*Article* **Preparation of Emulsifier-Free Styrene–Acrylic Emulsion via Reverse Iodine Transfer Polymerization**

**Tao Huang and Shuling Gong \***

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China; 2017102030109@whu.edu.cn

**\*** Correspondence: gongsl@whu.edu.cn

**Abstract:** Styrene–acrylic emulsions containing hydroxyl functional monomer unit's component are widely used for maintenance coating. In this paper, a stable emulsifier-free styrene–acrylic emulsion with solid content over 43% could be obtained in 210 min via reverse iodine transfer polymerization (RITP). By adding a mixture of methacrylic acid (MAA) and poly(ethylene glycol)methyl ether methacrylate (PEGMA) into a system containing a high content of hydroxyl functional monomer component (19.4 wt.% of the total monomer mass), styrene (St) could be copolymerized with methyl methacrylate (MMA); the modified film exhibited good hardness properties, good adhesive properties, and low water absorption. An increase in the amount of PEGMA decreased the glass transition temperature (*T*g). When 1.4 times the reference amount of initiator was added, the highest molecular weight *<sup>M</sup>*<sup>n</sup> could reach 40,000 g.·mol−<sup>1</sup> with 0.25 times the reference amount of iodine in the emulsion. The largest tensile strength of the dried emulsion film over 5.5 MPa endowed the material with good mechanical properties. Living polymerization was proven by the kinetics of RITP emulsion and chain extension reaction. TEM micrographs manifest the emulsification of the seed random copolymer. This paper may provide a potential methodology for preparing polymer materials with excellent mechanical properties.

**Keywords:** styrene–acrylic emulsion; cooperated; reverse iodine transfer polymerization; polymeric emulsifier

#### **1. Introduction**

Styrene–acrylic emulsions are widely used as industrial maintenance coating for the acrylate unit's resistance to photodegradation and the styrene unit's resistance to hydrolysis [1,2]. Furthermore, styrene–acrylic emulsions are also used for preparing nanoparticles [3–9], which are applied in the treatment of bacterial infections [10,11] or encapsulation medicine [12,13]. The optionality of the monomer endows emulsion polymers with some special properties. In emulsion polymerization, methacrylic Acid (MAA), methacrylic acid-β-hydroxyethyl ester (HEMA), methyl methacrylate (MMA), styrene (St), *n*-butyl acrylate (BA), and *n*-butyl methacrylate (BMA) are widely used monomers [14,15]. For example, the carboxyl group from MAA units or acrylic acid (AA) units endows the polymer with adhesive properties [16]; the hydroxyl group from methacrylic acid (MAA) units or methacrylic acid-β-hydroxyethyl ester (HEMA) can be crosslinked with amino resin [17], whereby the modified film with the crosslinked structure exhibits good mechanical properties [18]. MMA or St are used as hard monomers, which can increase the glass transition temperature of the polymer [1]. BA is used as a soft monomer [19]. Thus, styrene–acrylic emulsion polymers with functional groups such as carboxyl or hydroxyl in the pendant group have wide application prospects.

Styrene–acrylic emulsions can be prepared via emulsion polymerization with or without an emulsifier. Emulsion polymerization with a nonpolymeric emulsifier is conducted via the polymerization of weak water-soluble monomers within the emulsion of a nonpolymeric emulsifier [11]. In emulsifier-free polymerization, an initiator [20] or water-soluble

**Citation:** Huang, T.; Gong, S. Preparation of Emulsifier-Free Styrene–Acrylic Emulsion via Reverse Iodine Transfer Polymerization. *Polymers* **2021**, *13*, 3348. https:// doi.org/10.3390/polym13193348

Academic Editors: Edina Rusen and Eduardo Guzmán

Received: 6 August 2021 Accepted: 26 September 2021 Published: 29 September 2021

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monomer [21] can act as a polymeric emulsifier. HEMA is a water-soluble monomer; however, polymers with HEMA units may not be stably dispersed in water easily. A high content of HEMA units or hydroxyl groups in a polymer favors the crosslinking reaction between HEMA units and amino resin [22], and this can guarantee high crosslinking density that enhances mechanical properties [18]. However, there exists difficulty in preparing an acrylic emulsion polymer with a high content of HEMA for the low stability of the emulsion, and the low stability is derived from crosslinked polymer led by the crosslinking, which is caused by transesterification among the pendant hydroxyl groups of the polymer chain [23,24] and the interpolymer complex. Our group identified coagulum over 0.5 wt.% and flocculation when an excessive amount of HEMA (≥25 wt.% total monomer mass) was added in the polymerization [25]. For this reason, the content of HEMA should not be too high when preparing a high-solid-content acrylic emulsion to restrain the crosslinked polymer. Thus, it is necessary to study a synthetic methodology to prepare a stable emulsion with HEMA units in the polymer chain. An interpolymer complex is formed in acidic conditions because of the hydrogen bonding between the hydroxyl and carboxyl groups in monomer units [26,27], and the complex decreases the stability of the emulsion. For this reason, the ammonium salt of MAA [28] was used as a polymeric surfactant to restrain the formation of the interpolymer complex in our previous paper, and a stable emulsion with a high content of hydroxyl functional monomer HEMA (19.4 wt.% of the total monomer mass) units was prepared; the highest *<sup>M</sup>*<sup>n</sup> of the polymer could reach 34,100 g·mol<sup>−</sup>1, but the content of MAA in the total monomer mass was 12.3 wt.%. Furthermore, the emulsion viscosity with St units in the polymer chain was high, and the fluidity of the emulsion was not good. The carboxyl group in MAA units may corrode metal when ammonia solution is volatilized; hence, choosing a polymeric emulsifier that does not corrode metal and can stabilize an emulsion with St units in the polymer chain is necessary. Nonionic polymeric emulsifier PEGMA was added to stabilize the emulsion with St units [29–31], and the copolymer particles were uniform. In terms of the chemical structure, the group of poly(ethylene glycol) ethyl ether in PEGMA does not corrode metal. On the basis of the above understanding, PEGMA was chosen to study the preparation of a styrene–acrylic emulsion in this paper.

Emulsion polymerization can be conducted via controlled radical polymerization (CRP) [32]. In CRP, the controlled mechanism is based on the reversible deactivation of growing radicals [32,33]. An emulsion copolymer with PEGMA units and St units in the polymer chain can be prepared via CRP such as nitroxide-mediated polymerization (NMP) [21] and reversible addition fragmentation chain transfer polymerization (RAFT) [34–36]. The monomer benzyl methacrylate (BnMA) is analogous to St in chemical structure, and an emulsion copolymer containing PEGMA units and BnMA units can be prepared via photo-controlled iodine-mediated green reversible deactivation radical polymerization (RDRP) [37]. In our previous paper, an emulsion copolymer containing HEMA units and MAA units was prepared via RITP, and the emulsion was stable. Compared with NMP polymerization and RAFT polymerization, RITP does not require complicated chemicals to regulate the polymerization, and the chain transfer angents are synthesized in situ during the polymerization [38]. On the other hand, only a few of the chain transfer agents in RAFT are commercially available, and disadvantages of the polymer prepared by RAFT polymerization include its odor and color [39]. Iodine is commercially available, and there is no odor in the dried emulsion film. Thus, iodine transfer polymerization was chosen. Furthermore, PEGMA can be copolymerized with HEMA [24] or a monomer mixture comprising MAA and St [40], forming a stable emulsion. Therefore, it is probable that an emulsion copolymer containing PEGMA units, HEMA units, MAA units, and St units may be prepared via RITP emulsion polymerization.

In this paper, the copolymerization of St and an acrylate-based monomer (Figure 1) was studied via RITP emulsion polymerization. The research was conducted by changing the mass ratio of methyl methacrylate/St, the mass ratio of PEGMA/MAA, the amount of PEGMA, and the amount of iodine. Kinetics experiments of the random copolymerization

and chain extension reaction with St units and BA units proved the living polymerization of the chain. The emulsion polymer was characterized by monomer conversion, viscosity, particle size, molecular weight, *T*g, particle morphology, and Fourier-transform infrared (FTIR) spectra. The protective properties of the modified film were measured by pencil hardness rank, adhesive property, toluene absorption, and water resistance. The mechanical properties of the dried emulsion film or modified film were measured by tensile experiment.

**Figure 1.** (**A**) Synthetic route of styrene–acrylic emulsion copolymer via RITP and (**B**) chain extension reaction via RITP.

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

#### *2.1. Materials*

Ammonia solution (25–28 wt.%), *n*-butyl acrylate (BA), methyl methacrylate (MMA), styrene (St), *n*-butyl methacrylate (BMA), methacrylic acid (MAA), *p*-toluene sulfonic

acid (TsOH), and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), as analytical reagent grade (AR). Methacrylic acid-β-hydroxyethyl ester (HEMA) was purchased from Tianjin Institute of Chemical Reagents, AR. Poly (ethylene glycol) methyl ether methacrylate (PEGMA, average molecular weight = 475 g·mol<sup>−</sup>1) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). 4,4 –Azobis (4-cyanovaleric acid) (ACPA, 98%, AR), containing ca. 20% water, was purchased from Energy Chemical. *N*, *N*-Dimethylethanolamine (DMEA) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), AR. Hexamethylolmethymelamine (HMMM, MF Resin) was provided by H. J. Unkel Co., Ltd. (Zhuhai, China). The monomers BA, MMA, St, and BMA were extracted by washing four times with 10 wt.% aqueous sodium hydroxide solution in a separatory funnel, followed by washing with deionized water four times in a separatory funnel. Other materials were used as received.
