**1. Introduction**

Water-soluble polymers have become increasingly important in oilfield applications such as hydraulic fracturing, enhanced oil recovery, and drilling [1]. As the petroleum industry taps deeper reservoirs, there is an urgent requirement that chemicals should maintain performances in harsh reservoirs with high temperatures and salinities [2,3]. Polyacrylamide and partially hydrolyzed polyacrylamide (HPAM) are widely used in polymer flooding and hydraulic fracturing [4]. However, HPAM with poor heat resistance and salt tolerance cannot withstand the harsh reservoir [5–8].

In addition, scholars have paid increased attention in recent decades to hydrophobically associating water-soluble polymers (HAWSPs), which have good thickening and salt resistance. Hydrophobic association between the hydrophobic groups of HAWSP increases the hydrodynamic volume and increases viscosity significantly [9,10]. Compared with conventional water-soluble polymers, HAWSPs have poor solubility due to the presentence of hydrophobic groups, which one of the factors restricting its application in oil fields.

The host–guest effect has attracted attention in recent years because it can improve the performance of polymers. *β*-cyclodextrin (*β*-CD) is the most used in this field [11–14]. *β*-CD can form supramolecular structures with guest chains by hydrogen bonding and van der Waals forces [15,16]. There is a network structure in such a polymer solution caused by the inclusion of the *β*-CD and the guest such as phenyls, long-chain alkyls, adamantyl, and azobenzene [17–21]. Weickenmeier et al. obtained a polymer solution with an apparent supramolecular network structure attributed to the host–guest effect between *β*-CD polymers and hydrophobically modified polymers [22]. Wei Bing et al. grafted a

**Citation:** Xu, T.; Mao, J.; Zhang, Y.; Yang, X.; Lin, C.; Du, A.; Zhang, H.; Zhang, Q.; Mao, J. Experimental Study on High-Performers Quaternary Copolymer Based on Host–Guest Effect. *Polymers* **2021**, *13*, 2972. https://doi.org/10.3390/ polym13172972

Academic Editor: Edina Rusen

Received: 29 May 2021 Accepted: 27 August 2021 Published: 1 September 2021

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*β*-CD and an adamantane ring structure on the side chain of the polymer; the superior properties make this novel polymer promising in enhancing oil recovery [23]. Zou CJ et al. prepared a new supramolecular retarded acid system by *β*-CD and 2-Phosphonobutane-1, 2, 4-Tricarboxylic Acid, which can decrease the corrosion rate with formations [24]. Due to the spatial position of the host–guest molecules on the polymer chain and the difference in their molecular activation energy, the rheological behavior of the polymer solution system is significantly different. According to previous studies, grafting of *β*-CD groups in a HAWSP can produce host–guest interactions, improving properties such as solubility and temperature, salt, and shear resistance [25,26].

Inspired by the above, a quaternary polymer (HGP) was designed by copolymerizing acrylamide (AM), acrylic acid (AA), and two kinds of monomer: *N*-(3-methacrylamidopropyl)- *N,N*-dimethylnaphthalen-1-aminium chloride (NAP) and modified maleic anhydride (MAH*β*-CD). NAP and MAH-β-CD were selected as functional monomers for the following reasons: (1) the hydrophobic cationic monomer NAP can form hydrophobic microdomains in an aqueous solution due to hydrophobic association, which significantly increases the viscoelasticity. (2) Due to the poor solubility of hydrophobically associating polymers, the introduction of cyclodextrin can speed up the dissolution of the polymer. At the same time, NAP and MAH-β-CD have host–guest effects in the solution. It is reported that the envelope constants between cyclodextrin and naphthalene are 4.67 × 103 molL−<sup>1</sup> (Figure 1) [27]. (3) In addition, NAP and MAH-*β*-CD are rigid groups, which not only increase the rigidity of the backbone of the polymer but also improve the temperature resistance of the polymer.

**Figure 1.** Cyclodextrin and naphthalene have host–guest effects.

HGP embraces good solubility, thickening performance, shear tolerance, and salt and temperature resistance due to the host–guest effect.
