INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 43

CARBOXYL-CONTAINING POLYDIMETHYLSILOXANES: SYNTHESIS AND PROPERTIES

Cite this: INEOS OPEN, V. V. Gorodov,*a,b S. A. Milenin,a N. V. Demchenko,a and A. M. Muzafarova,b 2020, 3 (2), 43–54 DOI: 10.32931/io2011r a Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia Received 28 April 2020, b Accepted 4 June 2020 Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia http://ineosopen.org

Abstract The present review highlights the synthetic approaches to carboxyl-containing polydimethylsiloxanes (PDMSs). The main properties of these copolymers are described. The potential of their application in different fields of science and technology is demonstrated. The promising routes for further development of this group of copolymers are suggested.

Key words: carboxyl-containing polymers, polydimethylsiloxanes, carboxylic acids.

Introduction 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 The development of modern technologies, in particular, the interactions owing to the formation of hydrogen bonds. The creation of new highly efficient ecologically friendly and variation of the content of carboxyl groups in a polymer can economically reasonable technological solutions implies the use afford elastomers, thermoplastic elastomers, polymers with of materials that possess a complex of unique properties (heat fiber-forming properties, or solid resins [1 12]. The carboxyl- and frost resistance, resistance to aging in the air and under the – functionalized polysiloxanes bearing spacers between a siloxane action of UV light, secondary processability, inertness to living framework and carboxyl groups shorter than or organisms, etc.). The last two decades have witnessed –СН2–СН2– CH OCH decompose, whereas those with the longer spacers considerable progress in the creation of new classes of smart – 2 2– are stable [13]. materials. They include the materials that exhibit shape memory Carboxyl-containing PDMSs can be divided into three large effect and the materials that are prone to self-healing of damages groups (Fig. 1): under certain conditions. These systems may extend the service (a) polymers bearing carboxyl groups in side chains, lives of the products from polymeric materials that are subjected (b) , -terminal (telechelic) polymers, to microcuttings and scratches, which would positively affect α ω (c) polymers bearing carboxyl groups both in side chains the ecological situation in the world. The polymeric materials and at , -positions. featuring shape memory effect are of particular interest for the α ω potential application in medicine (surgery, gynecology, rehabilitation of muscles), robotics (artificial muscles), and radioelectronics (fire detectors, relays). Taking this into account, we turned our attention to carboxyl-containing polydimethylsiloxanes. Our interest in these Figure 1. Carboxyl-containing polydimethylsiloxanes (A acid moiety). 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 Organosiloxane polymers bearing carboxyl groups are used the most promising polymer matrices for wide use. Secondly, in water-proof compositions for impregnation of natural and this is the diversity of synthetic approaches to carboxyl- synthetic fabrics [14, 15] since the siloxane backbone displays containing PDMSs and, meanwhile, the lack of their mass hydrophobic properties and the presence of a small amount of production. The latter suggests that, despite the diversity of polar groups can provide adhesion to the material. The reactive synthetic approaches, there is no versatile method for their carboxyl groups interact with metal oxides, resulting in core– production. Finally, the unique properties of carboxyl- shell structures, and are used for the preparation of stable containing silicones serve as a starting point for the production suspensions of magnetic fluids and magnetic elastomers [16, of new polymeric materials and devices that meet modern 17]. PDMSs with carboxyl groups are also utilized as surfactants requirements. Of note is the propensity of siloxanes [1, 2] and for the production of foams [11], in emulsion polymerization mixtures of carboxyl-containing siloxanes with amino- [18–25], in cosmetic compositions for skin and hair treatment containing analogs [3] to self-healing under certain conditions. [26], in compositions with low surface energies as antifouling Recent investigations established that the mixtures of carboxyl- coatings [27], and for creation of chelate polymers [28]. containing PDMSs with poly(ethylene glycol) diglycidyl ether Carboxyl-containing PDMSs partially or fully neutralized with

V. V. Gorodov et. al., INEOS OPEN, 2020, 3 (2), 43–54 INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 44 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]). Figure 2. Polydimethylcarbosiloxanes. 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 (a) 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 (b) 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 (c) 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], (d) resulting in the formation of the siloxane bond. The synthesis of organosilicon compounds bearing carboxylic acid residues was described by Sommer as early as Figure 3. Main synthetic approaches to the carboxyl-containing the 1950s [35, 36]. However, these were monomeric polydimethylsiloxanes. 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 ° and above, the structure formation occurs С Figure 4. Syntheses of 5-allyloxyisophthalic and 3-allyloxyglutaric with a transition from the liquid state to the rubbery one. In this acids. case, an increase in the temperature, the duration of polymer heating, and the content of carboxyl groups led to the growth of the synthesis of 5-allyloxyisophthalic acid were compared. the elastic modulus. The observed phenomenon was reversible: According to the first one, diethyl 5-hydroxyisophthalate was when the heated sample was kept at room temperature, it obtained at the first step; then, it was allylated and hydrolyzed to converted to the initial liquid state. The authors assumed that give the protected product in 95% yield. The ethyl protecting these changes in the polymer properties are associated with the groups were removed by hydrolysis in hydrochloric acid. The changes in the bonding type (transformation of intramolecular second method implied the preliminary synthesis of a potassium hydrogen bonds into intermolecular ones) or with the formation salt which was reacted with allyl bromide in the presence of of clusters at physical network cross-link points. benzyltributylammonium bromide. The yield of the product in The synthetic approaches to carboxyl-containing this case composed 67%. Then, it was hydrolyzed by the first polydimethylsiloxanes can be divided into four groups: (а) method. In the third protocol, 5-hydroxyisophthalic acid and copolymerization of cyclosiloxanes, (b) hydrolysis of potassium hydroxide were dissolved in a water–ethanol mixture; chlorosilanes and silanols, (c) hydrosilylation or hydrothiolation then, allyl bromide was added, and the resulting mixture was of different functional groups with polydimethylsiloxanes, (d) refluxed for 24 h. The yield of the product in this case was 34%. miscellaneous methods, in particular, those based on the To obtain 3-allyloxyglutaric acid, starting diethyl 3- addition of a carboxyl-containing moiety to the functional group hydroxyglutarate was allylated with allyl bromide in the already introduced in PDMS (Fig. 3). presence of copper and DMF. The yield of the product Kawakami et al. [41] showed that aliphatic and aromatic composed 56%. Then, it was hydrolyzed to acid. dicarboxylic acids can be introduced into polydimethylsiloxanes At the final step, both allyl-containing acids were protected bearing terminal dimethylhydride siloxane units via with trimethylsilyl groups using hexamethyldisilazane; then, hydrosilylation. The starting narrow-dispersed hydride- PDMS was added across the vinyl groups in the presence of a containing PDMSs were obtained by the living anionic platinum catalyst followed by the deprotection (Fig. 5). Thus, a polymerization of hexamethylcyclotrisiloxane, which was series of copolymers with the molar masses ranging from 2000 terminated by introducing dimethylchlorosilane. The modifying to 9000 were synthesized. The yields at the hydrosilylation step agents were prepared from 5-hydroxyisophthalic acid and composed 40–78% and 74–95% in the case of aryl- and diethyl 3-hydroxyglutarate (Fig. 4). Three different methods for alkyldicarboxyl derivatives, respectively.

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Figure 5. Hydrosilylation of 5-allyloxyisophthalic and 3- Figure 6. Synthesis of the carboxyl-containing PDMS from methacrylic allyloxyglutaric acids. acid esters. Cheng et al. [42] used hydrosilylation of methyl and tert- product was converted to an ionomer under the action of zinc butyl methacrylates to obtain α,ω-terminal (telechelic) PDMSs. chloride and, thus, afforded a hard thermoplastic polymer, which Then, the ester groups were removed upon alkaline (in the case tensile strength was about 9 MPa. The polymer exhibited self- of methyl ester) or acid (in the case of tert-butyl ester) healing properties during annealing of two contacting halves hydrolysis. The latter changed the molar-mass characteristics of already at 40 °С, whereas annealing at 80 °С led to almost the final polymer relative to the initial one. Hence, the polymers complete healing and more than 90% strength recovery relative with the molar masses ranging from 1000 to 2000 and to the initial value. The possibilities of application of this polydispersity varying from 1.32 to 1.44 were obtained. The material in 3D printing and creation of conducting materials resulting telechelic carboxyl-containing PDMSs were introduced upon filling with graphene were also demonstrated. into a two-step reaction with diethylenetriamines and urea to In the second work [4], PDMS bearing methyl methacrylic produce new supramolecular elastomers [43–45]. According to acid residues at each silicon atom was partially cross-linked the results of DSC and X-ray diffraction studies, these using poly(ethylene glycol) diglycidyl ether (Fig. 7). The glass elastomers appeared to be completely amorphous. Investigations transition point of the resulting product was 0.5 °С. It was on their biocompatibility showed that the copolymers do not thermally stable up to 200 °С. possess cytotoxicity and do not provoke skin irritation; the films Furthermore, the films obtained from this material displayed based on these elastomers were found to exhibit self-healing self-healing and shape memory effects. The self-healing properties as well as wound-healing ability at the level properties were manifested already at room temperature. The comparable to that of the commercially available materials. sample obtained over 20 min of contacting of two material Knebelkamp et al. [46] synthesized PDMS bearing pieces at 25 °С can be extended up to 200% without fracture. methacrylic acid moieties distributed along the chain by Complete healing of the cut was observed in 6 h. Moreover, the hydrosilylation of tert-butyl methacrylate with material remembered its shape upon sample deformation at polydimethylsiloxane bearing methylhydride units (Fig. 6). The room temperature and subsequent cooling below a glass- authors suggest methanesulfonic acid (or p-toluenesulfonic acid) transition point and recovered the initial shape upon further as a catalyst for the deprotection. The degree of conversion of heating. The authors explained the shape memory mechanism by tert-butyl groups composed 98–100%. There are no data on the formation of hydrogen bonds between dimers of carboxyl whether the molar-mass distribution changes after the moieties upon cooling and their rupture upon heating, resulting deprotection, but the authors state that the experimental contents in the recovery of the initial shape. of the units with carboxyl groups coincide with the calculated Matisons and Provatas [48] introduced different amino acids ones. The molar masses of the polymers described in the bearing double bonds into a PDMS chain via hydrosilylation mentioned work did not exceed 2000 g/mol. The resulting (Fig. 8). To accomplish the hydrosilylation of the –С=С– bonds, polymers were used as emulsifiers for silicone and coconut oils the tert-butyl-protected amino acids were used. The protecting in water. In the case of coconut oil, a stable emulsion was groups were subsequently removed under the action of obtained that did not tend to immediate breaking. trifluoromethanesulfonic acid. Two polymers were used as the The Chinese research groups used hydrosilylation to starting hydride-containing PDMSs: poly(methylhydrosiloxane) introduce methyl methacrylate into PDMS bearing hydride units with the molar mass of 2000 and poly[(methylhydro)(di- in the chain using Karstedt's catalyst at 85 °С in toluene [4, 47]. methyl)siloxane] copolymer with the molar mass of 12000 and Starting commercially available polymethylhydrosiloxane had the content of hydride units of 13 mol %. Five amino acids were the molar mass of Mw = 1800–2100. The methyl protecting used as the second component: alanine, glycine, leucine, group was subsequently removed upon alkaline hydrolysis in phenylalanine, and valine. The stability of the THF in the presence of lithium hydroxide at 85 °С. polydimethylsiloxane in the presence of trifluoroacetic acid was In one of the performed investigations [47], the resulting confirmed using the following procedure. Dow Corning 200

Figure 7. Partial cross-linking of the carboxyl-containing PDMS with poly(ethylene glycol) diglycidyl ether.

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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 Figure 8. Synthesis of the PDMS with amino acid moieties. significant elongation. Carboxyl-containing PDMSs can also be obtained by the polydimethylsiloxane fluid (0.5 g) was dissolved in chloroform reaction of thiol–ene addition (hydrothiolation) which features a (10 mL); then, an excess of trifluoroacetic acid was added. The free radical mechanism (Fig. 9) [16, 53]. In the reported reaction mixture was stirred for four days. The target product examples, azobis(isobutyronitrile) was used as a radical initiator. was extracted with diethyl ether and rinsed with water. The The reactions afforded selectively the anti-Markovnikov resulting transparent oil was characterized by NMR products, namely, β-adducts. The molar masses of tri- and spectroscopy but its molar mass characteristics were not studied. hexafunctional polymers obtained by O'Вrien [16] ranged from The amino acid-functionalized polymers derived from the 2000 to 9000. The conversion of vinyl groups was over 98%. silylation appeared to be thermally unstable: the beginning of Subsequently, these polymers were used for the production of mass loss was observed already at 100 °С and 50% mass loss magnetic fluids with nanoscale magnetite particles. The tri- and was detected at about 200 °С. This result was anticipated since hexafunctional derivatives were shown to cover the same areas such behavior had already been described [49]. Although the on magnetite but the hexafunctional derivatives contained free introduction of urea moieties in PDMSs does not reduce their acid units, which resulted from incomplete grafting during the thermal stability and these polymers are stable up to 300 °С synthesis. Ścibiorek et al. [53] used block copolymers as vinyl- [50]. The polymers obtained from poly[(metylhydro)(di- containing PDMSs. These copolymers were obtained by the methyl)siloxane] were amorphous; the glass-transition point of living anionic polymerization of hexamethylcyclotrisiloxanes, the alanine derivative was –80 °С. vinylpentamethylcyclotrisiloxanes, and trivinyltrimethylcyclo- Horstman et al. [51] pursued a comprehensive study of trisiloxanes. The molar masses of the resulting copolymers were carboxyl-containing siloxane polymers and outlined a broad 17000, 8600, and 3600, and the contents of vinyl units were 50, spectrum of their potential application. Therefore, this study 25, and 30 mol %, respectively. After the thiolation, the 1Н merits detailed consideration. The carboxyl-containing PDMSs NMR did not contain the signals of the vinyl groups. The were synthesized by hydrosilylation of trimethylsilyl polymers were studied by DSC. The DSC curves showed a step undecenoate with polydimethylsiloxane bearing methylhydride in the thermal capacity at –123 °С which corresponds to the siloxane units. The main research object was the PDMS polymer glass-transition point and an endothermic peak at –40 °С, which featuring the molar mass of 15000 and the content of hydride can be attributed to glass-transition and melting of the PDMS units of 33 mol %. The carboxyl groups of the resulting phase in the block copolymer, respectively. A step in the copolymers were partially or fully neutralized with metal thermal capacity at –19 °С corresponded to the glass-transition acetylacetonates. The ionomers obtained represented of the carboxyl-containing block. Therefore, the resulting thermoplastic elastomers. The higher was the degree of carboxyl copolymers exhibited microphase separation. group neutralization, the wider was the rubbery plateau of these Guan and Dong [54] explored the possibility of synthesis of elastomers. According to the stress–strain nomograms, these β-carboxyethyl-substituted PDMSs from β-cyanoethyl PDMS ionomers are very close to the classical rubbers based on derivatives. The latter were obtained by the combined anionic PDMSs but, as well as thermoelastoplastics, offer an advantage polymerization of octamethylcyclotetrasiloxane with of processability above the yield point. For example, the authors cyanoethylheptamethylcyclotetrasiloxane in the presence of compared the high-molecular PDMS (M = 100000 g/mol) w tetramethylammonium siloxanolate. Screening the reaction bearing 1 mol % of carboxyl units neutralized with magnesium conditions for the complete conversion of cyanoethyl groups ions and the chemically cross-linked elastomer based on PDMS during acid hydrolysis, the authors managed to achieve only (Table 1). The resulting ionomers were suggested as gelling 26.7 mol % conversion. The molar masses of the resulting agents in cosmetic compositions and in mixtures with MQ resins polymers ranged from 6000 to 34000; the content of the for the production of thermoplastic adhesives. cyanoethyl groups varied from 2 to 9 mol %. 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

Table 1. Comparison of the PDMS elastomer and ionomer

Yield Appearance Modulus Strength, Material point, Strain at 25 °C G', Pa kPa °С PDMS transparent, 133000 150 350 180% ionomer hard elastic PDMS transparent, Figure 9. Synthesis of the carboxyl-containing PDMSs by 19000 – 350 180% elastomer hard elastic hydrothiolation.

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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 Figure 10. Synthesis of the carboxyl-containing PDMS from acidic conditions (Fig. 10). The authors noted that the degree of (cyanoethyl)dichloromethylsilane. conversion of cyanoethyl groups to carboxyl ones reached 98%.

The resulting polymer had the molar mass of Mn = 6300 and the polydispersity of 1.6. 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. Figure 11. Intermolecular interactions between poly(2- Cooling to room temperature led to a white rubbery residue. The carboxyethylmethylsiloxane) and PDMS. 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). The results of DSC analysis showed that poly(2- carboxyethylmethylsiloxane) has a melting point of 67 °С but, Figure 12. Synthesis of the carboxyl-containing PDMS by the after its mixing with conventional PDMS, the DSC curve did not condensation of chlorosilane with the oligomeric hydroxy-terminated reveal any thermal effect up to 217 °С; above this temperature, PDMS. the decomposition of the carboxyl moiety began. Resulting poly(2-carboxyethylmethylsiloxane) was used to obtained the molar masses of 12000 and 19000 and the contents of the membranes by cross-linking with ethylene glycol. The oxygen ester units of 1.8 and 0.6 mol %, respectively. According to the GPC analysis, the molar masses of the polymers obtained after permeability (O2/N2) of this membrane was compared to those of other PDMS derivatives (simple PDMS, PDMS copolymers deprotection were 33000 and 43000, respectively. This suggests with poly(2-carboxyethylmethylsiloxane) and methyl ester of that the deprotection was accompanied by chemical poly(2-carboxyethylmethylsiloxane)). Among all the samples modification of the polymers; therefore, this method for the explored, the best results were achieved just with poly(2- removal of protecting groups is controversial. The resulting carboxethylmethylsiloxane). copolymers represented viscous liquids prone to a transition to Due to the absence of any groups capable of interacting in the rubbery state upon heating above 70 °С. During prolonged PDMSs, they usually do not mix with polymers of other natures. storage at room temperature, the polymers again converted to The introduction of polar groups improves their mixing ability the state of viscous flow. The authors explained this by the [61, 62]. Li et al. studied a mixture of carboxyethyl-PDMS with ordering of carboxyl groups, resulting in the formation of polar poly(vinylpyridine) (PVP) [63] and later with poly(1- domains at elevated temperatures, which led, in turn, to physical vinylimidazole) (PVI) [64]. The unmodified PDMSs did not mix cross-linking. The moisture strongly affects the decomposition with PVI and PVP, but the introduction of 23 mol % and more of these physical interactions: the higher its content is, the faster carboxyl units enabled the mixing of the resulting polymers with the polymer returns to the initial state. The addition of zinc PVI and PVP in any ratios. Most of the mixtures with PVI acetylacetonate to these polymers gave rise to an elastomer exhibited positive deviations of glass-transition points. Neither network even at room temperature. mixture featured the lower critical solution temperature. X-Ray Subsequently, the synthetic scheme was modified [10]. At photoelectron and IR spectroscopic studies showed that the the first step, dichlorosilane with a tert-butyl propanoate hydrogen bonds in these mixtures were formed between the substituent was obtained. Then, it was introduced into carboxyl groups of PDMS and the nitrogen atoms. condensation with α,ω-hydroxy-PDMS. At the final step, the A series of works [9, 10, 65, 66] were devoted to the protecting groups were removed in the presence of a catalytic preparation of polydimethylsiloxanes bearing carboxyl groups in amount of trifluoromethanesulfonic acid at 120 °C. The polymer chains, which were used to obtain ionomers via application of trifluoromethanesulfonic acid in the amount of 2– interaction with metal salts. At the first step, dichlorosilane with 3 μM per 10 g of the polymer allowed the authors to avoid the a tert-butyl butyrate substituent was synthesized [9]. Then, it evacuation procedure and reduce the processing temperature to was introduced into condensation with oligomeric α,ω-hydroxy- 120 °С. It is stated that the catalyst at these concentrations does PDMS. At the final step, the protecting tert-butyl groups were not affect the siloxane chain and acts only on the tert-butyl removed by heating to 210 °C under vacuum (Fig. 12) [9]. This groups. The molar masses of the resulting polymers ranged from afforded the polymers with tert-butyl butyrate residues featuring 9000 to 80000 and the contents of carboxyl groups varied from

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0.27 to 1.23 mol %. During heating, the viscosity of these under acidic or alkaline conditions is a more promising way. carboxyl-containing PDMSs firstly reduces and then increases, The hydrothiolation and hydrosilylation are also very attractive which leads to gelling. The higher is the processing temperature, methods because they proceed through the addition reactions the faster is the rearrangement of hydrogen bonds from intra- to without the release of side products, and the vinyl- and hydride- intermolecular. containing PDMSs are commercially available. This means that Berger and Fost [11] suggested an interesting method for the the processes can be carried out without a solvent and, after production of PDMS with carboxyl groups using itaconic acid completion, do not require additional purification and isolation which is involved in the industrial production of carboxylate procedures. The same reason prompts an interest to the synthesis rubbers. The synthesis was accomplished by the interaction of by the Michael reaction from available itaconic acid and amino-containing polydimethylsiloxanes with itaconic acid, aminosiloxanes. which resulted in the formation of a pyrrolidone ring by the The challenges that we formulated starting investigations in Michael reaction. The method holds great promise in the future this field were simultaneously simple and very complex. We since it does not require the use of a catalyst, does not lead to the aimed, first of all, at entering this field, having harnessed the release of side products, and, what is more important, affords a most promising routes for the synthesis of carboxyl-containing polar group that provides strong intermolecular interaction. polydimethylsiloxanes and adopted them for an emerging area Exploring the effect of hydrogen bonds in siloxane systems, of green silicone technologies and new applications. After a Xing et al. [67, 68] suggested the method for producing literature survey, this part of the work appeared to be not so carboxyl-containing PDMSs by the interaction of amino- complicated. Furthermore, it was important to understand why containing PDMS with succinic aldehyde (Fig. 13). the unique properties of the carboxyl-containing This method afforded three polymers with the molar masses polydimethylsiloxanes did not find adequate practical of 2000, 4000, and 5500 g/mol. The authors noted that the application and widespread use. And the second challenge was introduction of terminal carboxyl groups does not affect the much more complicated since it implied full immersion into this glass-transition points of the resulting polymers compared to field and a search for bottlenecks within the whole carboxyl– PDMSs with terminal hydroxy groups featuring analogous siloxane project. Our program implied finding the solutions to molar masses. However, the introduction of these groups the following problems: strongly affected the rheological behavior of the polymers, – sequential synthesis of the carboxyl-containing which was reflected in a broadened rubber-like plateau in their polydimethylsiloxanes differing in the arrangement of structural rheological spectra. It was assumed that owing to the formation units bearing carboxyl groups (telechelic polymers, polymers of hydrogen bonds, the terminal groups facilitate the formation with carboxyl groups distributed along the chain, polymers of dimers and clusters, which leads to such unusual properties. containing different types of spacers); Another interesting approach is based on the oxidation of – investigation of the rheological and thermal characteristics tolyl groups connected with the silicon atom [69]. The methyl of the resulting carboxyl-containing PDMSs (cc-PDMS), groups of the tolyl moieties were converted to the carboxyl ones assessment of the effect of the type of a spacer and the upon oxidation with oxygen under mild conditions (at 40–60 °С concentration of carboxyl groups in the samples under and the ambient pressure), resulting in the target carboxyl- investigation; containing derivatives in 80–96% yields. The suggested – systematization of the synthesized cc-PDMSs, selection of catalytic system allows one to retain the siloxane bond intact. the promising fields of their practical application, and The investigations were performed mainly with short siloxane construction of the corresponding carboxyl-containing structures; however, this approach has much room for the polysiloxanes. application in the synthesis of siloxane polymers provided that To solve the formulated problems, we synthesized, at the an appropriate catalytic system and solvents are selected. first step, hydride-containing oligomers with variable In general, researchers have been expending considerable arrangement and concentrations of silyl hydride groups. Their efforts on the synthesis and application of the carboxyl- syntheses were carried by the cationic polymerization combined containing polydimethylsiloxane polymers for more than 50 with catalytic rearrangement (Fig. 14). The low-molecular years, which evidences the high applied potential of these products were removed by distillation under reduced pressure. polymers. The detailed description of the synthetic procedures is published The most promising method seems to be the hydrolysis of β- elsewhere [70]. cyanoethyl groups since it utilizes available precursors and The performed experiments afforded two series of hydride- allows one to achieve the maximum content of carboxyl groups containing PDMSs with hydride units either statistically in a polymer calculated per a methylsiloxane unit. Of course, the distributed along the macromolecule chain or located only at its method requires further development, especially in a sequence ends (telechelic polymers). The molar masses of the latter of chemical steps. Presumably, the hydrolysis of a cyanoethyl ranged from 2500 to 9300 g/mol. In the case of the statistical group in the monomer followed by hydrolytic polycondensation polymers, the molar masses varied from 3000 to 24000 g/mol

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

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appeared which contain a considerable amount of the carboxyphenyl groups at the silicon atom [69, 73]. To study (1) their properties, simple models are required, some examples of which are presented in Figs. 17 and 18. The interaction of amino-substituted precursors with (2) 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 Figure 14. Cationic polymerization combined with catalytic starting from itaconic acid and alkylamines is widely used in the rearrangement. production of surfactants [74]. with various contents of hydride units. The telechelic aminosiloxanes were obtained by the ring- We chose for modification commercially available opening anionic polymerization of octamethylcyclotetrasiloxane undecylenic acid which is derived from natural raw materials. (D4) using 1,3-bis(3-aminopropyl)tetramethyldisiloxane Undecylenic acid is the only one ω-unsaturated carboxylic acid (APDS) as a chain-growth terminating agent and the catalytic that is produced on an industrial scale [71]. To perform amount of α,ω-bis(tetramethylammoniumoxy)polydimethyl- hydrosilylation of the С=С bond, the carboxyl moieties were siloxane (TMAS) (Fig. 19). The conditions for the synthesis of a preliminarily protected with two types of groups: trimethylsilyl and tert-butyl (Fig. 15). 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 15. Synthesis of undecylenic acid esters. 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. Quite a complicated multistep approach for the synthesis of the cc-PDMSs bearing the phenylene spacer is justified by the Figure 16. Synthesis of the organosilicon analog of benzoic acid. fact that in recent years the unique multifunctional models have

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

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

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Figure 19. Synthesis of the telechelic amino-containing PDMS. series of polymers with the molar masses ranging from 600 to Residue mass, mas.% 3500 are presented in Ref. [75]. It should be noted that the 100 presence of aminopropyl groups in the composition of oligomers hampered GPC analysis of the products and assessment of their 75 molar mass distribution. The resulting oligomeric 2 aminosiloxanes were used to obtain telechelic 4- 50 3 1 carboxypyrrolidone derivatives of PDMS via the interaction 25 with itaconic acid in о-xylene in the presence of anhydrous 4 magnesium sulfate as a dehydrating agent (Fig. 20). 0 One of the main characteristics of polydimethylsiloxane 200 400 600 о polymers is their thermal properties; therefore, DSC and TGA Т, С studies of the resulting polymers were the primary goals. The a effect of the introduced groups, which assignment was to strengthen the intermolecular interaction, on the behavior of the 5 macromolecules at high and low temperatures was a key factor 4 for their further use. Of particular interest was a comparative study of the polymers featuring different types of modifying 3 groups, different contents and arrangement of these groups 2 relative to the main chain. The investigations on the thermal behavior of the resulting 0,2 W/g 1 polymers showed that crystallization of PDMS is suppressed at -140 -120 -100 -80 -60 -40 -20 0 20 40 60 °C the length of a siloxane chain ≤ 33 when the tert-butyl ester b moieties occupy the telechelic positions and at the content of the Figure 21. (a) TGA curves of PDMSs bearing 2.3 (1, 2) or 8 mol % (3, ester groups > 0.9 mol % when they are distributed along the 4) of benzoic acid residues in the air (1, 3) or in an argon atmosphere (2, chain. The undecylenic acid moieties at the telechelic positions 4) at the heating rate of 10 °С/min. (b) DSC curves of PDMSs bearing suppress the crystallization of PDMS but are prone to the following concentrations of benzoic acid residues: 0.8 (1), 2.3 (2), 8 crystallization in a separate phase; the same moieties distributed (3), 19 (4), 36 (5) mol %. along the chain suppress the crystallization of PDMS at the content ≥ 0.9 mol % but crystallize in a separate phase at the content > 16 mol %. The introduction of the benzoic acid moieties into a PDMS chain suppresses its crystallization even at the content of only 0.8 mol %. The glass-transition point shifts to the positive temperatures with an increase in the content of carboxyl moieties, reaching the value of 28 °С at 36 mol %. The introduction of the benzoic acid moieties reduces the thermal and thermooxidative stability of PDMS. The higher is the content of these moieties, the stronger is the effect (Fig. 21). а Therefore, the fragments of benzoic acid suppress the crystallization of PDMS at the content ≥ 0.8 mol % at any position (at the chain ends or statistically distributed along the chain), but they don't crystallize in a separate phase. The investigation of PDMSs bearing carboxypyrrolidone moieties revealed that they can form liquid crystals (Fig. 22). It should be noted that the kinetics of the formation of a liquid crystalline phase depends on the thermal prehistory of the sample. If the sample was obtained from solution, it contains a

b

Figure 22. (a) Optical images at crossed polaroids for the sample with b the molar mass of 1300 at 64 °C; ( ) DSC curves during the first heating (1), second heating (3), and cooling (2) of the sample with the molar Figure 20. Reaction of aminosiloxanes with itaconic acid. mass of 1300 g/mol.

V. V. Gorodov et. al., INEOS OPEN, 2020, 3 (2), 43–54 INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 51 liquid crystalline phase; if the sample was heated above the E ,kJ/mol a melting point of the liquid crystalline phase, then its recovery 28 will take several months. The activation energy of viscous flow 26 for these polymers ranged from 61 to 37 kJ/mol depending on carboxyl the length of the polymer chain (Table 2). The DSC data showed 24 that 4-carboxypyrrolidone moieties at the ends of the polymer 22 chain suppress the crystallization of PDMS but crystallize in a 20 separate phase. The rheological properties are also important characteristics 18 ester of materials and define their potential. The next step in our 16 investigations on the properties of the resulting telechelic 1 2 3 4 5 6 polymers and copolymers bearing carboxylic acid and ester mol.% units was to explore their rheological properties. In this case, the a availability of several series of polymers with different parameters also facilitated obtaining informative and especially , Pa•s 80 2 interesting results directly associated with the strengthening 70 effect caused by the introduced substituents. 60 50 The rheological properties of the resulting polymers were 40 30 explored. The comparison of two homologous series of the 20 polymers bearing tert-butyl undecenoate and undecylenic acid 10 moieties revealed that both of the fragments enhance the 1 2 activation energy of viscous flow with an increase in the contents of these groups in the polymer (Fig. 23a). As the 1 viscous flow activation energy increases, the undecylenic acid moiety begins to make a greater contribution to the 50 100 150 200 strengthening of intermolecular interactions than the tert-butyl Time, min ester units both in the case of the telechelic polymers [70] and b the polymers with modifying agents distributed along the Figure 23. (a) Value of Ea as a function of the content of modifying polymer chain. units. (b) Change of ηef in the course of heating of the PDMS telechelic Particular attention was drawn to the structure formation at polymer with the molar mass of 5000 (1) and its analog with the molar elevated temperatures [76]. PDMSs with the undecylenic acid mass of 16000 (2) at 160 °C. moieties were studied upon their annealing at the temperatures above 80 °С during different time periods. The higher was the nifested by the tert-butyl undecenoate derivatives (Tables 2, 3). annealing temperature and the higher was the content of The deviations from the Newtonian character of flow are carboxyl units in the chain, the faster was the conversion of the manifested in the case of benzoic acid at 8 mol %, and in the polymer bearing carboxyl units in the chain to a gel. The reverse case of undecylenic acid—at 40 mol %. The optimal content of conversion of the polymer to a viscous liquid is a long process the modifying groups ranges from 0.8 to 8 mol %. The polymers that also depends on the presence of moisture in the air. In the bearing carboxyl groups with any type of spacers, distributed case of the telechelic polymers, no gelling was detected; instead along the polymer chain, are prone to reversible gelling at only an increase in the viscosity was observed (Fig. 23b). This elevated temperatures, while the telechelic arrangement leads to can be very useful for the application of analogous derivatives an increase in the viscosity with a temperature rise. as lubricants operating at elevated temperatures when the The comparison of different methods for the introduction of classical agents based on hydrocarbons reduce their viscosity. carboxyl groups into a polydimethylsiloxane structure leads to The investigation of the rheological properties of the the following general conclusions. The introduction of the telechelic polydimethylsiloxanes bearing pyrrolidone units with carboxyl groups is an efficient method for controlling carboxyl groups showed that the suggested modifying system is (strengthening) the level of intermolecular interactions in PDMS more efficient than those used in the production of PDMS oligomers and polymers. The glass-transition point and the homologs bearing tert-butyldecanoate, 10-carboxydecyl groups, activation energy of viscous flow reflected rather adequately the as well as the benzoic acid residues. The activation energies of differences between the sample types explored. The sensitivity viscous flow for the polymers with the carboxypyrrolidone units of these methods enables assessment of not only the are higher than those for the PDMS analogs with tert- concentration effects of the carboxyl-containing moieties but butyldecanoate, 10-carboxydecyl, and benzoic acid moieties, also the role of the spacer structural units. For the first time, we which evidences that the carboxypyrrolidone groups strengthen revealed the liquid crystalline ordering of telechelic polymers the intermolecular interaction of siloxane chains more efficiently with the carboxypyrrolidone moieties at the chain ends. This than the others. enables further expansion of the opportunities to control the Among a series of the polymers bearing modifying groups properties of telechelic cc-PDMSs owing to the selection of in the chain, the most efficient one for the strengthening of terminal groups prone to self-arrangement. Of particular intermolecular interactions appeared to be the system with the attention is the development of the effects of viscosity increase benzoic acid moieties, the lower activity was demonstrated by for the compositions of cc-PDMSs with the carboxyl-containing the undecylenic acid analogs, and the least efficiency was ma- fragments distributed along the polymer chain. The use of this

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Table 2. Comparison of the copolymers with various functional groups between a siloxane core and carboxyl groups. The goal was to at the telechelic positions create a core–shell structure to enhance the suspension stability Visco- of the magnetic compositions based on iron carbonyl in the Ea, sity at siloxane matrix (Fig. 24). Modifier Тg, °С Mn Mw/Mn kJ/mol 20 °С, (Pa∙s) carboxypyrrolidone 42.0 –123 2300 2.4 5.943 carboxypyrrolidone 37.0 –124 3800 1.9 6.254

benzoic acid 25.5 –123 2800 2.4 0.540 benzoic acid 26.6 –123 4600 2.6 1.127 Figure 24. Scheme of grafting of iron particles to carboxyl-containing tert-butyl PDMSs (R is any spacer: alkyl, aryl). 17.0 120 2900 2.1 0.088 undecenoate – tert-butyl 16.6 123 5200 2.3 0.165 The results of the energy-dispersive X-ray microanalysis undecenoate – confirmed the grafting of siloxanes on the surface of iron undecylenic acid 20.0 –124 2100 3.5 0.335 particles. However, the shell was not integral; it appeared to be undecylenic acid 20.5 –125 5200 3.5 1.118 insular. Nevertheless, the suggested approach is promising for Table 3. Comparison of the rheological properties of the copolymers the synthesis of magnetic particles with a controlled shell with modifying groups in the chains provided that the optimal conditions and components are selected. Content of Visco- Turning to the development of cc-PDMSs, let us outline the Ea, modifying sity at Modifier Тg, °С Mn Mw/Mn kJ/mol units, 20 °С, closest prospects that literally wait for researchers. First of all, mol % (Pa∙s) these include the modifiers for lubricating compositions. The benzoic acid a – –112 4300 8.0 – – future steps in this field are obvious: the production of PDMS benzoic acid 23.9 –122 2800 2.3 2.9 0.785 compositions bearing simultaneously decyl and carboxydecyl benzoic acid 20.0 –125 3500 0.8 2.3 0.267 substituents at the silicon atoms. In this case, the control of the undecylenic acid 27.0 –117 2400 6.5 2.8 2.035 lubricant viscosity will be combined with the improvement of undecylenic acid 20.8 –125 5700 2.5 2.1 0.284 the lubricating properties. Of note are also the derivatives undecylenic acid 18.5 –126 6400 0.9 1.6 0.179 bearing sulfide bridges in the organic framework of the silicon tert-butyl 19.,3 115 4800 7.7 2.4 0.213 atom. This, in turn, will allow one to use both hydride- undecenoate – tert-butyl containing and vinyl-containing siloxane polymer matrices. In 17.6 124 6100 1.7 1.7 0.150 undecenoate – both cases, the modifying agents can be obtained, as it was tert-butyl 15.7 –125 5400 0.9 1.5 0.056 already mentioned, by solvent-free technologies and without undecenoate concomitant side processes in full consistence with the green without a 15.0 123 4100 1.2 0.053 modifier [77] – – chemistry approaches. a Particular attention should be drawn to telechelic and Insoluble in toluene for determining by GPC; Mn of the precursor was given. monofunctional cc-PDMS structures. They can become the constructional elements of polymer compositions owing to the phenomenon in lubricating compositions is restricted by low interaction of carboxyl groups with each other. The presence of lubricating properties of PDMS—the basis. Therefore, new a large variety of the cc-PDMSs with carboxyl groups prospects of the use of cc-PDMS in lubricating compositions distributed along the polymer chain along with telechelic and will become obvious upon a combination of cc-PDMS with monofunctional beams allows one to expect a wide diversity of modified PDMS bearing long alkyl substituents in the molecule structural forms which can be obtained by combinatorial composition [78, 79]. methods. The potential of these approaches has been discussed Our studies did not encompass such a direction as the for a long time and mainly in the patent literature, but only now, modification of organic polymer compositions of cc-PDMS, with the efficient approaches to the cc-PDMSs in hands, we can although, as it was already mentioned, cc-PDMSs can readily proceed with the molecular construction in this area. mix with different polymer matrices. The cc-PDMS samples The data obtained on the self-organization of cc-PDMSs obtained by our research group are excellent examples of these bearing different spacer moieties can be used to strengthen the modifiers, which enable fine-tuning of a hydrophilic– effect of spacers for other siloxane polymer matrices bearing hydrophobic balance of the polymer compositions. We are going phenyl groups at the silicon atoms, such as to fill this gap in the nearest future because the interaction of cc- methylphenylsiloxanes or polyphenylsilsesquioxanes [81]. In PDMS with different surfaces plays a key role in a whole range these cases, the effect of the phenyl-containing spacers can of fields, for example, magnetic elastomers [80]. strongly increase. We have already mentioned that the phenyl- The investigations on the magnetic elastomers based on containing spacers have very serious potential owing to the polysiloxanes are of particular interest because they can be used appearance of new direct methods for the generation of carboxyl in medicine owing to the biological inertness of silicones. Thus, groups via selective oxidation of alkyl groups at the aromatic O'Brien [16] treated magnetite particles with the carboxyl- substituents [69, 73]. containing PDMSs to create magnetic fluids and use them in the We would like to conclude this highlight devoted to the treatment of retinal detachment. Iron carbonyl particles were most promising trends with a reminder about a vast area also processed analogously [17] with the carboxyl-containing connected with the conversion of cc-PDMSs to ionomeric polysiloxanes of various structures bearing different spacers forms. This field has great potential from many points of view

V. V. Gorodov et. al., INEOS OPEN, 2020, 3 (2), 43–54 INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 53 since the ionomeric concept provides cc-PDMSs with additional 10. A. Batra, C. Cohen, T. M. Duncan, Macromolecules, 2006, 39, degrees of freedom for the regulation of properties, construction 426–438. DOI: 10.1021/ma051418q of new structures, and production of new materials with the 11. US Patent 5596061, 1997. 12. US Patent 8633478, 2014. improved complex of properties. First of all, this refers to 13. Silicon Based Polymers: Advances in Synthesis and lubricating and sealing compositions, coatings and free films, Supramolecular Organization, F. Ganachaud, S. Boileau, B. and, what is more important, all this can be achieved within the Boury (Eds.), Springer, Dordrecht, 2008. DOI: 10.1007/978-1- green chemistry approaches. Certain results in this field 4020-8528-4 published to date had a sporadic character and, as it was already 14. US Patent 6379751, 2002. 15. US Patent 7250456, 2007. mentioned, did not change the game. However, after the 16. K. W. O'Brien, PhD Dissertation (Chem.), Blacksburg, 2003. completion of libraries of cc-PDMS oligomers of various http://hdl.handle.net/10919/28869 structures, the ionomeric systems would become a subject for 17. V. V. Gorodov, S. A. Kostrov, R. A. Kamyshinskii, E. Yu. the most careful research. Kramarenko, A. M. Muzafarov, Russ. Chem. Bull., 2018, 67, 1639–1647. DOI: 10.1007/s11172-018-2271-8 18. I. A. Gritskova, D. B. Adikanova, V. S. Papkov, N. I. Prokopov, Acknowledgements D. I. Shragin, S. A. Gusev, S. M. Levachev, E. V. Milushkova, A. A. Ezhova, A. D. Lukashevich, Polym. Sci., Ser. B, 2016, 58, This work was supported by the Russian Foundation for 163–167. DOI: 10.1134/S1560090416020019 Basic Research, project no. 18-03-00637, and the Ministry of 19. RU Patent 2611629, 2017. Science and Higher Education of the Russian Federation, grant 20. RU Patent 2610272, 2017. 21. I. A. Gritskova, A. A. Ezhova, A. E. Chalikh, S. M. Levachev, S. of the Government of the Russian Federation no. N. Chvalun, Russ. Chem. Bull., 2019, 68, 132–136. DOI: 14.W03.31.0018. 10.1007/s11172-019-2428-0 GPC and NMR analyses were made using the equipment of 22. I. A. Gritskova, V. G. Lakhtin, D. I. Shragin, A. A. Ezhova, I. B. the Collaborative Access Center "Сenter for Polymer Research" Sokolskaya, I. N. Krizhanovsky, P. A. Storozhenko, A. M. of ISPM RAS. Muzafarov, Russ. Chem. Bull., 2018, 67, 1908–1914. DOI: Rheological measurements were performed with the 10.1007/s11172-018-2306-1 23. I. A. Gritskova, A. A. Amelichev, O. A. Satskevich, A. V. financial support from the Ministry of Science and Higher Shkolnikov, A. A. Ezhova, G. A. Simakova, V. A. Vasnyov, N. A. Education of the Russian Federation at INEOS RAS. Lobanova, B. A. Izmaylov, Fine Chem. 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