**1. Introduction**

Natural antioxidants offer a host of benefits, especially for our health and general well-being, including anti-aging, anti-carcinogenic, anti-inflammatory, and anti-microbial properties, which promote a growing use in cosmetic products. The antioxidant activities of natural antioxidants have been largely ascribed to the presence of phenolic content [1,2].

Additionally, antioxidants possess very strong preservative properties and they may prevent lipid oxidation in cosmetic products. Besides, sunlight, air and vehicle pollution, and other environmental factors produce free radicals, which could be neutralized by antioxidants. The use of antioxidants in cosmetics aims to create a barrier that helps protect the skin against free radicals produced by oxidative stressors [3].

Among phenols, which are a well-known group of biologically active compounds, the best known are flavonoids, tannins, and phenolic acids. The last ones belong to aromatic secondary plant metabolites, are widespread in vegetation, and provide a wealth of anti-aging benefits. They can be readily absorbed in the human body and demonstrate strong preserving properties, which include anti-bacterial and anti-fungal effects [4]. The main function of phenolic antioxidants is retardation oxidation of unsaturated oils that may affect the final color and smell of cosmetic products [5].

p-Anisic acid (4-Methoxybenzoic acid) reveals antioxidant, anti-inflammatory, and anti-tumor properties, and is a proven antiseptic, which makes it suitable as a preservative in cosmetic products [5]. Over the past few years, p-AA became progressively substantial when used as a multi-functional material in the cosmetics and food sectors [6]. Among various cosmetic ingredients with antimicrobial properties, there are some commercially available ones (p-anisic acid and levulinic acid), which were found in number in several herbs (e.g., *Pimpinella anisum*) and as a by-product in the preparation of diosgenin from wild yam (*Dioscorea villosa*) [7].

The other developed cosmetic products, consisting of these alternative anti-microbial substances, are characterized as preservative-free or naturally preserved cosmetics [8].

Nevertheless, antioxidants encounter some difficulties, especially concerning the stability problems when they penetrate the transdermal barrier [9]. The use of controlled release systems in cosmetics contributes to the improvement of antioxidants' skin penetration. Moreover, it is a further challenge that the developed delivery systems (DSs) are readily integrated into the final cosmetic product formulation, thus leading to a consistent product for the customers' use. Furthermore, the use of DS enhances the protection of sensitive active ingredients and guarantees their targeted controlled release. It was proved that cosmetic DSs demonstrate a positive impact on the permeation of the bioactive compounds throughout the skin layers, and thus the concentration of the active species may be well controlled in both the obtained formulation and in the skin, respectively. So far, several types of DSs (e.g., niosomes, transfersomes, liposomes, lipid nanoparticles, polymeric microparticles, and nanoparticles) have been applied in cosmetic products. Skin interaction with the various DSs depends mostly on such factors as size, flexibility, and composition [10].

Many works have demonstrated that the improvement of the skin penetration of indigenously applied drugs can be achieved using particle-based formulations. However, a more rational approach needs to be considered when the transfer of the promising results to the clinical application is planned. Moreover, the different properties of diseased skin and the fate of these polymeric materials should be taken into account. Ideally, depending on the nature of the polymer, decomposition of a biodegradable polymer into non-toxic products and its degradation in a controlled manner after topical applications on the skin surface is highly desired [11].

The emergence of a broad spectrum of biodegradable polymers dedicated specifically for biomedical applications has resulted from the development of macromolecules with well determined structure and properties. The synthetic aliphatic polyesters possess a unique position among biodegradable polymers. They show the possibility of many health-related applications thanks to their good biodegradability and biocompatibility [12,13].

β-Lactones are considered as attractive substrates in organic synthesis [14] that can be used as monomers for the synthesis of aliphatic polyesters via ring-opening polymerization (ROP) [15,16]. Quite recently, selective and highly active catalysts that might be used for epoxide carbonylation have been described [17–19]. Thus, a new source of bioactive β-lactone monomers that contain in their structure a biologically active substance has been established. They proved to be very valuable and helpful for the synthesis of new polyesters, including polymeric DS with bioactive moieties. Polymer-bioactive compound conjugates offer countless benefits over free drugs such as increased water solubility, controlled and targeted drug delivery, more bioavailability, and thus enhanced therapeutic efficacy [20].

In our previous studies, we provided an overview of the approach for synthesizing bioactive (co)oligoesters on the basis of anionic homo- and co-polymerization of selected β-substituted β-lactones that contain bioactive species selectively chosen from compounds commonly used in cosmetic industry. The molecular-level structure of the obtained bioactive (co)oligoesters, as well as their abilities in the prospective application in cosmetology, were also demonstrated [21–25].

In the present contribution, we present a further extension study on bioactive (co)oligoesters that contain p-anisic acid moieties bound along oligomer chains. Considering the future use of developed biomaterials as controlled DS of p-AA, the current research has been focused on detailed studies of hydrolytic degradation of the resulting bioactive (co)polyesters.

The results of studies on the hydrolytic degradation of the obtained materials allowed us to gain full knowledge of the hydrolysis process and examine the p-AA release profile. In addition, applying mass spectrometry has enabled to identify the products of degradation of the biomaterials studied and determine their molecular structure.

Moreover, both bioactive (homo)- and (co)oligoesters were found to be cytocompatible when in contact with human HaCaT cells and they seem to be optimal to support keratinocyte functions. Especially, (p-AA-CH2-HP)n oligoester did not exert any inhibitory impact on cell growth, even at a relatively high concentration. Incubation with (co)oligoester had beneficial effects on HaCaT cells at concentrations below 100 μg/mL.
