroxyls. Bulk polycondensation with or without a core molecule (glycerol) was then carried out using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst. The degree of branching (DB) was in the range 0.40-0.47 for the resulting HBPEs. In another work by Testud and coworkers (Testud et al., 2017), four kinds of AB_n-type monomers (n = 2 or 3) based on unsaturated fatty acid esters with different carbon chain lengths (C11-C22) were obtained via a two-step procedure that involved the epoxidization of double bond and subsequent oxirane ring-opening with water in the presence of phosphoric acid. These bio-sourced AB_n monomers were then polymerized into HBPEs with DB values of 0.07∼0.41, depending on the type of condensation catalysts and polymerization temperature. In another work by Sun and coworkers (Sun et al., 2017), the HBPEs were prepared from castor oil and soybean oil-based triol or tri(carboxylic acid) monomers, respectively, via the ozonolysis-reduction (or oxidative) treatment followed by the A₂+B₃ polycondensation. However, these reported HBPEs are usually obtained as polyol oligomers with relatively low molecular weights 2,000∼9,200 g/mol. And their T_g values are generally higher than ―30 °C and exist in the appearance of semi-crystalline solid, which limit the applications in some occasions requiring excellent low-temperature flexibility and/or low viscosity, such as liquid toughener, sealants, and injectable carrier of drugs.

Methyl ricinoleate (MR) is a kind of commercially available 18-carbon mono-hydroxylated unsaturated fatty acid ester that is derived from renewable castor oil (Kunduru et al., 2015). Unlike other mono-functional fatty acid ester derivatives, MR contains an inherent hydroxyl group at the C12 position as well as a cis-configured CC bond between C9 and C10 positions in its molecular structure, as illustrated in Scheme 1. And the internal double bond can be potentially converted into one or two hydroxyl functions, and AB_n monomer for the synthesis of HBPEs can be thus obtained. In addition, the unique C6 dangling alkyl chain in the MR unit can play a role of internal plasticization to lower T_g of MR-based polymers, and meanwhile prevent the crystallization even at very low temperature (Petrović et al., 2008). It has been reported that MR is able to self-polymerize to result in an amorphous polyricinoleate with a T_g as low as ―74.8 °C (Ebata et al., 2007) and has similar thermomechanical properties to polybutadiene and polyisoprene rubbers (Domb and Nudelman, 1995; Lebarbé et al., 2013). These structural features make MR or its acid derivative (i.e. ricinoleic acid, RA) become an attractive building block for the synthesis of biobased polymers. Although linear co-polyesters (Kunduru et al., 2015; Lebarbé et al., 2013; Shikanov and Domb, 2006; Slivniak and Domb, 2005; Slivniak et al., 2005), polyanhydride (Domb and Nudelman, 1995; Teomim et al., 2001; Krasko et al., 2003), polyurethanes (Petrović et al., 2008; Xu et al., 2008; Petrović et al., 2010; Palaskar et al., 2010) based on MR and RA have been intensively investigated for a wide range of applications, there is few reports regarding the synthesis of HBPEs from them until now. It is known that there are many pathways to introduce hydroxyl function onto the structure of VO and its derivatives by treating the CC bond, such as epoxidation/hydroxylation, hydroformylation/reduction, and ozonolysis/hydrogenation (Desroches et al., 2012; Pfister et al., 2011). As compared to these above processes, the thiol-ene click reaction owns several advantages that make it a particularly attractive, facile and versatile one, such as the simplicity in procedure, high efficiency with almost qualitative yields and relatively less side reaction, fast reaction rate, the availability of a broad range of enes, and tolerance to the presence of air/oxygen and moisture (Lowe, 2010).

In this work, we synthesized an AB₂ monomer from MR via an efficient UV-initiated thiol-ene addition with 2-mercaptoethanol in the absence of solvents. And effects of thiol/ene molar ratios, dosages of DMPA as a photoinitiator and UV irradiation time on the progress of thiol-ene coupling were examined to obtain the optimum reaction condition. Subsequently, the self-polycondensation of AB₂ monomer yielded a series of HBPEs under different transesterification catalysts and polymerization temperature. The molecular structure of resulting HBPEs was characterized in terms of NMR, FT-IR, and GPC, respectively. In addition, their thermal and dynamic rheological properties were investigated, respectively, with the polyricinoleate (LPE) as a linear analogue for comparison. To our knowledge, the use of MR to synthesize biobased HBPE with excellent chain flexibility and thermal stability, has not been reported elsewhere.