2020 Volume 3 Issue 2

Taking this into account, we turned our attention to carboxyl-containing polydimethylsiloxanes. Our interest in these objects stems from a whole range of factors. First of all, this is the need for expanding the global market of silicones as one of the most promising polymer matrices for wide use. Secondly, this is the diversity of synthetic approaches to carboxyl-containing PDMSs and, meanwhile, the lack of their mass production. The latter suggests that, despite the diversity of synthetic approaches, there is no versatile method for their production. Finally, the unique properties of carboxyl-containing silicones serve as a starting point for the production of new polymeric materials and devices that meet modern requirements. Of note is the propensity of siloxanes [1, 2] and mixtures of carboxyl-containing siloxanes with amino-containing analogs [3] to self-healing under certain conditions. Recent investigations established that the mixtures of carboxyl-containing PDMSs with poly(ethylene glycol) diglycidyl ether exhibit not only self-healing but also shape memory effect [4].

The introduction of carboxyl groups into the composition of polymer molecules strengthens the intra- and intermolecular interactions owing to the formation of hydrogen bonds. The variation of the content of carboxyl groups in a polymer can afford elastomers, thermoplastic elastomers, polymers with fiber-forming properties, or solid resins [1–12]. The carboxyl-functionalized polysiloxanes bearing spacers between a siloxane framework and carboxyl groups shorter than –СН₂–СН₂– or –CH₂OCH₂– decompose, whereas those with the longer spacers are stable [13].

Carboxyl-containing PDMSs can be divided into three large groups (Fig. 1):

(a) polymers bearing carboxyl groups in side chains,

(b) α,ω-terminal (telechelic) polymers,

(c) polymers bearing carboxyl groups both in side chains and at α,ω-positions.

Figure 1. Carboxyl-containing polydimethylsiloxanes (A acid moiety).

Organosiloxane polymers bearing carboxyl groups are used in water-proof compositions for impregnation of natural and synthetic fabrics [14, 15] since the siloxane backbone displays hydrophobic properties and the presence of a small amount of polar groups can provide adhesion to the material. The reactive carboxyl groups interact with metal oxides, resulting in core–shell structures, and are used for the preparation of stable suspensions of magnetic fluids and magnetic elastomers [16, 17]. PDMSs with carboxyl groups are also utilized as surfactants for the production of foams [11], in emulsion polymerization [18–25], in cosmetic compositions for skin and hair treatment [26], in compositions with low surface energies as antifouling coatings [27], and for creation of chelate polymers [28]. Carboxyl-containing PDMSs partially or fully neutralized with metal salts can be used in the manufacture of golf ball cores [29] and electronic devices [30], production of composite materials with other polymers [31] and membranes for separation of gas mixtures (for example, CO₂/CH₄ [32]).

Analysis of the state-of-the-art

The compounds with carboxyl groups attached to the silicon atom are not described; presumably, because the carboxyl group cannot be formed directly at the silicon atom. Recently, it was shown [33] that even the reactions of sodium diethoxy(methyl)silanolate with carbon dioxide both in an autoclave and in solution lead to the formation of a siloxane bond. The proposed mechanism implies that firstly a carbon dioxide molecule is inserted between silicon and sodium atoms. However, the carboxyl group directly attached to the silicon atom generates an uncompensated partial positive charge on it, which strongly increases its reactivity. This leads to the interaction with another molecule of the Rebrov salt [34], resulting in the formation of the siloxane bond.

The synthesis of organosilicon compounds bearing carboxylic acid residues was described by Sommer as early as the 1950s [35, 36]. However, these were monomeric compounds. One of the first attempts to synthesize PDMSs with carboxyl groups in organic substituents at the silicon atom was made by the group of Zhdanov in 1980, who described the synthesis and properties of polydimethylcarbosiloxanes bearing carboxyl groups as side substituents (Fig. 2) [37–40]. The authors studied the physicochemical properties of these polymers (viscosity, elastic modulus, and thermomechanical properties) in the temperature range of 20–160 °C. It was revealed that at 80 °С and above, the structure formation occurs with a transition from the liquid state to the rubbery one. In this case, an increase in the temperature, the duration of polymer heating, and the content of carboxyl groups led to the growth of the elastic modulus. The observed phenomenon was reversible: when the heated sample was kept at room temperature, it converted to the initial liquid state. The authors assumed that these changes in the polymer properties are associated with the changes in the bonding type (transformation of intramolecular hydrogen bonds into intermolecular ones) or with the formation of clusters at physical network cross-link points.

Figure 2. Polydimethylcarbosiloxanes.

The synthetic approaches to carboxyl-containing polydimethylsiloxanes can be divided into four groups: (а) copolymerization of cyclosiloxanes, (b) hydrolysis of chlorosilanes and silanols, (c) hydrosilylation or hydrothiolation of different functional groups with polydimethylsiloxanes, (d) miscellaneous methods, in particular, those based on the addition of a carboxyl-containing moiety to the functional group already introduced in PDMS (Fig. 3).

Figure 3. Main synthetic approaches to the carboxyl-containing polydimethylsiloxanes.

Kawakami et al. [41] showed that aliphatic and aromatic dicarboxylic acids can be introduced into polydimethylsiloxanes bearing terminal dimethylhydride siloxane units via hydrosilylation. The starting narrow-dispersed hydride-containing PDMSs were obtained by the living anionic polymerization of hexamethylcyclotrisiloxane, which was terminated by introducing dimethylchlorosilane. The modifying agents were prepared from 5-hydroxyisophthalic acid and diethyl 3-hydroxyglutarate (Fig. 4). Three different methods for the synthesis of 5-allyloxyisophthalic acid were compared. According to the first one, diethyl 5-hydroxyisophthalate was obtained at the first step; then, it was allylated and hydrolyzed to give the protected product in 95% yield. The ethyl protecting groups were removed by hydrolysis in hydrochloric acid. The second method implied the preliminary synthesis of a potassium salt which was reacted with allyl bromide in the presence of benzyltributylammonium bromide. The yield of the product in this case composed 67%. Then, it was hydrolyzed by the first method. In the third protocol, 5-hydroxyisophthalic acid and potassium hydroxide were dissolved in a water–ethanol mixture; then, allyl bromide was added, and the resulting mixture was refluxed for 24 h. The yield of the product in this case was 34%. To obtain 3-allyloxyglutaric acid, starting diethyl 3-hydroxyglutarate was allylated with allyl bromide in the presence of copper and DMF. The yield of the product composed 56%. Then, it was hydrolyzed to acid.

Figure 4. Syntheses of 5-allyloxyisophthalic and 3-allyloxyglutaric acids.

At the final step, both allyl-containing acids were protected with trimethylsilyl groups using hexamethyldisilazane; then, PDMS was added across the vinyl groups in the presence of a platinum catalyst followed by the deprotection (Fig. 5). Thus, a series of copolymers with the molar masses ranging from 2000 to 9000 were synthesized. The yields at the hydrosilylation step composed 40–78% and 74–95% in the case of aryl- and alkyldicarboxyl derivatives, respectively.

Figure 5. Hydrosilylation of 5-allyloxyisophthalic and 3-allyloxyglutaric acids.

Cheng et al. [42] used hydrosilylation of methyl and tert-butyl methacrylates to obtain α,ω-terminal (telechelic) PDMSs. Then, the ester groups were removed upon alkaline (in the case of methyl ester) or acid (in the case of tert-butyl ester) hydrolysis. The latter changed the molar-mass characteristics of the final polymer relative to the initial one. Hence, the polymers with the molar masses ranging from 1000 to 2000 and polydispersity varying from 1.32 to 1.44 were obtained. The resulting telechelic carboxyl-containing PDMSs were introduced into a two-step reaction with diethylenetriamines and urea to produce new supramolecular elastomers [43–45]. According to the results of DSC and X-ray diffraction studies, these elastomers appeared to be completely amorphous. Investigations on their biocompatibility showed that the copolymers do not possess cytotoxicity and do not provoke skin irritation; the films based on these elastomers were found to exhibit self-healing properties as well as wound-healing ability at the level comparable to that of the commercially available materials.

Knebelkamp et al. [46] synthesized PDMS bearing methacrylic acid moieties distributed along the chain by hydrosilylation of tert-butyl methacrylate with polydimethylsiloxane bearing methylhydride units (Fig. 6). The authors suggest methanesulfonic acid (or p-toluenesulfonic acid) as a catalyst for the deprotection. The degree of conversion of tert-butyl groups composed 98–100%. There are no data on whether the molar-mass distribution changes after the deprotection, but the authors state that the experimental contents of the units with carboxyl groups coincide with the calculated ones. The molar masses of the polymers described in the mentioned work did not exceed 2000 g/mol. The resulting polymers were used as emulsifiers for silicone and coconut oils in water. In the case of coconut oil, a stable emulsion was obtained that did not tend to immediate breaking.

Figure 6. Synthesis of the carboxyl-containing PDMS from methacrylic acid esters.

The Chinese research groups used hydrosilylation to introduce methyl methacrylate into PDMS bearing hydride units in the chain using Karstedt's catalyst at 85 °С in toluene [4, 47]. Starting commercially available polymethylhydrosiloxane had the molar mass of M_w = 1800–2100. The methyl protecting group was subsequently removed upon alkaline hydrolysis in THF in the presence of lithium hydroxide at 85 °С.

In one of the performed investigations [47], the resulting product was converted to an ionomer under the action of zinc chloride and, thus, afforded a hard thermoplastic polymer, which tensile strength was about 9 MPa. The polymer exhibited self-healing properties during annealing of two contacting halves already at 40 °С, whereas annealing at 80 °С led to almost complete healing and more than 90% strength recovery relative to the initial value. The possibilities of application of this material in 3D printing and creation of conducting materials upon filling with graphene were also demonstrated.

In the second work [4], PDMS bearing methyl methacrylic acid residues at each silicon atom was partially cross-linked using poly(ethylene glycol) diglycidyl ether (Fig. 7). The glass transition point of the resulting product was 0.5 °С. It was thermally stable up to 200 °С.

Furthermore, the films obtained from this material displayed self-healing and shape memory effects. The self-healing properties were manifested already at room temperature. The sample obtained over 20 min of contacting of two material pieces at 25 °С can be extended up to 200% without fracture. Complete healing of the cut was observed in 6 h. Moreover, the material remembered its shape upon sample deformation at room temperature and subsequent cooling below a glass-transition point and recovered the initial shape upon further heating. The authors explained the shape memory mechanism by the formation of hydrogen bonds between dimers of carboxyl moieties upon cooling and their rupture upon heating, resulting in the recovery of the initial shape.

Matisons and Provatas [48] introduced different amino acids bearing double bonds into a PDMS chain via hydrosilylation (Fig. 8). To accomplish the hydrosilylation of the –С=С– bonds, the tert-butyl-protected amino acids were used. The protecting groups were subsequently removed under the action of trifluoromethanesulfonic acid. Two polymers were used as the starting hydride-containing PDMSs: poly(methylhydrosiloxane) with the molar mass of 2000 and poly[(methylhydro)(di-methyl)siloxane] copolymer with the molar mass of 12000 and the content of hydride units of 13 mol %. Five amino acids were used as the second component: alanine, glycine, leucine, phenylalanine, and valine. The stability of the polydimethylsiloxane in the presence of trifluoroacetic acid was confirmed using the following procedure. Dow Corning 200 polydimethylsiloxane fluid (0.5 g) was dissolved in chloroform (10 mL); then, an excess of trifluoroacetic acid was added. The reaction mixture was stirred for four days. The target product  was extracted with diethyl ether and rinsed with water. The resulting transparent oil was characterized by NMR spectroscopy but its molar mass characteristics were not studied. The amino acid-functionalized polymers derived from the silylation appeared to be thermally unstable: the beginning of mass loss was observed already at 100 °С and 50% mass loss was detected at about 200 °С. This result was anticipated since such behavior had already been described [49]. Although the introduction of urea moieties in PDMSs does not reduce their thermal stability and these polymers are stable up to 300 °С [50]. The polymers obtained from poly[(metylhydro)(di-methyl)siloxane] were amorphous; the glass-transition point of the alanine derivative was –80 °С.

Figure 8. Synthesis of the PDMS with amino acid moieties.

Horstman et al. [51] pursued a comprehensive study of carboxyl-containing siloxane polymers and outlined a broad spectrum of their potential application. Therefore, this study merits detailed consideration. The carboxyl-containing PDMSs were synthesized by hydrosilylation of trimethylsilyl undecenoate with polydimethylsiloxane bearing methylhydride siloxane units. The main research object was the PDMS polymer featuring the molar mass of 15000 and the content of hydride units of 33 mol %. The carboxyl groups of the resulting copolymers were partially or fully neutralized with metal acetylacetonates. The ionomers obtained represented thermoplastic elastomers. The higher was the degree of carboxyl group neutralization, the wider was the rubbery plateau of these elastomers. According to the stress–strain nomograms, these PDMS ionomers are very close to the classical rubbers based on PDMSs but, as well as thermoelastoplastics, offer an advantage of processability above the yield point. For example, the authors compared the high-molecular PDMS (M_w = 100000 g/mol) bearing 1 mol % of carboxyl units neutralized with magnesium ions and the chemically cross-linked elastomer based on PDMS (Table 1). The resulting ionomers were suggested as gelling agents in cosmetic compositions and in mixtures with MQ resins for the production of thermoplastic adhesives.

Table 1. Comparison of the PDMS elastomer and ionomer

PDMSs bearing undecylenic acid residues were used as inhibitors in silicone compositions cured at room temperature via the condensation mechanism using a tin catalyst [52]. To improve the processability of these compositions, it was necessary to retard the growth of the main diorganopolysiloxane chain at the first step. The inhibitors in use were as follows: a telechelic polymer with the molar mass of about 1200 bearing undecylenic acid residues and PDMS with the molar mass of 30000 and the content of carboxyl units of 2 mol %. This afforded the compositions with excellent processing properties and, after curing, high strength, low elastic modulus, and significant elongation.

Carboxyl-containing PDMSs can also be obtained by the reaction of thiol–ene addition (hydrothiolation) which features a free radical mechanism (Fig. 9) [16, 53]. In the reported examples, azobis(isobutyronitrile) was used as a radical initiator. The reactions afforded selectively the anti-Markovnikov products, namely, β-adducts. The molar masses of tri- and hexafunctional polymers obtained by O'Вrien [16] ranged from 2000 to 9000. The conversion of vinyl groups was over 98%. Subsequently, these polymers were used for the production of magnetic fluids with nanoscale magnetite particles. The tri- and hexafunctional derivatives were shown to cover the same areas on magnetite but the hexafunctional derivatives contained free acid units, which resulted from incomplete grafting during the synthesis. Ścibiorek et al. [53] used block copolymers as vinyl-containing PDMSs. These copolymers were obtained by the living anionic polymerization of hexamethylcyclotrisiloxanes, vinylpentamethylcyclotrisiloxanes, and trivinyltrimethylcyclo-trisiloxanes. The molar masses of the resulting copolymers were 17000, 8600, and 3600, and the contents of vinyl units were 50, 25, and 30 mol %, respectively. After the thiolation, the ¹Н NMR did not contain the signals of the vinyl groups. The polymers were studied by DSC. The DSC curves showed a step in the thermal capacity at –123 °С which corresponds to the glass-transition point and an endothermic peak at –40 °С, which can be attributed to glass-transition and melting of the PDMS phase in the block copolymer, respectively. A step in the thermal capacity at –19 °С corresponded to the glass-transition of the carboxyl-containing block. Therefore, the resulting copolymers exhibited microphase separation.

Figure 9. Synthesis of the carboxyl-containing PDMSs by hydrothiolation.

Guan and Dong [54] explored the possibility of synthesis of β-carboxyethyl-substituted PDMSs from β-cyanoethyl derivatives. The latter were obtained by the combined anionic polymerization of octamethylcyclotetrasiloxane with cyanoethylheptamethylcyclotetrasiloxane in the presence of tetramethylammonium siloxanolate. Screening the reaction conditions for the complete conversion of cyanoethyl groups during acid hydrolysis, the authors managed to achieve only 26.7 mol % conversion. The molar masses of the resulting polymers ranged from 6000 to 34000; the content of the cyanoethyl groups varied from 2 to 9 mol %.

The modified polydimethylsiloxanes are widely used in the production of membranes [55–57]. Of particular attention are a series of works devoted to the synthesis of carboxyl-containing PDMSs and their application as membranes for gas separation [58–60]. The syntheses were carried out according to the scheme of hydrolysis of methyldichlorocyanoethylsilane under acidic conditions (Fig. 10). The authors noted that the degree of conversion of cyanoethyl groups to carboxyl ones reached 98%. The resulting polymer had the molar mass of M_n = 6300 and the polydispersity of 1.6.

Figure 10. Synthesis of the carboxyl-containing PDMS from (cyanoethyl)dichloromethylsilane.

Poly(2-carboxyethylmethylsiloxane) displays strong intermolecular interaction between its segments. It also has a high viscosity and forms a milky rubber-like latex. The product derived from the blocking of carboxyl groups with methanol under acidic conditions did not display strong intermolecular interactions and represented a transparent liquid. To prepare the mixtures of the resulting carboxyl-containing PDMS, it was melted at 70 °С and then mixed with conventional PDMS. Cooling to room temperature led to a white rubbery residue. The fact that the resulting sample dissolves in THF testified that its flexibility is not caused by the covalent cross-linking and stems from the high degree of intermolecular interactions between the segments of poly(2-carboxyethylmethylsiloxane) and PDMS, which is based on the hydrogen bonds (Fig. 11).

Figure 11. Intermolecular interactions between poly(2-carboxyethylmethylsiloxane) and PDMS.

The results of DSC analysis showed that poly(2-carboxyethylmethylsiloxane) has a melting point of 67 °С but, after its mixing with conventional PDMS, the DSC curve did not reveal any thermal effect up to 217 °С; above this temperature, the decomposition of the carboxyl moiety began. Resulting poly(2-carboxyethylmethylsiloxane) was used to obtained membranes by cross-linking with ethylene glycol. The oxygen permeability (O₂/N₂) of this membrane was compared to those of other PDMS derivatives (simple PDMS, PDMS copolymers with poly(2-carboxyethylmethylsiloxane) and methyl ester of poly(2-carboxyethylmethylsiloxane)). Among all the samples explored, the best results were achieved just with poly(2-carboxethylmethylsiloxane).

Due to the absence of any groups capable of interacting in PDMSs, they usually do not mix with polymers of other natures. The introduction of polar groups improves their mixing ability [61, 62]. Li et al. studied a mixture of carboxyethyl-PDMS with poly(vinylpyridine) (PVP) [63] and later with poly(1-vinylimidazole) (PVI) [64]. The unmodified PDMSs did not mix with PVI and PVP, but the introduction of 23 mol % and more carboxyl units enabled the mixing of the resulting polymers with PVI and PVP in any ratios. Most of the mixtures with PVI exhibited positive deviations of glass-transition points. Neither mixture featured the lower critical solution temperature. X-Ray photoelectron and IR spectroscopic studies showed that the hydrogen bonds in these mixtures were formed between the carboxyl groups of PDMS and the nitrogen atoms.

A series of works [9, 10, 65, 66] were devoted to the preparation of polydimethylsiloxanes bearing carboxyl groups in polymer chains, which were used to obtain ionomers via interaction with metal salts. At the first step, dichlorosilane with a tert-butyl butyrate substituent was synthesized [9]. Then, it was introduced into condensation with oligomeric α,ω-hydroxy-PDMS. At the final step, the protecting tert-butyl groups were removed by heating to 210 °C under vacuum (Fig. 12) [9]. This afforded the polymers with tert-butyl butyrate residues featuring the molar masses of 12000 and 19000 and the contents of the ester units of 1.8 and 0.6 mol %, respectively. According to the GPC analysis, the molar masses of the polymers obtained after deprotection were 33000 and 43000, respectively. This suggests that the deprotection was accompanied by chemical modification of the polymers; therefore, this method for the removal of protecting groups is controversial. The resulting copolymers represented viscous liquids prone to a transition to the rubbery state upon heating above 70 °С. During prolonged storage at room temperature, the polymers again converted to the state of viscous flow. The authors explained this by the ordering of carboxyl groups, resulting in the formation of polar domains at elevated temperatures, which led, in turn, to physical cross-linking. The moisture strongly affects the decomposition of these physical interactions: the higher its content is, the faster the polymer returns to the initial state. The addition of zinc acetylacetonate to these polymers gave rise to an elastomer network even at room temperature.

Figure 12. Synthesis of the carboxyl-containing PDMS by the condensation of chlorosilane with the oligomeric hydroxy-terminated PDMS.

Subsequently, the synthetic scheme was modified [10]. At the first step, dichlorosilane with a tert-butyl propanoate substituent was obtained. Then, it was introduced into condensation with α,ω-hydroxy-PDMS. At the final step, the protecting groups were removed in the presence of a catalytic amount of trifluoromethanesulfonic acid at 120 °C. The application of trifluoromethanesulfonic acid in the amount of 2–3 μM per 10 g of the polymer allowed the authors to avoid the evacuation procedure and reduce the processing temperature to 120 °С. It is stated that the catalyst at these concentrations does not affect the siloxane chain and acts only on the tert-butyl groups. The molar masses of the resulting polymers ranged from 9000 to 80000 and the contents of carboxyl groups varied from 0.27 to 1.23 mol %. During heating, the viscosity of these carboxyl-containing PDMSs firstly reduces and then increases, which leads to gelling. The higher is the processing temperature, the faster is the rearrangement of hydrogen bonds from intra- to intermolecular.

Berger and Fost [11] suggested an interesting method for the production of PDMS with carboxyl groups using itaconic acid which is involved in the industrial production of carboxylate rubbers. The synthesis was accomplished by the interaction of amino-containing polydimethylsiloxanes with itaconic acid, which resulted in the formation of a pyrrolidone ring by the Michael reaction. The method holds great promise in the future since it does not require the use of a catalyst, does not lead to the release of side products, and, what is more important, affords a polar group that provides strong intermolecular interaction.

Exploring the effect of hydrogen bonds in siloxane systems, Xing et al. [67, 68] suggested the method for producing carboxyl-containing PDMSs by the interaction of amino-containing PDMS with succinic aldehyde (Fig. 13).

Figure 13. Synthesis of the carboxylic acid-terminated polydimethylsiloxane (PDMS-COOH) from the amine-terminated PDMS (PDMS-NH₂).

This method afforded three polymers with the molar masses of 2000, 4000, and 5500 g/mol. The authors noted that the introduction of terminal carboxyl groups does not affect the glass-transition points of the resulting polymers compared to PDMSs with terminal hydroxy groups featuring analogous molar masses. However, the introduction of these groups strongly affected the rheological behavior of the polymers, which was reflected in a broadened rubber-like plateau in their rheological spectra. It was assumed that owing to the formation of hydrogen bonds, the terminal groups facilitate the formation of dimers and clusters, which leads to such unusual properties.

Another interesting approach is based on the oxidation of tolyl groups connected with the silicon atom [69]. The methyl groups of the tolyl moieties were converted to the carboxyl ones upon oxidation with oxygen under mild conditions (at 40–60 °С and the ambient pressure), resulting in the target carboxyl-containing derivatives in 80–96% yields. The suggested catalytic system allows one to retain the siloxane bond intact. The investigations were performed mainly with short siloxane structures; however, this approach has much room for the application in the synthesis of siloxane polymers provided that an appropriate catalytic system and solvents are selected.

In general, researchers have been expending considerable efforts on the synthesis and application of the carboxyl-containing polydimethylsiloxane polymers for more than 50 years, which evidences the high applied potential of these polymers.

The most promising method seems to be the hydrolysis of β-cyanoethyl groups since it utilizes available precursors and allows one to achieve the maximum content of carboxyl groups in a polymer calculated per a methylsiloxane unit. Of course, the method requires further development, especially in a sequence of chemical steps. Presumably, the hydrolysis of a cyanoethyl group in the monomer followed by hydrolytic polycondensation under acidic or alkaline conditions is a more promising way. The hydrothiolation and hydrosilylation are also very attractive methods because they proceed through the addition reactions without the release of side products, and the vinyl- and hydride-containing PDMSs are commercially available. This means that the processes can be carried out without a solvent and, after completion, do not require additional purification and isolation procedures. The same reason prompts an interest to the synthesis by the Michael reaction from available itaconic acid and aminosiloxanes. 

The challenges that we formulated starting investigations in this field were simultaneously simple and very complex. We aimed, first of all, at entering this field, having harnessed the most promising routes for the synthesis of carboxyl-containing polydimethylsiloxanes and adopted them for an emerging area of green silicone technologies and new applications. After a literature survey, this part of the work appeared to be not so complicated. Furthermore, it was important to understand why the unique properties of the carboxyl-containing polydimethylsiloxanes did not find adequate practical application and widespread use. And the second challenge was much more complicated since it implied full immersion into this field and a search for bottlenecks within the whole carboxyl–siloxane project. Our program implied finding the solutions to the following problems:

– sequential synthesis of the carboxyl-containing polydimethylsiloxanes differing in the arrangement of structural units bearing carboxyl groups (telechelic polymers, polymers with carboxyl groups distributed along the chain, polymers containing different types of spacers);

– investigation of the rheological and thermal characteristics of the resulting carboxyl-containing PDMSs (cc-PDMS), assessment of the effect of the type of a spacer and the concentration of carboxyl groups in the samples under investigation;

– systematization of the synthesized cc-PDMSs, selection of the promising fields of their practical application, and construction of the corresponding carboxyl-containing polysiloxanes.

To solve the formulated problems, we synthesized, at the first step, hydride-containing oligomers with variable arrangement and concentrations of silyl hydride groups. Their syntheses were carried by the cationic polymerization combined with catalytic rearrangement (Fig. 14). The low-molecular products were removed by distillation under reduced pressure. The detailed description of the synthetic procedures is published elsewhere [70].

Figure 14. Cationic polymerization combined with catalytic rearrangement.

The performed experiments afforded two series of hydride-containing PDMSs with hydride units either statistically distributed along the macromolecule chain or located only at its ends (telechelic polymers). The molar masses of the latter ranged from 2500 to 9300 g/mol. In the case of the statistical polymers, the molar masses varied from 3000 to 24000 g/mol with various contents of hydride units.

We chose for modification commercially available undecylenic acid which is derived from natural raw materials. Undecylenic acid is the only one ω-unsaturated carboxylic acid that is produced on an industrial scale [71]. To perform hydrosilylation of the С=С bond, the carboxyl moieties were preliminarily protected with two types of groups: trimethylsilyl and tert-butyl (Fig. 15).

Figure 15. Synthesis of undecylenic acid esters.

To compare the effect of the spacer nature, a synthetic route to an analog of benzoic acid bearing the C=C bond at the silicon atom, introduced for hydrosilylation, was devised (Fig. 16). The carboxyl group was protected with a trimethylsilyl moiety. The detailed description of the synthetic procedures is presented in Ref. [72].

Figure 16. Synthesis of the organosilicon analog of benzoic acid.

Then, the above-mentioned modifiers were introduced into the hydride-containing PDMSs by the hydrosilylation over Karsted's platinum catalyst (Figs. 17 and 18). The trimethylsilyl protecting groups were removed, while the tert-butyl moieties were left to study as references.

Figure 17. Synthesis of the teleсhelic PDMSs with carboxyl groups.

Figure 18. Synthesis of the PDMSs with carboxyl groups distributed along the polymer chains.

Quite a complicated multistep approach for the synthesis of the cc-PDMSs bearing the phenylene spacer is justified by the fact that in recent years the unique multifunctional models have appeared which contain a considerable amount of the carboxyphenyl groups at the silicon atom [69, 73]. To study their properties, simple models are required, some examples of which are presented in Figs. 17 and 18.

The interaction of amino-substituted precursors with itaconic acid (the Michael reaction) was chosen as an alternative strategy for the production of the PDMSs bearing carboxyl groups. The synthesis of the carboxyl-containing derivatives starting from itaconic acid and alkylamines is widely used in the production of surfactants [74].

The telechelic aminosiloxanes were obtained by the ring-opening anionic polymerization of octamethylcyclotetrasiloxane (D4) using 1,3-bis(3-aminopropyl)tetramethyldisiloxane (APDS) as a chain-growth terminating agent and the catalytic amount of α,ω-bis(tetramethylammoniumoxy)polydimethyl-siloxane (TMAS) (Fig. 19). The conditions for the synthesis of a series of polymers with the molar masses ranging from 600 to 3500 are presented in Ref. [75]. It should be noted that the presence of aminopropyl groups in the composition of oligomers hampered GPC analysis of the products and assessment of their molar mass distribution. The resulting oligomeric aminosiloxanes were used to obtain telechelic 4-carboxypyrrolidone derivatives of PDMS via the interaction with itaconic acid in о-xylene in the presence of anhydrous magnesium sulfate as a dehydrating agent (Fig. 20).

Figure 19. Synthesis of the telechelic amino-containing PDMS.