STUDY OF THE RADICAL COPOLYMERIZATION REACTION OF UNSATURATED POLYESTERS WITH ACRYLIC ACID

ABSTRACT

Unsaturated polyesters, with their various compositions, offer a promising and cost-effective raw material for modern sealing and adhesive technologies. The purpose of this study is to investigate the radical copolymerization reaction of polyethylene (propylene) glycol fumarate with acrylic acid in dioxane solution at various molar ratios of the initial polymer-monomer mixture. The composition of the copolymers was identified by HPLC spectroscopy through the analysis of mother liquors, and the structure was confirmed by IR spectroscopy. The kinetics of copolymerization were studied by dilatometric method. The swelling degree of copolymers was determined by gravimetric method, and their unsaturation degree - by bromide-bromate method. The radical copolymerization constants were calculated by the integral Mayo-Lewis method.  The obtained results indicate that regardless of the composition of the initial polymer-monomer mixture, the copolymer composition is enriched with acrylic acid units, and as the concentration of acrylic acid increases in the initial polymer-monomer mixture, the reaction rate significantly accelerates. Higher unsaturated polyester content in the initial polymer-monomer mixture reduces the copolymer’s moisture absorption ability, making them suitable as base materials for sealing and adhesive production.  By analyzing the numerical values of copolymerization constants, the higher activity of acrylic acid compared to unsaturated polyesters has been demonstrated, thereby confirming the previously proposed assumption of their comparatively lower reactivity.

АННОТАЦИЯ

Одним из перспективных и недорогих видов сырья с учетом применения современных технологий получения герметизирующих и клеевых материалов являются ненасыщенные полиэфиры различного состава. Целью данной работы является исследование реакции радикальной сополимеризации полиэтилен(пропилен-)гликольфумарата с акриловой кислотой в растворе диоксана при различных мольных соотношениях исходной полимер-мономерной смеси. Состав сополимеров идентифицировали методом ВЭЖХ-спектроскопии путем анализа маточных растворов, структура подтверждена ИК-спектроскопией. Кинетика сополимеризации исследована дилатометрическим методом. Степень набухания сополимеров определена гравиметрическим методом, а их степень ненасыщенности – бромид-броматным методом. Константы радикальной сополимеризации рассчитаны интегральным методом Майо-Льюиса. Полученные результаты свидетельствуют о том, что при любом составе исходной полимер-мономерной смеси состав сополимера обогащается звеньями акриловой кислоты, а с увеличением ее содержания в исходной полимер-мономерной смеси скорость реакции существенно возрастает. Увеличение содержания ненасыщенного полиэфира в исходной полимер-мономерной смеси приводит к снижению способности сополимера к влагопоглощению, что дает возможность предположить их применение в качестве основ при получении герметизирующих и клеевых материалов. Путем анализа численных значений констант сополимеризации доказана более высокая активность акриловой кислоты в сравнении с ненасыщенными полиэфирами, и тем самым подтверждено выдвинутое ранее предположение о сравнительно более низкой реакционной способности последних.

Keywords: unsaturated polyesters, radical copolymerization, kinetics, sealant, adhesives.

Ключевые слова: ненасыщенные полиэфиры, радикальная сополимеризация, кинетика, герметик, клеи.

1. Introduction

Currently, due to the expanding application scope of sealing compounds in construction, capable of swelling in water to some extent, the key focus now lies in dedicating extra attention to developing and modifying their composition for synthesis. The compositions of these materials, which are utilized not only in the construction industry but also in various other practical fields, are constantly evolving due to their specific application and operational requirements. This continuous improvement not only enhances their quality but also lowers costs by introducing novel compositions in their production. At present, there is a notable emphasis on advancements that utilize readily available, cost-effective materials as primary coreagents in sealant synthesis. These materials exhibit a range of practical properties, addressing the primary objectives for new sealing materials – enhancing durability and reliability. Thus, unsaturated polyesters of various structures become promising materials for obtaining sealants, possessing good dielectric properties, resistance to ultraviolet (UV) radiation, stability of optical characteristics, resistance to chemicals, and aggressive environments. Their good compatibility with fillers and the ability to undergo radical copolymerization reactions with vinyl monomers are also important, resulting in compounds with high resistance to mechanical impact, load and adhesion [1, p.117].

Sealants based on unsaturated polyesters of various structures have been previously obtained [2]; however, the application of copolymers of polyethylene (propylene) glycol fumarate with acrylic acid as the polymeric base for sealant materials has not been previously investigated, which determines the novelty of this study.

2. Experimental section

The following reagents were used in the study: ethylene glycol, propylene glycol, fumaric acid, zinc chloride ("Reakhim"), acrylic acid (Aldrich), benzoyl peroxide (Aldrich). All reagents were of "AR" grade and were used without further purification.

The initial unsaturated polyesters (UP) - polyethylene glycol fumarate (p-EGF) and polypropylene glycol fumarate (p-PGF) were obtained by polycondensation reaction of the corresponding reagents, taken in a ratio of 1.05:1 mole of ethylene (propylene) glycol to fumaric acid according to the standard procedure [3] in the presence of the catalyst - AlCl3, taken in an amount of 2% of the initial reaction mass. The reaction temperature was lowered to 423–433 K due to the introduction of the catalyst. The polycondensation reaction was monitored by determining the acid number and indirectly by the amount of water released during the reaction.

The radical copolymerization reaction of the above-mentioned UPs with acrylic acid (AA) yielded copolymers of different molar concentrations. The radical copolymerization reaction was carried out in a dioxane solution at a solvent to polymer-monomer mixture ratio of 1:1 by mass, at a temperature of 333 K in the presence of a radical initiator – benzoyl peroxide (BP) for 52 hours. The kinetics of radical copolymerization were investigated by dilatometric method.

To purify the synthesized copolymers of various compositions from residual unreacted initial polymer-monomer mixture, they were washed with dioxane, and the obtained mother liquor was analyzed by HPLC method on an Agilent 1260 Infinity LC chromatograph, which allowed determining the actual composition of the obtained compounds. After washing with dioxane, the copolymers were also washed with distilled water and transferred to Petri dishes for drying in a vacuum oven until constant weight was achieved.

Identification of the copolymers was carried out by IR spectroscopy using a “FSM 1201” spectrophotometer in the range of 450–4000 cm^–1. The radical copolymerization constants and parameters of p-EGF and p-PGF with AA were calculated by the integral Mayo-Lewis method [4].

3. Results and discussion

The obtained polyesters, p-EGF and p-PGF, were high-molecular compounds with a honey-like texture, matte milky-white, and light-yellow color, with relatively low softening temperatures, soluble in chloroform and dioxane, and insoluble in water.

The molecular weight of p-EGF was approximately ~2500 Da, whereas for p-PGF, this parameter did not exceed ~1488 Da, indicating that the formed compounds are polymers, according to [5, p.18]. The numerical values of the molecular weights of the synthesized polyesters determined by light scattering method correlated well with the values calculated by indirect methods - by determining the acid number and measuring the volume of water released during polycondensation, confirming the reliability of the obtained results. The structure of the synthesized UPs was identified by IR spectroscopy. The IR spectra exhibited characteristic absorption bands in the range of 1570–1590 cm–1, corresponding to the unsaturated bonds of fumarate groups.

Subsequently, network copolymers of the above-mentioned UPs with AA were obtained by radical copolymerization. This process is schematically represented in figure 1:

R_I– radical initiator, R = H, CH₃ (groups of unsaturated polyester).

Figure 1. Scheme of copolymer synthesis of p-EGF-AA and p-PGF-AA

Next, figures 2 (a, b) show the kinetic curves of copolymerization of the above-mentioned UPs with AA, obtained by the dilatometric method [6].

Figure 2. Kinetic curves of copolymerization of p-EGF and p-PGF with acrylic acid

Analysis of the obtained kinetic curves shows that the reaction rate directly depends on the composition of the initial polymer-monomer mixture - the initial solution of the polymer (p-EGF or p-PGF) and the monomer (AA) in dioxane. Specifically, with an increase of AA in the initial mixture, the reaction rate increases (Figure 2a, b), the numerical values of which are presented below in Table 1. This is explained by the greater reactivity of AA compared to the above-mentioned unsaturated polyesters [7, pp.105-107].

It is worth noting that there is lower reaction rate with p-EGF compared to p-PGF. This can be explained by the structure of p-PGF, which was synthesized based on fumaric acid and propylene glycol, indicating a greater branching of the glycol molecule, leading to additional steric hindrances in the copolymerization process with AA.

The identification of copolymers was also performed by IR spectroscopy. The IR spectrum of the p-PGF copolymer exhibited characteristic bands in the range of 1570–1590 cm^–1, indicating the presence of some unreacted double bonds characteristic of the inclusion of UP units in the copolymer composition. Analysis of the peak area numerical value of ​​this interval indicated its reduction by 2–3.5 times compared to the corresponding numerical value of the peak area in the IR spectrum of the p-PGF itself, suggesting a decrease in the number of unsaturated double bonds in the composition of p-PGF as a result of the copolymerization reaction with AA. Similar data were obtained by comparing the peak areas of IR spectra for the examined binary system of p-EGF with AA. Additionally, the formation of the p-PGF-AA copolymer is confirmed by the presence of absorption regions in the range of 1700–1780 cm^–1 in the IR spectra of this copolymer, characteristic of the –COOH groups.

Furthermore, the composition of the two-component systems of UP-AA synthesized at different molar ratios of comonomers, determined by HPLC analysis of the mother liquors on a residual basis, is presented in Table 1 [8]. Moreover, parallel determination of the composition of the copolymers synthesized by us using potentiometry yielded results that correlated well with the data obtained by HPLC analysis, which serves as confirmation of their reliability [9].

Table 1.

Dependence of the copolymer composition on the composition of the initial mixture during the copolymerization of UP (p- EGF, p-PGF) (М₁) with AA (М₂), BP [I]=8 mol/m³, T = 333 K

The analysis of the obtained data indicates that the composition of the synthesized copolymers in all cases, regardless of the initial polymer-monomer mixture ratios, is somewhat enriched with AA units, indicating its higher reactivity compared to UP [7, p.107]. The yield of the obtained copolymers in all examined cases is inversely proportional to their degree of unsaturation determined by the bromide-bromate method [10]. This can be explained by the insufficient concentration of AA (taking into account the difference in the molecular weights of the coreagents) for branching and crosslinking reactions, leading to an increase in the number of unreacted fumarate groups [10, p.15, p.22-24].

It's noteworthy that the obtained network copolymers exhibit relatively low moisture sorption capacity, attributed to their dense polymer network. However, this characteristic presents an opportunity for incorporating fillers in the production of sealants and adhesives based on them.

Next, table 2 presents the copolymerization constants of UP with AA of various compositions [6, p. 105-106], calculated by the integral Mayo-Lewis method [4].

Table 2.

Constants and parameters of radical copolymerization of UP-AA

The analysis presented in table 2 indicates that in both examined cases, the copolymerization constant r₂, which determines the activity of AA relative to UP, exceeds one, indicating the predominant occurrence of addition reactions to the radical based on AA's "own" radical or monomer.  This case also directly confirms the occurrence of partial homopolymerization at high AA contents in the copolymer composition [11, p. 35]. Regarding the copolymerization constant r₁, which determines the relative activity of UP, it should be noted that in both examined binary systems, it does not exceed one, indicating greater activity of the radical based on UP towards the "foreign" monomer or radical. This case confirms the formation of alternating or block structures during copolymerization. It is worth noting that the expected tendency of the r₁ constant to approach zero did not materialize due to the inability of fumarate groups to undergo homopolymerization; instead, its value approaches one, which can be explained by the participation of fumarate groups in termination reactions [12].

The product of copolymerization constants (r₁∙r₂) for all examined binary systems is close to one, indicating the possibility of forming structures with a freely distributed units [13].

Thus, we can say that the chemical properties of the copolymers we obtained based on p-EGF and p-PGF with AA are significantly influenced not only by the nature of the unsaturated polyester but also by its quantitative content in the initial polymer-monomer mixture, allowing for the control of the exhibited properties of the final copolymer through compositional variation.

4. Conclusion

Copolymers of p-EGF and p-PGF with AA were obtained by radical copolymerization in solution. The kinetics of the process were investigated, the actual composition of the copolymers was determined, and their chemical properties were studied.

It has been established that by varying not only the choice of the coreagent (unsaturated polyester) but also its content in the initial polymer-monomer mixture, it is possible to control the properties of the synthesized copolymers, in particular, the frequency of the polymer network, which directly affects the manifestation of the copolymer's adhesive properties. This aspect could play a crucial role in the potential application of p-EGF and p-PGF copolymers with AA as the basis for sealant materials.

Funding: This work was carried out under the research project RK № AP23488036 "Development of thermoreactive polyester binders for the production of poured decorative products and building materials" within the framework of grant funding for scientific and (or) scientific-technical projects for 2024-2026, implemented by the Ministry of Science and Higher Education of the RK.

Conflict of interest: There is no conflict of interest among the authors.

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