Kinetics of radical telomerization of acrylic acid in the presence of 1-octadecanethiol

Introduction

The study of the processes underlying the synthesis of amphiphilic polymers and oligomers is one of the most intensively developing interdisciplinary directions at the forefront of the science of polymers, chemical kinetics, physical surface chemistry and Green Chemistry. Amphiphilic polymers are of significant interest as surface active agents (SAA) with low critical micelle concentrations (CMC) [1], which determines the cost-effectiveness of their application and allows calculating the reduction of SAA emissions into the environment. Since the complete withdrawal from the use of SAA is impossible [2], it is necessary to develop synthetic approaches that minimize the costs of materials and energy for their production, reduce the concentration of SAA in working solutions and reduce SAA emissions into the environment. It is also necessary to replace SAA-containing aromatic fragments that are responsible for the formation of highly toxic substances when they degrade in the environment [3], [4] by expanding the range of SAA with aliphatic hydrophobic fragments.

The expansion of the range of amphiphilic polymers is mostly achieved by involving new monomers in the polymerization and telomerization processes. In addition, the approach associated with varying the nature of the end-groups by selecting suitable initiating systems and (or) chain transfer agents is also of great importance. At the same time, the method of radical polymerization provides the ability to synthesize a wide range of polymers from a wide range of monomers while it does not impose strict requirements on the conditions, which is particularly attractive in practical terms.

Addition of active chain-transfer agents is one of the ways of end-group control in radical polymerization or telomerization [5]. It can be assumed that an excess of the chain-transfer agent, as compared to the initiator, provides the possibility of initiation almost exclusively by radicals that are formed via interactions between primary products of the radical break-down of the initiating and chain-transfer agents as well as the restriction of the chain by transfer [6], [7]. This method has been previously applied for the production of amphiphilic oligomers with hydrophilic units consisting of residual N-vinyl-2-pyrrolidone with mercaptans and carboxylic acid chlorides used as chain-transfer agents [8], [9], [10], [11]. Acrylic acid is another monomer capable of providing hydrophilic units. The efficiency of the use of a number of mercaptans as chain transfer agents in radical polymerization of acrylic acid was shown and the constants of chain transfer thereto were determined [12], [13]. Kinetic patterns of the radical polymerization of acrylic acid have been investigated in a number of publications [14], [15], [16], [17], [18], [19]. It was shown that the degree of monomer ionization had an effect on the polymerization rate [15], [16], [19] as well as the fact that the abnormal reaction order with respect to the monomer concentration was reported [14], [16], [18].

The abnormally high order of the reaction with respect to the monomer concentration, which lies in the range of 1.5–1.7 according to previously reported studies, is the subject of discussion and is explained either by the participation of the monomer in the initiator decay [14], [16], [18] or by the formation of ion pairs in case the polymerization is carried out in an aqueous medium [17]. At the same time, it is of great interest the study of the kinetics of radical polymerization of acrylic acid in non-polar (low-polarity) media in the presence of significant amounts of active chain transfer agents when the formation of ion pairs is unlikely and the participation of the monomer in the initiator decay event is minimized. Furthermore, low molecular weight products are formed in these conditions, which assures the absence of a gel effect that can introduce significant errors into the calculated kinetic parameters the presence of which is reported in the radical polymerization of acrylic acid in paper [19].

In the present study the kinetics of polymerization (telomerization) of acrylic acid initiated by 2,2′-Azobis(2-methylpropionitrile) (AIBN) in the presence of octadecyl mercaptan as the chain-transfer agent in the medium of 1,4-dioxane was studied. The structure of the obtained product was confirmed by ¹H NMR and ¹³C NMR spectroscopy and the critical micelle concentration (CMC) was determined. A quantitative approach was proposed for the determination of kinetic parameters of free-radical telomerization of acrylic acid in low-polar media under the assumption that both unassociated and associated forms of acrylic acid participate in the chain propagation, the latter stabilized through formation of intermolecular hydrogen bonds. The telomerization reaction studied corresponds to the atom economy principle and leads to the formation of a product with an aliphatic hydrophobic fragment having a low CMC in water.

Experimental part

Results and discussion

The proposed equation of radical telomerization of acrylic acid in the presence of octadecyl mercaptan is shown in Scheme 1.

Scheme 1:

Synthesis of a telomere of acrylic acid with thiooctadecyl end-group.

The presence of signals in ¹H NMR spectrum with chemical shifts at 0.84 ppm and 1.22 ppm that are associated with CH₃ and CH₂ groups of the end octadecyl unit, indicated the formation of an acrylic acid oligomer with thiooctadecyl end-group (Fig. 1).

Fig. 1:

¹H NMR spectrum of the acrylic acid oligomer obtained in the presence of octadecyl mercaptan as the chain-transfer agent.

The chemical shift at 3.55 ppm is attributed to the CH-groups of acrylic acid residues present in-between the first and the last links of the chain. The chemical shifts at 1.75 ppm and 2.20 ppm are associated with the absorption of protons of the CH₂-groups of the acrylic acid residues. The wide absorption maximum with a chemical shift at 1.49 ppm is indicative of the overlapping of the CH₂-signals of the octadecyl fragment groups and acrylic acid residues. It is assumed that the 2.61 ppm chemical shift represents the protons of the CH-groups of the first acrylic acid residue linked with the thiooctadecyl end-group, whereas the 2.44 ppm chemical shift may be attributed to the protons of the CH₂-groups linked with sulfur atoms. The absence of an intense singlet in chemical shifts between 1.3 ppm and 1.4 ppm suggests the absence of residues of isobutyronitrile at the chain end, which confirms the initiation almost exclusively by the thiooctadecyl radical [21].

The ¹H NMR spectrum of the acrylic acid telomere with a thiooctadecyl end-group is in good correlation with the results of the ¹³C NMR spectrum (Fig. 2). The chemical shift at 14 ppm is associated with the methyl end-group, while the absorption at 22 ppm corresponds to the adjacent methylene group of the thiooctadecyl fragment. The remaining methylene groups of the thiooctadecyl fragment other than the one directly bonded with the sulfur atom (31 ppm) have chemical shifts at around 30 ppm. Finally, the chemical shifts at 34 ppm and at 41 ppm are related with the carbon atoms of the CH₂- and CH-groups of the acrylic acid residues. The downfield shift at 179 ppm is indicative of a carboxyl group. On the contrary, no signal in the range of 120 ppm indicates the absence of a nitrile group [21]. Thus, the direct interaction of acrylic acid with AIBN and the decomposition products thereof is suppressed in the presence of a significant amount of octadecyl mercaptan as suggested in the introduction.

Fig. 2:

¹³C NMR spectrum of the acrylic acid oligomer obtained in the presence of octadecyl mercaptan as the chain-transfer agent.

The presence of thiooctadecyl end-groups is also indicative of the ability of the obtained products to form micelles at concentrations exceeding 2.5×10⁻² g×l⁻¹ (Fig. 3a). Numerical differential distribution of micelles of the amphiphilic oligomers of acrylic acid with thiooctadecyl end-group in aqueous medium obtained by dynamic laser light scattering is shown in Fig. 3b. The number-average diameter of micelles is 8.0±0.5 nm at 298 K and pH=5.1 with the concentration of the amphiphilic oligomer of acrylic acid being equal to 0.05 g×l⁻¹. The number-average molecular weight of the obtained polymer was determined by back titration of the sulfide end-group with m-chloroperbenzoic acid and amounted to 2425, which corresponds to the number-average polymerization degree of 29.7. Thus, it can be concluded that the product was an oligomer (telomere). Thus, the radical telomerization of acrylic acid in the presence of octadecyl mercaptan allows for one-stage synthesis of SAA with a low CMC. Furthermore, aside from the small amounts of AIBN used as an initiator, all the atoms of acrylic acid and octadecyl mercaptan were fully included into the oligomeric product.

Fig. 3:

Determination of CMC and micelle size distribution. (a) Plot of the optical absorption density (A) of the amphiphilic acrylic acid oligomer versus concentration (C) at 298 K, (b) numerical size distribution of micelles in aqueous medium (pH=5.1) at 298 K and at concentration of the amphiphilic oligomer of acrylic acid equal to 0.05 g×l⁻¹ as determined by dynamic laser light scattering.

It is known that low molecular weight oligomers of acrylic acid can degrade in the environment. Oligomers of acrylic acid with molecular weight below 500 undergo rapid degradation, in case of molecular weights ranging from 1000 to 4500 the rate of biodegradation is much lower, and in cases of higher molecular weights biodegradation appears practically impossible [22]. At the same time, certain bacteria are capable of degrading oligomers of acrylic acid with molecular weights in the range from 1000 to 4500 [23]. Although the degradability of polyacrylic acid in the environment is low there are indications that the toxicity of polyacrylic acid is rather low as well [24] and it is deemed environmentally safe [25].

Further studies of telomerization of acrylic acid in the presence of octadecyl mercaptan were aimed at investigation of the kinetics of this process as that being of potential technological importance on one hand and of considerable theoretical interest on the other, since the interpretation of the abnormally high reaction order with respect to the monomer concentration remains a matter of debate.

It is known that the kinetics of radical polymerization of vinyl monomers in solution adheres to the equation (1):

where k is the observed rate constant, [M] is the monomer concentration, [I] is the initiator concentration, t is the polymerization time, n is the order with respect to the initiator concentration and m is the order with respect to the monomer concentration.

Typically, the order with respect to the initiator concentration is close to 0.5 and the order with respect to the monomer concentration is around 1.0 [26], which is the result of the bimolecular termination in the radical polymerization. The order with respect to the monomer concentration in radical polymerization was determined by measuring the half-transformation period of acrylic acid. As the thermal decomposition of AIBN was the slowest stage, the concentration of the initiator can be assumed to be almost constant ([I]=[I]₀). Therefore, the effective rate constant k[I]ⁿ=k_ef can be introduced into equation (1) yielding equation (2) as follows:

By taking an integral of the equation (2) from t=0 to t=τ_1/2 and from [M]₀ to [M]₀/2 with subsequent logarithmation the equation (3) can be obtained that shows the relation of the half-transformation period with the initial monomer concentration:

The experimental data appear linear in the “lnτ_1/2–ln[M]₀” coordinates with a slope ratio of −0.6, which corresponds to the order with respect to the monomer (m) equal to 1.6 (Fig. 4).

Fig. 4:

The plot of “lnτ_1/2–ln[M]₀” for acrylic acid polymerization at 343 K and 1.46×10⁻² mol×l⁻¹ AIBN.

To determine the order with respect to the initiator concentration, equation (1) can be written as follows including the given order with respect to the monomer concentration and keeping in mind that the initiator conversion was low during the first 15 min. In this case the current concentration of the initiator can be considered equal to the initial concentration thereof and the kinetic equation shall look as follows (4):

where [I]₀ is the initial concentration of the initiator.

Integration of equation (4) yields equations (5) and (6), which can be used to determine the order with respect to the AIBN concentration:

The k′ values can be calculated for various initial concentrations of AIBN as a slope of linear anamorphosis of kinetic curves in the “[M]^−0.6–t” coordinates (Fig. 5).

Fig. 5:

Linear anamorphosis for acrylic acid polymerization (2.88 mol×l⁻¹) at 343 K and various initial AIBN concentrations: 1 – 0.97×10⁻², 2 – 1.46×10⁻², 3 – 2.92×10⁻², 4 – 3.88×10⁻² mol×l⁻¹.

The plot “[M]^−0.6–t” is linear with curve intersection at a point that corresponds to the initial concentration of the monomer. Furthermore, the rate constant k′=0.051 l^0.6×mol^−0.6×min⁻¹ calculated at 343 K and at AIBN concentration of 1.46×10⁻² mol×l⁻¹ should match that calculated based on the free term of the linear relationship (3) (Fig. 4).

k′= 0.6(2m−1−1)ea(m−1)=0.054 l0.6×mol−0.6×min−1

m=1.6, a=2.25 – length of the intercept segment on the line in the “lnτ_1/2–ln[M]₀” coordinates (Fig. 5).

As can be seen, the values of the rate constant k′ calculated using two independent methods agree with an accuracy of ca. 6%, which indicates the correctness of the method used for determination of the order with respect to the monomer concentration.

The values of the rate constant k′ calculated for various concentrations of the initiator (Fig. 5) can be used to verify the experimental data versus the theoretical order with respect to the AIBN concentration, which is equal to 0.5 (Fig. 6a) as well as for its determination through linear relationship (7) in logarithmic coordinates “ln k′–ln[I]₀” (Fig. 6b).

Fig. 6:

Determination of the order with respect to the AIBN concentration (343 K): A – coordinates “k′–ln[I]₀^0.5”, B –coordinates “lnk′–ln[I]₀”.

The obtained plots were linear in nature and congruent (Fig. 6). Thus, the assumption about the theoretical order with respect to the initiator concentration was highly precise (Fig. 6a) and the determination thereof via the slope of the linear anamorphosis of equation (7) (Fig. 6b) yields a value of 0.55. Therefore, mainly bimolecular chain termination can be observed in polymerization of acrylic acid in the presence of AIBN as the initiator and octadecyl mercaptan as the chain-transfer agent. Since mercaptans are known to be active chain-transfer agents [12], [13] and octadecyl mercaptan was used in significant excess versus AIBN, one may suggest the material chain restriction occurs almost exclusively by interactions between the growing radicals and octadecyl mercaptan, while the termination of the kinetic chain occurs mainly due to recombination of radicals produced by octadecyl mercaptan (Scheme 2).

Scheme 2:

Chain transfer and termination reactions.

The order with respect to the acrylic acid concentration appeared higher than theoretical and is in good agreement with data found in literature [14], [16], [18], which indicates significant cooperativity of the chain propagation events. In aqueous media, the cooperativity of the monomer attachment events may be explained by the formation of ion pairs [17], however their contribution in low-polar media is insignificant.

The increase of the order with respect to the monomer concentration in course of polymerization in the low-polar 1,4-dioxane medium can be explained by contribution of linear and cyclic associates of acrylic acid (Scheme 3) stabilized by hydrogen bond formation, the existence of which has been recognized in literature [27], [28], [29].

Scheme 3:

Chain propagation during polymerization of acrylic acid in low-polar media.

If the chain propagation occurs with the contribution of an unassociated form of acrylic acid (Scheme 3a) one may expect the first order with respect to the monomer concentration. In contrast, contribution of the acrylic acid associates (Scheme 3b, c) yields the second order with respect to the monomer concentration. Therefore, the following equation may be devised (8):

where k – the observed rate constant; k₁, k₂, k₃ – the polymerization rate constants with participation of linear and cyclic associates as well as unassociated form of acrylic acid; α₁, α₂ – the fraction of linear and cyclic associates of acrylic acid.

Assuming that at each given temperature the equilibrium between different forms of acrylic acid associates and its non-associated form is established rapidly, the following equation may be obtained (9):

k_A=(k₁α₁+k₂α₂) – polymerization rate constant with participation of associated forms of acrylic acid; k_M=(1−α₁−α₂)k₃ – polymerization rate constant with participation of monomeric (unassociated) form of acrylic acid.

The equation (9) can be transformed into the linear form in “[M]^0.6–[M]” coordinates (10) that is valid for each current moment of time at given temperature:

The kinetics of polymerization of acrylic acid at various temperatures was studied to verify experimentally the equation (10). Using the equations (5) and (6) while keeping in mind the known concentration and the order with respect to the initiator concentration, it is possible to determine rate constants k at various temperatures (Fig. 7) and the rate constants k_M and k_A (Fig. 8).

Fig. 7:

The plot of “[M]^−0.6 – t” for various temperatures: 1 – 333 K, 2 – 343 K, 3 – 353 K (the concentrations of acrylic acid and AIBN are 2.88 mol×l⁻¹ and 1.46×10⁻² mol×l⁻¹, respectively).

Fig. 8:

The plot of “[M]^0.6–[M]” at different temperatures: 333 K (red), 343 K (green), 353 K (blue) (the concentrations of acrylic acid and AIBN are 2.88 mol×l⁻¹ and 1.46×10⁻² mol×l⁻¹, respectively).

Evidently, the observed linear dependencies (Fig. 8) cannot be interpreted under the assumption of monomer participation in the act of initiator decay or under the assumption that during initiator decay two radical are formed that are trapped in the solvent cage and are capable of initiating the polymerization as a result of the acrylic acid diffusion. These concepts are often used to explain the abnormally high reaction order with respect to acrylic acid concentration [14], [16], [30]. Furthermore, the papers [14], [16], [18], [30], [31] indicate that replacing an azo-based initiator with peroxydisulfate does not affect the reaction order with respect to acrylic acid concentration, which also contradicts the interpretation of the abnormally high reaction order with respect to acrylic acid concentration as a result of the specific aspects of the act of initiation. At the same time, the results presented above are consistent with the assumption that the associates of acrylic acid stabilized by hydrogen bonds participates in the act of chain propagation, which was proposed in [28].

The calculated kinetic parameters are listed in Table 1. The relative errors calculated for all of the values listed in Table 1 varied between 4 and 10%, which is in good correlation with the error of the rate constant k′ being 6% as previously determined in two independent experiments.

It can be seen that the polymerization process with the unassociated form of acrylic acid had a lower activation energy compared to its associated form. It was assumed that this due to the higher degree of stabilization of the associates of acrylic acid by hydrogen bonds in the ground state as compared to the activated complex. The pre-exponential factor corresponding to the polymerization with associated acrylic acid forms was 37 times higher than that of the unassociated form, that can be explained by the much lower concentration of the latter. The significant difference of pre-exponential factor values was almost completely compensated by the difference between activation energies of the competing processes, which in the end leads to an almost thermally invariant character of the relationships “[M]^0.6–[M]” obtained at different temperatures. Therefore, the summary of the obtained kinetic data for telomerization of acrylic acid in the 1,4-dioxane medium approved the participation in the chain propagation events of both unassociated and associated forms of acrylic acid stabilized by hydrogen bonding.

Conclusions

The synthesis of amphiphilic oligomers of acrylic acid with thiooctadecyl end-groups by radical telomerization of acrylic acid induced by AIBN in the presence of octadecyl mercaptan as the chain-transfer agent was accomplished, the oligomers being capable of micelle formation in aqueous solutions event at low concentrations. It was shown that the radicals formed during thermal decomposition of AIBN interact almost exclusively with 1-octadecanthiol to form thiooctadecyl radicals, which initiate the polymerization of acrylic acid. Thus, all the atoms of acrylic acid and 1-octadecanthiol are fully included into the oligomer product, which suggests atom economy in this reaction.

It was determined that the order with respect to the initiator concentration is close to 0.5 indicating bimolecular termination of the kinetic chain, while the order with respect to the monomer concentration is equal to 1.6, which can be explained by concurrent participation in the chain propagation of the unassociated form of acrylic acid and the associates thereof stabilized by hydrogen bonds. The kinetic model was suggested and the kinetic parameters of the polymerization with associated and unassociated forms of acrylic acid were determined.