Coupling agents with 2,4,6,8-tetramethylcyclotetrasiloxane core – synthesis and application in styrene–butadiene rubber production

Abstract

A facile one-pot procedure for the functionalization of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ to branched organosilicon compounds DSP₃ or DS₃P bearing various, but clearly defined, numbers of silyl groups with an affinity to silica (S) and organic groups (alkyl, alkenyl, perfluoroaryl, epoxy, ether and ester) capable of bonding with polymers (P) has been demonstrated. Sequential hydrosilylation of allyltrimethoxysilane in the first step and olefin or alkyne in the second (and reverse order of reagents) has been applied in the presence of Karstedt's catalyst, which is commercially available and commonly used in industry. The developed procedure permitted the synthesis of 13 new, branched organosilicon compounds DSP₃ and DS₃P with excellent yields and precise distribution of functional groups. All products were easily isolated by simple filtration followed by drying under reduced pressure and fully characterized by spectroscopic methods (¹H, ¹³C, ²⁹Si NMR, IR) and mass spectrometry (ESI or MALDI). The obtained compounds, because of the presence of two different functional groups with an affinity to inorganic filler and polymer matrix, are regarded as versatile silica modifiers, silane coupling agents for various applications, and starting materials in the preparation of polymer networks and vapor-deposited coatings. The vulcanizates prepared using cyclosiloxane-functionalized rubber showed enhanced rolling resistance and wet skid resistance, proving the compound's potential in producing novel “Green Tire” treads.

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Introduction

Over the years, silica and other inorganic fillers have become important reinforcing agents in the synthesis of organic–inorganic hybrid materials.^1,2 The main problem those siliceous fillers generate for industrial applications is related to their hydrophilic nature, which causes difficulties in processing with hydrophobic polymers. Silica particles easily adhere to each other through hydrogen bonding between silanol groups and create agglomerates. Therefore, modification of the silica surface is required to overcome filler–filler interactions and improve filler-polymer compatibility. For this purpose, silane coupling agents (SCAs), which are bifunctional organosilicon compounds, are used. They belong to the most frequently used silica modifiers.³ These compounds contain two functional end-groups capable of forming covalent bonds with both organic and inorganic substances. One of these groups is a silicon atom with easily hydrolyzable groups (typically alkoxy, acyloxy, or halogen atoms). The other constitutes an organic moiety with an affinity to polymers. Functionalization of the silica surface with SCAs greatly increases the hydrophobicity of the filler and facilitates its dispersion in polymers. Consequently, the introduction of a silica/silane system into polymer matrices improves the mechanical, dynamic, and heat resistance of filled resins and rubbers.⁴ On the other hand, SCAs are also commonly applied as adhesion promoters, which are used in the production of adhesives, coatings, dental fillings, films, paints, and inks. Other applications involve cross-linking action (polymer networks, wire insulations), catalyst immobilization, and binding of biomaterials. The global SCAs market is expected to grow from USD 1.2 billion in 2021 to USD 1.6 billion by 2026, indicating the huge applicability of these organosilanes in academia and various branches of industry.⁵ Accordingly, a constant demand for new molecules with interesting properties has been observed in recent years.^6,7

The catalytic hydrosilylation reaction, which refers to the addition of silicon hydrides across multiple bonds, is a fundamental transformation in the synthesis of organosilicon fine chemicals, silicones, and SCAs.⁸ Bifunctional organosilanes able to act as coupling agents can be obtained by the hydrosilylation of two different substrates with silane, which has more than one Si–H bond. Several methodologies for the synthesis of such reagents with various alkenes and alkynes using different Si–H donors, including 1,1,3,3-tetramethyldisiloxane (having two Si–H bonds)⁹ and silsesquioxanes (having eight Si–H bonds)^10,11 have recently been described. Herein, we would like to expand the library of new bifunctional silanes obtained by the functionalization of 2,4,6,8-tetramethylcyclo-tetrasiloxane (DH₄) (bearing four Si–H bonds) with two different unsaturated substrates: allyltrimethoxysilane and another olefin or alkyne. Such molecules have potential applications as silica modifiers, SCAs or substrates in the synthesis of polymer materials.

DH₄ is an example of a cyclic siloxane. This class of compounds is described by the general formula (SiR₂O)_n. They might be regarded as silicon analogs of crown ethers, since they form different-sized rings of alternating silicon and oxygen atoms, with various side chains (R = hydrogen atoms, alkyl, or vinyl groups) attached to silicon.¹² These compounds are characterized by flexibility, high strength of Si–O bonds, and high reactivity under specified conditions in miscellaneous organic reactions, i.e., hydrolysis,¹³ halogenation,¹⁴ dehydrogenative couplings,^15,16 silylative coupling,^17,18 alkylative cleavage,¹⁹ Piers–Rubinsztajn reaction,²⁰ ring-opening polymerizations,^21,22 and hydrosilylation.^23–39 8-Membered compounds such as DH₄, because of the possible modification of peripheral Si–H groups by a hydrosilylation reaction, have opened access to several functional materials. While cyclosiloxanes have been utilized as substrates in the synthesis of dendritic molecules,⁴⁰ films^41,42 and coatings,^30,43 sensors,^44,45 liquid crystallines,^46,47 electrolytes,⁴⁸ ceramics,⁴⁹ and membranes,⁵⁰ there are no reports of their use as precursors of silica modifiers or coupling agents. The addition reactions of DH₄ to various olefins have been the subject of numerous reports. The most widely described group of products is DR₄ derivatives, which have four identical groups pendent from the cyclosiloxane core and are obtained by hydrosilylation between DH₄ and unsaturated compounds, including non-functional aliphatic olefins,^38,51 perfluoroalkenes,²³ cycloalkenes,³² alkenylbenzenes,^24,27,39 allyl alcohol derivatives,^28,34 allylamines,^35,37 alkenyl acid esters,^31,33 vinylsilanes,^25,29,36 and alkynes.²⁶ Hydrosilylation reactions focused on the modification of DH₄ with two different substrates (olefin or alkyne) leading to products with different content of the groups DSP₃, DS₂P₂ or DS₃P are a more challenging task, definitely less frequently described in the scientific journals, and mainly disclosed in the patent literature.^52–58 Nevertheless, it must be noted that the distribution of both groups coming from two starting materials in the structures of synthesized products is often random. Moreover, detailed characterization of these compounds by NMR methods has not been provided in the majority of these reports. Therefore, it is impossible to elucidate the precise structure of bifunctional products and the distribution of both groups attached to the cyclosiloxane core. As a result, there has been no reliable method of the synthesis of 2,4,6,8-tetramethylcyclotetrasiloxane derivative with DRR′₃ structure reported in the literature.

Bearing in mind the above-quoted facts about the application and synthesis of modified 2,4,6,8-tetramethylcyclo-tetrasiloxane, as well as the poor selectivity and unclear synthetic protocols for providing bifunctional compounds, here we present a one-pot synthetic protocol for the preparation of a wide gamut of functional organocyclotetrasiloxanes by platinum-catalyzed hydrosilylation of olefins and alkynes. The developed strategy involves the modification of DH₄ in two steps: (i) reaction of DH₄ with olefin possessing alkoxysilyl groups that allow for binding with silica (S), leading to a mono-decorated silyl derivative DH₃R with three active Si–H bonds, which then in the second step (ii) react with alkenes or alkynes, which have groups capable of bonding with polymers (P). This pathway leads to derivatives with a DSP₃ structure. Another procedure using both reagents in the reverse order produced coupling agents with a DS₃P structure.

This work constitutes the first example of the synthesis of 2,4,6,8-tetramethylcyclotetrasiloxanes DSP₃ or DS₃P with a well-defined number of functional groups supported by comprehensive NMR, IR, and MS characterization of the synthesized compounds. Moreover, application tests of a selected compound in the production of “Green Tires” are presented.

Experimental

General procedure for the one-pot hydrosilylation of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ with olefin 2a–c and allyltrimethoxysilane 2d – synthesis of silane coupling agents 5a–5c

The syntheses of compounds were conducted in two steps in a one-pot mode.

Step I. Anhydrous toluene (2 mL) was placed in the oven-dried Schlenk vessel equipped with a magnetic stirrer. Then, 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ (6 equiv., 1.5 mmol, 370 μL), olefin 2a–c (0.25 equiv., 0.25 mmol), and Karstedt's catalyst solution (2.5 × 10⁻⁴ mmola Pt, 3 μL) were added sequentially. The reaction was carried out at 50 °C for 24 hours. The total conversion of olefin was confirmed by GC-MS analysis. Volatiles (the solvent and the excess of cyclic siloxane) were evaporated and the residue was dried under reduced pressure and used directly in step II.

Step II. The intermediates 4a/4b/4c were redissolved in a new portion of toluene (2 mL). Next, allyltrimethoxysilane 2d (3 equiv., 0.75 mmol, 130 μL), and Karstedt's catalyst solution (7.5 × 10⁻⁴ mmola Pt, 9 μL) were injected into the Schlenk vessel and the reaction was conducted at 100 °C for 24 hours. The post-reaction mixture was then passed through a plug of Cellite®. The final products were isolated after concentration on a rotatory evaporator and dried under reduced pressure.

Hydrosilylation of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ with allyltrimethoxysilane 2d and second olefin 2a–c, 2e–h – synthesis of silane coupling agents 5d–5j

The syntheses were carried out using an analogous procedure to that described in the previous section, but allyltrimethoxy-silane 2d was applied in step I (leading to intermediate 4d), and another olefin 2a–c, 2e–h (3 equiv.) in step II.

Hydrosilylation of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ with allyltrimethoxysilane 2d and alkyne 2i–k – synthesis of silane coupling agents 5k–5m

The syntheses were carried out using an analogous procedure to that described in the previous section, but alkyne 2i–k (3 equiv.) was used instead of olefin in step II, and both steps were conducted at 50 °C.

Results and discussion

The selective functionalization of DH₄ with two different substrates is challenging, due to the presence of four reactive Si–H bonds. Moreover, hydrosilylation of alkenes or alkynes with this reagent can lead to different constitutional isomers. Dehydrogenative silylation, olefin isomerization, or oligomerization are possible side reactions promoted by transition metal catalysts. Another problem constitutes the control of Si–H polyaddition to two different substrates, which may result in the formation of mixtures of products with different (very random) distributions of both functional groups (DR₂R′₂, DR₃R′, DRR′₃, and others with Si–H bonds that are not fully reacted). Taking into consideration the process's complexity, the proper selection of the catalytic system seems to be of major importance. Due to its excellent effectivity and high functional group tolerance, Pt-catalyzed hydrosilylation has been recognized as one of the most important reactions of homogeneous catalysis, allowing for the synthesis of a plethora of organosilanes and silicons. Therefore, based on reports in the literature and our previous experience^59–65 in the synthesis of functionalized silanes via hydrosilylation reactions, we selected Karstedt's catalyst [Pt₂(dvs)₃, platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane] for our research.

In the first stage of our studies, to prove the effectivity of Karstedt's catalyst, we carried out the reaction of DH₄ with an excess of 1-hexene 2a (5 equiv.), leading to DR₄ derivative 3. The reaction was conducted in anhydrous toluene in the presence of [Pt₂(dvs)₃] (1 × 10⁻³ mmol Pt/1 mmol of Si–H bond) at 100 °C for 24 hours (Table 1, entry 1). Tetrahexyl derivative (3) was isolated by filtration through a short plug of Cellite® and dried under reduced pressure, giving the product in excellent 97% yield. ¹H and ¹³C NMR analyses of the pure product confirmed the exclusive formation of the anti-Markovnikov product. On the other hand, the multiplicity of signals around 20 ppm on ²⁹Si NMR may imply that the product exists as a mixture of cis–trans isomers with different spatial arrangements of methyl and n-hexyl groups on silicon centers. Next, our efforts were focused on the introduction of only one aliphatic chain into the cyclosiloxane ring. Substrates with more than one Si–H bond suffer from the possible uncontrolled polyaddition of Si–H bonds to the unsaturated substrates, giving mixtures of products with a different number of hydrogens and alkyl groups attached to the silicon atoms. Therefore, the hydrosilylation of 1-hexene (2a) with an excess of 2,4,6,8-tetramethylcyclotetrasiloxane (DH₄) was conducted to verify the feasibility of incorporation of only one n-hexyl group into the cyclotetrasiloxane ring and to check the stability of targeted compounds with reactive Si–H functionalities. Indeed, applying 6 equiv. excess of DH₄ to 1-hexene (2a) resulted in the formation of the hydrosilylation product DH₃(n-hexyl) decorated with one n-hexyl chain 4a in 94% yield (Table 1, entry 2). In this case, milder conditions compared to the synthesis of the tetra-n-hexyl product 3 (lower temperature of 50 °C and lower catalyst concentration of 2.5 × 10⁻⁴ mmol Pt/1 mmol of Si–H bond) were applied. Moreover, we observed that 6-fold excess of DH₄ was required to prevent the formation of adducts with more than one Si–H bond reacted. Decreasing the volume of DH₄ resulted in a higher content of DH₂(n-hexyl)₂ in the post-reaction mixture. The excess of DH₄ was removed by fast and simple trap-to-trap distillation which allows its reuse in further tests. GC-MS and NMR techniques confirmed that distillate collected in a cold trap contained only DH₄, and other low boiling cyclosiloxane derivatives or products of DH₄ decomposition were not observed.

Table 1 Modification of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄via hydrosilylation of alkenes 2a–d in the presence of Pt₂(dvs)₃ – synthesis of DR₄ model compound 3 and SCAs precursors DH₃R4a–d

Entry	Product	R	Isolated yield of 3 or 4 ^b [%]
Reaction conditions: [DH4] : [2] : [Pt] = 6 : 1 : 2.5 × 10^−4, 50 °C, 24 h.a Reaction conditions: [DH4] : [2] : [Pt] = 1 : 5 : 1 × 10^−3, 100 °C, 24 h, anhydrous toluene, total conversions of reagents were confirmed by ^1H NMR; the selectivity was determined by GC-MS, and ^1H, ^13C, ^29Si NMR.b The products exist as a mixture of stereoisomers.c Yields based on the amount of alkenes.
1	3		97^a
2	4a		94^c
3	4b		92^c
4	4c		90^c
5	4d		93^c

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On the other hand, analyses of the isolated product 4a showed that only traces of by-product DH₂(n-hexyl)₂ with two aliphatic arms in its structure had been formed. An analogous procedure was followed for the reactions with 1,2-epoxy-5-hexene 2b, allylpentafluorobenzene 2c, and allyltrimethoxysilane 2d. Targeted products 4b–d were obtained in 92%, 90%, and 93% yields respectively. The compounds 4a–c were next subjected to hydrosilylation reactions with allyltrimethoxysilane 2d (3 equiv.), giving potential silica modifiers (5) equipped with one arm with affinity to polymer matrices and three pendent organosilicon moieties (DS₃P modifiers) capable of hydrolysis and condensation with OH-rich materials, e.g., silica and other siliceous fillers (Scheme 1).

Scheme 1 Synthesis of new organocyclosiloxanes DS₃P from 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ by one-pot hydrosilylation of alkenes 2a–c (I step) and allyltrimethoxysilane 2d (II step). ^aReaction conditions: [DH₄] : [2a–c] : [Pt] = 6 : 1 : 1 × 10⁻³, 50 °C, toluene, 24 h. ^bReaction conditions: [4] : [2d] : [Pt] = 1 : 3 : 3 × 10⁻³, 100 °C, toluene, 24 h. Conversion and structure of products were confirmed by ¹H, ¹³C, ²⁹Si NMR, and MS (MALDI or ESI). The isolated yields are presented in brackets. The products exist as a mixture of stereoisomers.

Products 5a–c were isolated with excellent yields (94%, 88%, and 90% respectively). Similar results were obtained when the syntheses were repeated in a one-pot procedure, without isolation of the intermediates 4a–c. Therefore, the next experiments were carried out according to this method (Scheme 2). After completion of the first step, volatiles were removed under reduced pressure, and then a new portion of the solvent, alkene 2a–c, 2e–h or alkyne 2i–k (3 equiv.) and the catalyst solution were added, followed by immediate placement of the reaction vessel in an oil bath heated to 50 °C. Using the described method, three new organocyclosiloxanes with one silylpropyl group and three alkyl 5a, alkyl epoxide 5b or perfluoroaryl 5c chains were synthesized. The desired products with the DS₃P structure were obtained with an excellent yield of 88–94% after filtration through a pad of Cellite and drying under reduced pressure.

Scheme 2 Synthesis of new organocyclosiloxanes DSP₃ from 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ by one-pot hydrosilylation of allyltrimethoxysilane 2d (I step) and alkenes 2a–c, 2e–h or alkynes 2i–k (II step). ^aReaction conditions: [DH₄] : [2] : [Pt] = 6 : 1 : 1 × 10⁻³, 50 °C, toluene, 24 h. ^bReaction conditions: [4] : [2] : [Pt] = 1 : 3 : 3 × 10⁻³, 100 °C (50 °C when alkynes were applied in the second step), toluene, 24 h. Conversion and structure of products were confirmed by ¹H, ¹³C, ²⁹Si NMR, and MS (MALDI or ESI). The isolated yields are presented in brackets. The products exist as a mixture of stereoisomers.

In the next step of our research, we focused on the synthesis of new silanes equipped with one organosilicon moiety, capable of modifying the surface of inorganic fillers, and three moieties reactive towards polymers (DSP₃ modifiers). For this purpose, we applied the reverse order of reactions. First, the reaction of DH₄ with allyltrimethoxysilane (2d) furnished the intermediate 4d, which after the evacuation of volatiles was used as the substrate in the second step (Table 1, entry 5). Based on our previous work on the hydrosilylation of various substrates with different silanes, we selected olefins 2a–c, 2e–h with groups with an affinity to polymers (alkenyl, epoxy, perfluoroaryl, ester), which might react with compound 4d in the presence of Karstedt's catalyst, giving access to the new molecules. Such compounds have potential as attractive additives or modifiers in materials chemistry and in various branches of industry. All hydrosilylation experiments between compound 4d and olefins 2a–c, 2e–h (3 equiv.) were carried out in anhydrous toluene at 100 °C for 24 h. The total conversion of reagents was confirmed by ¹H NMR spectroscopy. Moreover, the equimolar ratio of reagents permitted simplification of the separation procedure. Products were isolated using the aforementioned procedure, giving viscous oils with excellent yields (Scheme 2). The hydrosilylation of 1-hexene 2a furnished compound 5d in an almost quantitative yield (95%). Although this compound does not possess a chemical group with an affinity to polymers, it may be considered a hydrophobing agent. Further reactions conducted with 1,2-epoxy-5-hexene 2b and allyl glycidyl ether 2e led to the cyclotetrasiloxanes 5e and 5f, equipped with a reactive epoxy group in 94% and 86% yields, respectively. Silanes functionalized with oxirane rings are essential components of modern resins, coatings, adhesives, and other epoxy-containing hybrid organic–inorganic materials.^66,67 The hydrosilylation of allylpentafluorobenzene 2c with 4d resulted in the formation of derivative 5g with three perfluoroaryl moieties in 88% yield. Fluorosilanes are commonly known for their wide applicability in the preparation of superhydrophobic materials.^68,69 The developed strategy was also compatible with compounds that have ester moieties in their structures. The hydrosilylation of methyl 10-undecenoate 2f furnished long-chain functionalized cyclotetrasiloxane 5h in 93% yield. We also investigated the hydrosilylation of unsymmetrical allyl ethers derived from citronellol 2g, and geraniol 2h, which have one and two trisubstituted C=C bonds in their structures, respectively. The addition of Si–H bonds of intermediate 4d to these bio-based substrates proceeded chemoselectively on allyl endings, producing β-isomers exclusively. Competitive hydrosilylation of more substituted terpene multiple bonds was not observed. The products 5i and 5j, which have alkenyl functionalities, were obtained with excellent isolated yields (95%). The synthesis of new additives of high implementation potential from naturally occurring compounds rather than petrochemical resources complies with the Sustainable Goals highlighted by the United Nations. Coupling agents with alkenyl moieties have been reported as valuable fiber modifiers in thiol–ene click reactions,⁷⁰ and were also used in the synthesis of nanocomposites by copolymerization.⁷¹

Another approach which also led to cyclosiloxane modifiers fitted with unsaturated moieties was the hydrosilylation of alkynes 2i–k with compound 4d. All reactions were performed using analogical reaction conditions, previously optimized for olefins; however, alkynes seemed to be more reactive and 50 °C was sufficient to achieve the total conversion of reagents (100 °C was applied for olefins). Very good yields were observed for the three starting materials tested. Hydrosilylation of 3,3-dimethyl-1-butyne 2i with 4d led to regio- and stereoselective addition of Si–H across carbon–carbon multiple bonds with the predominant formation of (E)-isomer 5k (³J_H–H = 19.1 Hz) in 93% yield. On the other hand, the presence of a silicon atom in the alkyne structure impaired the selectivity of the reaction. Hydrosilylation of (triisopropylsilyl)acetylene 2j afforded product 5l bearing three alkenyl arms in 91% yield (mixture of geometric isomers). For internal alkenes, cis-hydrosilylation occurred, giving the product 5m in 87% yield, obtained by the reaction of compound 4d with 3 equiv. of 4-octyne 2k.

All organocyclosiloxanes obtained by the selective modification of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄ with two different substrates were synthesized with very good to excellent yields. They were fully characterized by spectroscopic methods (¹H, ¹³C, ²⁹Si NMR, IR) and mass spectrometry (ESI or MALDI). In particular, ²⁹Si NMR analysis is of crucial importance in the identification of such systems, as it permits fast differentiation between the possible products (Fig. 1). 2,4,6,8-Tetramethylcyclotetrasiloxane DH₄ displays one signal split into four resonance peaks around −32 ppm, while 2,4,6,8-tetrahexyl-2,4,6,8-tetramethylcyclotetrasiloxane 3 shows a signal shifted to ca. −20 ppm, which is also visible in the form of four peaks. On the other hand, cyclotetrasiloxane derivative decorated with one n-hexyl arm 4a is characterized by a signal at −16.6 ppm, representing the silicon atom substituted with two alkyl groups (Me and n-hex), and by a group of signals around −32 ppm, which correspond to three Si atoms with hydrogen and methyl groups bonded respectively (mixture of cis–trans stereoisomers). The ²⁹Si NMR spectrum of tetramethylcyclotetrasiloxane decorated with one propyltrimethoxy-silyl moiety 4d shows the same chemical shifts as the monohexyl-decorated compound. Additionally, it has a signal around −42 ppm, which comes from the trimethoxysilyl group. Ultimately, the final products with two types of silicon atoms in their structures 5d can be distinguished by the presence of the two signals at ca. −20 and −42 ppm, coming from silicon atoms in the cyclosiloxane ring and the aliphatic external chain respectively.

Fig. 1 ²⁹Si NMR analysis of products obtained by the modification of 2,4,6,8-tetramethylcyclotetrasiloxane DH₄via hydrosilylation reactions of 1-hexene 2a and allyltrimethoxysilane 2d.

The application of Karstedt's catalyst, which is simple, commercially available and of high industrial importance, allowed for the introduction of structurally different groups such as alkyl, alkenyl, epoxy, perfluoroaryl, ester and alkoxysilyl into the cyclosiloxane core. A series of new compounds have been synthesized for the first time (three compounds with the structure DS₃P, and 10 products with the structure DSP₃). The simple isolation procedure, which involves filtration of the post-reaction mixture through a short Cellite® plug and drying under reduced pressure is advantageous and reduces time-consuming purification steps. Moreover, the DH₄ used in excess in the first step of the protocol can easily be evaporated under vacuum, condensed in a cooling trap and reused in the next syntheses. The proper management of reagents is important for the possible application of the procedure on a larger scale. Due to the presence of two reactive groups, these newly synthesized silanes might be considered potential silica modifiers, SCAs for various applications, or additives in the synthesis of polymers or vapor-deposited coatings.

To check the application potential of the synthesized products, in the final stage of our research, a representative example (chosen with the Synthos S.A. company specialized in the production of rubbers), compound 5e, equipped with one organosilicon moiety (capable of forming stable bonds with silica) and three epoxy groups with an affinity to polymer was applied as an SCA in the synthesis of silica-filled styrene–butadiene rubber vulcanizates with the potential to produce “Green Tire” treads. Such tires are characterized by lower rolling resistance and enhanced wet grip compared to tires that only contain carbon black as a filler. Tires with low rolling resistance need less energy to keep rolling, which corresponds to decreased fuel consumption by mechanical vehicles and therefore contributes to the reduction of global emissions of CO₂ into the atmosphere. This is actually of a great importance due to the need to reduce the emission of greenhouse gases. On the other hand, the improvement in the dry handling and wet grip properties of car tire treads has a profound influence on driving comfort and safety. Tread parameters are highly dependent on the tire components and the coupling agents used in their production.

In this work, we decided to investigate the influence of the functionalization of Syntion 2150® rubber with cyclosiloxane SCA 5e on the properties of the final composites and compare them with unmodified commercially available rubber. Modified rubber was prepared via anionic polymerization using n-butyl-lithium as an initiator (the detailed procedure is described in ESI, section 4.1†). Rubber compounds were prepared based on a model recipe for a passenger car tire tread using Syntion 2150® and Synteca 44® rubbers, silica as the main filler, and various processing additives.⁷² Compounding of the rubbers was conducted in a Banbury-type internal mixer, while the final pass was completed on a lab-sized two-roll mill. Next, rubber compounds and vulcanizates were subjected to several tests to determine their dynamic and mechanical properties (the detailed rubber characterization, compound composition, mixing procedure, and analytical methods used to characterize the compounds and vulcanizates are presented in the ESI, sections 4.2.–4.4. and Tables S1–S5†).

After preparing the rubber compounds, the Payne effect was measured using a rotorless shear rheometer (please see ESI, section 4.5., Tables S6, S7 and Fig. S58†). The Payne effect is an important parameter which can be considered a measure of filler micro-dispersion, as it is attributed to strain-induced breakage and recovery of weak physical bonding between filler clusters. The results show that the Payne effect for rubber functionalized with 5e is lower than for unmodified rubber. The lower the value of the Payne effect, the better the micro-dispersion of the filler in the rubber compound, which usually corresponds to lower energy losses (lower values of the rolling resistance predictor). Next, rubber compounds were vulcanized at 170 °C (curing characteristics are presented in ESI, Table S8 and Fig. S59†) and mechanical and dynamic characterization of vulcanizates was carried out. Dynamic mechanical analysis (DMA) was performed with the aid of DMA 450 + Metravib with the attached 450 N load cell in shear. The loss factor (tan δ) and storage modulus (G′) for vulcanizates as a function of temperature are listed in ESI in Table S9† and depicted in Fig. S60.† The tire predictors were determined as follows: rolling resistance measured as tan δ at 60 °C, wet traction measured as tan δ at 0 °C, ice traction measured as tan δ at −10 °C and dry handling as G′ at 30 °C. Crucial parameters for the development of new “Green Tire” rubbers prepared with the use of the synthesized cyclosiloxane coupling agent have been depicted in the form of a triangle chart, including rolling resistance, dry handling, and wet grip predictors, while all measured parameters are presented as a radar chart in Fig. 2 (please see also ESI, Table S10†).

Fig. 2 Results of tests performed for compounds and vulcanizates prepared with the application of rubber functionalized with cyclosiloxane coupling agent 5e (in comparison with reference unmodified Syntion 2150® rubber.

It was observed that the rolling resistance of vulcanizates functionalized with 5e was improved by 6% compared to that of unmodified vulcanizates, showing the compound's potential in the preparation of fuel-saving tire treads. Moreover, the wet grip and ice traction predictors for the modified rubber turned out to be enhanced by 7% and 15% respectively, which would have a positive impact on driving safety. Vulcanizates modified with 5e were also characterized by improved filler dispersion and rebound at room temperature (by 11% for both parameters). Furthermore, the rubber's mechanical properties, i.e., hardness, tensile strength, and abrasion resistance, were comparable to those of the unmodified rubber.

The improvement of rolling resistance and wet skid properties (wet grip and ice traction predictors) of vulcanizates prepared with the rubbers functionalized with a newly synthesized cyclosiloxane coupling agent confirmed the possibility of its application in the production of “Green Tire” treads. Reduced rolling resistance causes lower fuel consumption since less energy is being used as the tires roll along the road and therefore has a positive impact on the environment by reducing a vehicle's carbon footprint. On the other hand, wet grip properties are crucial parameters which influence road traction, directional control, and deceleration on wet and icy surfaces. Enhanced wet grip predictors indicate a shorter braking distance on slippery roads, which would significantly improve driving safety.

Currently, we are working on the synthesis of new compounds intended to improve the wear resistance of rubbers. The results of this investigation will be the subject of another article in a more specialized journal.

Conclusions

In conclusion, we have developed a one-pot procedure for the synthesis of new organosilanes by selective modification of 2,4,6,8-tetramethylcyclotetrasiloxane via the subsequent hydrosilylation reaction of two different substrates: allyltrimethoxysilane and another olefin or alkyne. The application of Karstedt's catalyst, which is highly effective and commercially available, permitted the incorporation of a wide spectrum of functional groups, including alkenyl, perfluoroaryl, ether, ester, and epoxy moieties, into the cyclotetrasiloxane core, leading to 13 new products in very high, almost quantitative yields. All compounds were isolated by simple filtration through a thin pad of Cellite® and drying under reduced pressure, which makes the target products easily accessible. Hydrosilylation of alkenes proceeded in a regioselective manner, giving anti-Markovnikov isomers as major products. The addition of DH₃R intermediates to sterically hindered terminal alkyne led to the predominant formation of trans isomer, while hydrosilylation of silylacetylene gave a mixture of geometric isomers. All products were characterized by spectroscopic methods (¹H, ¹³C, ²⁹Si NMR, IR) and by mass spectrometry (ESI or MALDI).

Due to the presence of two groups of different chemical reactivity – readily hydrolyzable organosilicon moiety and organic functions with an affinity to polymers – the synthesized compounds might be considered novel, versatile silica modifiers, coupling agents, or substrates in the synthesis of polymer networks or vapor-deposited coatings, potentially leading to materials with improved physicochemical properties.

To prove its application potential, cyclosiloxane coupling agent 5e was applied in the functionalization of styrene–butadiene rubber (Syntion 2150®) via anionic polymerization. The obtained modified rubber was applied in the preparation of silica-reinforced vulcanizates intended for the production of “Green Tires”. The final composites exhibited improved rolling resistance and wet grip properties compared to unmodified rubber, showing the potential of this compound in the manufacture of novel tire treads for the car industry that could reduce fuel consumption and CO₂ emissions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financed by the National Science Centre in Poland (Grants No. UMO-2019/35/N/ST4/02594, UMO-2018/31/G/ST4/04012) and by the National Centre for Research and Development in Poland, Lider Programme No. LIDER/6/0017/L-9/17/NCBR/2018, and No. POWR.03.02.00-00-I026/16, co-financed by the EU through the European Social Fund under the Operational Program Knowledge Education Development. The authors acknowledge financial support from Synthos S.A.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00619k

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