Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties

New non-crystallizable low-dispersity star-shaped polydimethylsiloxanes (PDMS) containing stereoregular cis-tetra(organo)(dimethylsiloxy)cyclotetrasiloxanes containing methyl-, tolyl- and phenyl-substituents at silicon atoms and the mixture of four stereoisomers of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane as the cores were synthesized. Their thermal and viscous properties were studied. All synthesized compounds were characterized by a complex of physicochemical analysis methods: nuclear magnetic resonance (NMR), FT-IR spectroscopy, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), viscometry in solution, rheometry, and Langmuir trough study.


Introduction
Creation of polymers with new macromolecular architecture is one of the principal driving forces in the development of polymer science. The structure-properties relationship, which is the cornerstone of polymeric chemistry guarantees that polymer of unusual architecture would possess an unusual complex of properties [1,2]. Among a vast variety of macromolecular structures, a big class of branched high-molecular compounds is distinguished. Bright representatives of this class are the star-shaped polymers (SSP). These are branched macromolecules in which the arms (linear polymers) 'grow' from one branching center (core). So, an atom, a molecule or a macromolecule can act as the branching center. At the same time, it is supposed that the length of arms is identical [3]. An important parameter for such polymers is the number of arms, their functionality and molecular weight. The main feature of SSPs distinguishing them from linear analogs of identical molecular masses is their compact structure (smaller hydrodynamic volume, and, therefore, less viscosity) and possible wider functionality [4][5][6].
The study by DSC method was conducted on DSC-822e device (Mettler-Toledo, Greifensee, Switzerland) at 10 • C/min heating and cooling rates.
The study by TGA method was conducted on Derivatograph-C device, (MOM, Mateszalka, Hungary) in air and in argon at 10 • C/min heating rate.
Rheological studies were conducted on Anton Paar MCR 302 rheometer (Graz, Austria), in the mode of constant shear rate, plane-plane measuring mode, plane diameter 25 mm.
The reduced viscosity of diluted solutions of obtained polymers was measured with an Ubbelohde suspended level capillary viscometer in the concentrations range of 0.25-1 dl/g at 25 ± 0.05 • C.
Formation and study of Langmuir layer properties was carried out on Minitrough Extended (KSV, Espoo, Finland) with maximum area of the interphase surface equal to 558 cm 2 . Compression and expansion speed was 15 cm 2 min −1 . As a subphase, purified and demineralized water with a specific resistance of 18.2 MOhm cm (at 25 • C) thermostatically maintained at 20 • C with the use of Milli-Q (Millipore, Burlington, MA, USA) integrated water purification system was utilized. The studied star-shaped copolymers were dissolved in chloroform. Surface pressure was measured by Wilhelmy's method with the use of a rough platinum plate with 0.1 mN m −1 accuracy. Surface potential was measured by method of vibrating electrode (KSV, Espoo, Finland) with 1 mV accuracy. The Langmuir layers morphology directly on water surface was visualized by Brewster angle microscope BAM-300 (KSV, Espoo, Finland). The images obtained, corresponding to 200 × 200 µm 2 interface surface, were geometrically corrected taking into account Brewster angle of water (53.1 • ). Confidence intervals for the values obtained from surface pressure and surface potential isotherms are 0.1 mN m −1 for surface pressure, 30 Å 2 for the area per a molecule, 0.3 Å 2 for the area per unit of a dimethylsiloxane and 5 mV for surface potential.

Reaction of Cis-Tetra[phenyl(dimethylsiloxy)cyclotetrasiloxane Isomerization
Briefly, 0.3 g (0.4 mmol) of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane and 0.015 g (5 masses. %) of sulfocationic resin were loaded into a two-neck flask. The reaction in the ultrasonic bath continued for 4 h at 70 • C. To remove the sulfocationite, the reaction mass was dissolved in hexane and filtered through the paper filter. Briefly, 132 mL of hexane, 45.06 g (202.5 mmol) of D 3 and 18.1 mL of n-BuLi (28.9 mmol, 1.6 M solution in hexane) were loaded into a one-neck flask supplied with a magnetic stirrer. In 12 h, 75 mL of tetramethylene oxide (THF) was mixed into the system. In 6 h after adding THF, 7 g (57.9 mmol) of vinyldimethylchlorosilane was added dropwise. The reaction mass was filtered off from LiCl through the paper filter and solvents were removed to constant weight. The yield: 42.85 g (95%) of a viscous transparent liquid.

Cores Synthesis
Synthesis of cis-tetraorganocyclotetrasilsesquioxanes containing methyl-, tolyl-, and phenyl-groups was carried out according to Scheme 1. The respective organotrialkoxysilane was treated by equimolar quantity of sodium hydroxide or potassium hydroxide in presence of equimolar amount of water. n-Butanol was used as solvent in case of tolyl-and phenyl-substituent; in case of methyl-substituent, methanol and hexane mixture with 1/7 ratio was used. For further use of these cycles as SSP cores, they were treated by dimethylchlorosilane according to Scheme 1.
The isomerization of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane was carried out in mass in presence of sulfocationite (Amberlyst 15) at 70 °C for 4 h (Scheme 2). As a result, the mixture of all four isomers in equal quantities with 90% yield was formed. The respective organotrialkoxysilane was treated by equimolar quantity of sodium hydroxide or potassium hydroxide in presence of equimolar amount of water. n-Butanol was used as solvent in case of tolyl-and phenyl-substituent; in case of methyl-substituent, methanol and hexane mixture with 1/7 ratio was used. For further use of these cycles as SSP cores, they were treated by dimethylchlorosilane according to Scheme 1.
The isomerization of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane was carried out in mass in presence of sulfocationite (Amberlyst 15) at 70 • C for 4 h (Scheme 2). As a result, the mixture of all four isomers in equal quantities with 90% yield was formed. The kinetics of isomerization process was monitored by 1 H NMR method ( Figure 1).

Scheme 2. Scheme of cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane isomerization.
The kinetics of isomerization process was monitored by 1 H NMR method ( Figure 1). Reaction mass samples were taken after 1, 2, and 4 h from the beginning of reaction. According to the 1 H NMR spectroscopy data of initial compound, cis-tetra[phenyl (dimethylsiloxy)]cyclotetrasiloxane (a violet curve), in the field of 4.9 ppm we observe a signal that corresponds to SiH-group. As isomerization goes on, the emergence of new signals in the field of 4.5-5 ppm (turquoise and green curves), which correspond to SiH isomers groups is observed. The appearance of signals of equal intensity for all SiHgroups in the region of 4.5-5 ppm (burgundy curve) shows the moment when the reaction ends. The isomerization reaction is completely over (stopped) 4 h after the start. Reaction mass samples were taken after 1, 2, and 4 h from the beginning of reaction. According to the 1 H NMR spectroscopy data of initial compound, cis-tetra[phenyl (dimethylsiloxy)]cyclotetrasiloxane (a violet curve), in the field of 4.9 ppm we observe a signal that corresponds to SiH-group. As isomerization goes on, the emergence of new signals in the field of 4.5-5 ppm (turquoise and green curves), which correspond to SiH isomers groups is observed. The appearance of signals of equal intensity for all SiHgroups in the region of 4.5-5 ppm (burgundy curve) shows the moment when the reaction ends. The isomerization reaction is completely over (stopped) 4 h after the start.
Composition of mixture obtained and structure of isomers were identified by 1 H NMR-spectroscopy ( Figure 2).   According to GPC data, the hydrodynamic radius of isomers in the mixture and of initial cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane coincide ( Figure 3).
Composition of mixture obtained and structure of isomers were identified by 1 H NMR-spectroscopy ( Figure 2). According to GPC data, the hydrodynamic radius of isomers in the mixture and of initial cis-tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane coincide ( Figure 3).

Synthesis of Arm
The monofunctional PDMS-arm with n = 15 polymerization degree was synthesized by method of living anionic polymerization of hexamethylcyclotrisiloxane in presence of n-BuLi, with subsequent blocking by vinyldimethylchlorosilane (Scheme 3).

Synthesis of Arm
The monofunctional PDMS-arm with n = 15 polymerization degree was synthesized by method of living anionic polymerization of hexamethylcyclotrisiloxane in presence of n-BuLi, with subsequent blocking by vinyldimethylchlorosilane (Scheme 3). Scheme 3. Scheme of PDMS-arm synthesis.

Assembly of Star-Shaped Polydimethylsiloxanes
For SSP synthesis on the basis of various tetracyclic cores, the reaction of hydrosilylation was carried out in the presence of Karsted's catalyst in toluene (Scheme 4). The course of the reaction was monitored by 1 H NMR based on the disappearance of SiH signals in the initial organocyclosiloxane. Four SSPs with identical quantity and length of arms but with different cores were synthesized as a result. Molecular-mass characteristics of the polymers obtained are presented in Table 1. All polymers have narrow molecular-mass distribution.

Assembly of Star-Shaped Polydimethylsiloxanes
For SSP synthesis on the basis of various tetracyclic cores, the reaction of hydrosilylation was carried out in the presence of Karsted's catalyst in toluene (Scheme 4).

Synthesis of Arm
The monofunctional PDMS-arm with n = 15 polymerization degree was synthesized by method of living anionic polymerization of hexamethylcyclotrisiloxane in presence of n-BuLi, with subsequent blocking by vinyldimethylchlorosilane (Scheme 3). Scheme 3. Scheme of PDMS-arm synthesis.

Assembly of Star-Shaped Polydimethylsiloxanes
For SSP synthesis on the basis of various tetracyclic cores, the reaction of hydrosilylation was carried out in the presence of Karsted's catalyst in toluene (Scheme 4). The course of the reaction was monitored by 1 H NMR based on the disappearance of SiH signals in the initial organocyclosiloxane. Four SSPs with identical quantity and length of arms but with different cores were synthesized as a result. Molecular-mass characteristics of the polymers obtained are presented in Table 1. All polymers have narrow molecular-mass distribution. The course of the reaction was monitored by 1 H NMR based on the disappearance of SiH signals in the initial organocyclosiloxane. Four SSPs with identical quantity and length of arms but with different cores were synthesized as a result. Molecular-mass characteristics of the polymers obtained are presented in Table 1. All polymers have narrow molecular-mass distribution.

Thermal Properties
The synthesized SSP and the initial arm (PDMS-15) were studied by TGA and DSC methods.
In Thus, introduction of a cyclic fragment as the branching core suppresses the ability of polymeric chains of the target products to crystallize.
in organic substituent at Si atom and stereoregularity of the cyclic core, (Ph r 4-15 and Ph4-15 polymers) and initial PDMS-15 are presented. According to DSC data obtained, full suppression of PDMS-arm crystallization process is observed in all SSPs. Similarly, crystallization of side chains was not observed for SSPs with 21 PDMS units per arm [44]. Thus, introduction of a cyclic fragment as the branching core suppresses the ability of polymeric chains of the target products to crystallize.
Glass-transition temperatures for all SSPs are close and are within −124 to −122 °C that is characteristic of classical linear PDMS [51].

Rheological Properties
In Solution Intrinsic viscosity [η] depends on solvent quality, i.e., on its thermodynamic affinity to polymer. The macromolecular coil in various solvents swells differently. The "better" is the solvent the bigger is its size that, in turn, results in bigger hydrodynamic resistance to flow and [η] increase. Heptane is a "good" solvent for PDMS that is confirmed by value χ = 0.409 defined in [52]. In [53], the constants of Mark-Kuhn-Houwink equation for linear PDMS in heptane at 25 °C [η] = 1.207 × 10 −4 M 0.741 were found.
The measured values of intrinsic viscosity for SSP and calculated values of intrinsic viscosity of their linear analogs are presented in Table 3 and Figure 6.

Rheological Properties In Solution
Intrinsic viscosity [η] depends on solvent quality, i.e., on its thermodynamic affinity to polymer. The macromolecular coil in various solvents swells differently. The "better" is the solvent the bigger is its size that, in turn, results in bigger hydrodynamic resistance to flow and [η] increase. Heptane is a "good" solvent for PDMS that is confirmed by value χ = 0.409 defined in [52]. In [53], the constants of Mark-Kuhn-Houwink equation for linear PDMS in heptane at 25 • C [η] = 1.207 × 10 −4 M 0.741 were found.
The measured values of intrinsic viscosity for SSP and calculated values of intrinsic viscosity of their linear analogs are presented in Table 3 and Figure 6. In Figure 6, concentration dependencies of reduced viscosity of diluted solutions in heptane are presented. The values of intrinsic viscosity [η] defined at C → 0 are given in Table 3.  As presented data show (Table 3 and the Figure 6), all SSPs have small intrinsic viscosity values irrespective of macromolecule branching-out center structure that might evidence rather dense SSP macromolecule packing in solution.
The results of SSP rheological study in bulk are presented in Table 3. Low viscosity values in both solution and block are characteristic of practically all studied samples, and still viscous flow activation energy values "feel" the cyclic branching-out center. For all studied samples, viscous flow has high power consumption.
Flow curves of SSP and of their linear analog PMS-200 are presented in Figure 7. As Figure 7 shows, SSP viscosity practically does not depend on shift rate that indicates the Newtonian character of the flow. In Figure 6, concentration dependencies of reduced viscosity of diluted solutions in heptane are presented. The values of intrinsic viscosity [η] defined at C → 0 are given in Table 3.
As presented data show (Table 3 and the Figure 6), all SSPs have small intrinsic viscosity values irrespective of macromolecule branching-out center structure that might evidence rather dense SSP macromolecule packing in solution.
The results of SSP rheological study in bulk are presented in Table 3. Low viscosity values in both solution and block are characteristic of practically all studied samples, and still viscous flow activation energy values "feel" the cyclic branching-out center. For all studied samples, viscous flow has high power consumption.
Flow curves of SSP and of their linear analog PMS-200 are presented in Figure 7. As Figure 7 shows, SSP viscosity practically does not depend on shift rate that indicates the Newtonian character of the flow.
For linear polymer, the Newtonian flow begins at shear rates over 100 s −1 . At smaller shift rates, viscosity decreases as the shift rate grows that is related to gradual orientation of polymer macromolecules in the shear field. SSP viscosity in the whole interval of shear rates is less than for linear polymer, and Me 4 -15 has the least viscosity among SSPs.
Flow curves of Ph 4 -15 at various temperatures are presented in Figure 8. Similar dependencies were also obtained for other SSPs.
The viscosity temperature dependencies in Arrhenius equation coordinates (Figure 9) were plotted. For linear polymer, the Newtonian flow begins at shear rates over 100 s −1 . At smaller shift rates, viscosity decreases as the shift rate grows that is related to gradual orientation of polymer macromolecules in the shear field. SSP viscosity in the whole interval of shear rates is less than for linear polymer, and Me4-15 has the least viscosity among SSPs.
Flow curves of Ph4-15 at various temperatures are presented in Figure 8. Similar dependencies were also obtained for other SSPs. The viscosity temperature dependencies in Arrhenius equation coordinates ( Figure  9) were plotted. For linear polymer, the Newtonian flow begins at shear rates over 100 s −1 . At smaller shift rates, viscosity decreases as the shift rate grows that is related to gradual orientation of polymer macromolecules in the shear field. SSP viscosity in the whole interval of shear rates is less than for linear polymer, and Me4-15 has the least viscosity among SSPs.
Flow curves of Ph4-15 at various temperatures are presented in Figure 8. Similar dependencies were also obtained for other SSPs. The viscosity temperature dependencies in Arrhenius equation coordinates ( Figure  9) were plotted. They have linear shape that allows to calculate viscous flow activation energy (Еа) for all SSPs.
Based upon the data obtained, it can be assumed that the macromolecule coil size in They have linear shape that allows to calculate viscous flow activation energy (E a ) for all SSPs.
Based upon the data obtained, it can be assumed that the macromolecule coil size in bulk is smaller than for the polydimethylsiloxane analog. On the other hand, as PDMSarms have much smaller length, they orient quicker in the shift field that is confirmed by the Newtonian character of their flow at all shear rates.

Langmuir Layers
Amphiphilic siloxane SSPs form Langmuir monolayers at the air-water interface after spreading of the solutions and evaporation of the solvent. The macromolecules contain identical hydrophilic fragments and differ by hydrophobic substituents. Hydrophilic covalent-ionic Si-O bonds are directed into water subphase and form hydrogen bonds with water molecules. Hydrophobic butyl-, methyl-, phenyl-, or tolyl-groups are directed into air phase. SSPs studied in this paper differ from their analogs with bigger number of arms [44]. They have strictly four arms of 15 Si-O units instead of 21. Their cores contain both similar [44] phenyl-groups, and methyl-or tolyl-groups. Such SSP selection allows the discovery of the following: (1) whether SSP behavior in Langmuir monolayers is influenced by the type of hydrophobic groups in the core (among Dependencies of surface pressure (π) and surface potential (∆U) on the area of interphase surface per molecule (A) for Me 4 -15 are given in Figure 10, for Ph r 4 -15, Ph 4 -15, and Tol 4 -15 in Figure 11. A-D points on curve 1 in Figure 10 correspond to traditional [54][55][56] designation of π-A isotherm characteristic points of PDMS. Values of surface pressure, surface areas both per molecule and per repeating dimethylsiloxane unit in points A-D, and surface potential ranges (defined similarly as shown for curve 1 in Figure 10) are summarized in Table 4. Calculation of changes in surface areas per repeating dimethylsiloxane unit allows SSPs to be compared with each other, with PDMS, and with SSPs [44]. Brewster angle microscopy images for SSP Langmuir layers are shown in Figure 12.         Me 4 -15 contains butyl groups at the ends of arms as well as methyl groups as substituents at Si atoms similarly to PDMS. The shape of π-A isotherm and the range of surface pressure change between the points C and D (0.5 mN m −1 ) for Me 4 -15 are similar to the ones observed for linear PDMS. In Brewster angle microscopy images in the area of zero surface pressure (Figure 12a), the border of darker water surface, and lighter Me 4 -15 monolayer, is visible. At the same time, prior to surface pressure rise, the surface potential isotherm exhibits a jump from zero to positive ∆U values in a narrow interval of surface area change (curve 3 in Figure 10). On compression of a monolayer in the A-B region of π-A isotherm (curve 1 in Figure 10), the water surface is completely covered with a monolayer (Figure 12b); the conformational transition in siloxane arms in the B-C region of the π-A isotherm does not change Langmuir monolayer surface morphology (Figure 12c). Two hypotheses [57] of conformational transformations of siloxane chain in the B-C region of π-A isotherm of PDMS are known: the formation of horizontal folds from odd quantity of chain segments, or a helix with six dimethylsiloxane units per one turn (analog of Damaschun helicoid at crystallization). However, the surface pressure values in Me 4 -15 collapse point are lower than those typical for PDMS [57,58]. It can result from both Me 4 -15 rather low molecular mass as well as formation of less stable layer due to formation of helices consisting of only two coil turns assumed by each SSP arm, or steric difficulties caused by attachment of one of PDMS-arms end to the central cycle.
After the final collapse of Langmuir layer in point D (Figure 12d), Me 4 -15 excess accumulates into lenses, similar to [44]. Brewster angle microscopy image shows bright domains on a dark surface. In the range A-D, surface potential fluctuates irrespective of conformational transformations of the siloxane chain and Langmuir film collapse (curve 3 Figure 10).
An increase in size of organic substituents attached to the core, in Ph 4 -15, Ph r 4 -15, and Tol 4 -15 results in surface pressure decrease in points B and C of π-A isotherm together with its increase in point D in comparison with Me 4 -15 (Figures 10 and 11). At the same time, the surface-pressure jump between points C and D increases to 1.  Figures 10 and 11) are similar to those obtained under compression. The hysteresis in compression-expansion cycle is observed in the whole range of surface areas. After Langmuir layers expansion, the excess of polymers collapsed in lenses spreads in a monolayer with conformational transformations of helix or folded chains into straightened ones. The π-A and ∆U-A isotherm shapes, as well as the change in Langmuir layers surface morphology in compression-expansion cycle, indicate their liquid aggregate state.

Conclusions
The isomerization of cis-tetra[(phenyl)(dimethylsiloxane)]cyclotetrasiloxane was carried out for the first time and the mix of all four isomers in equal quantities was obtained with 90% yield. Four new narrow-dispersity non-crystallizable star-shaped polydimethylsiloxanes were synthesized. Their molecules contain identical number of arms of identical length, but have differences in the branching-out center. Cis-tetratolyl-, cis-tetraphenyl-, and cis-tetramethylsilsesquioxane cycles serve as the branching-out center in three SSPs, respectively. One SSP has the mix of four stereoisomers as a core. SSP viscosimetric research showed that their macromolecules are small-size dense coil in both solution and in bulk.
SSP study in Langmuir layers at the air-water interface showed that the increase in the size of organic substituents in cyclic core is a major factor for increased stability of Langmuir layer before a collapse. Thus, Me 4 -15 polymer forms less stable Langmuir layers by 1 mN m −1 than PDMS due to macromolecule structure change from linear to star-shaped and smaller molecular weight. Replacement of only four methyl groups in a SSP cyclic core to phenyl or tolyl without considerable change in molecular-mass characteristics, strikingly changes the form of the surface pressure isotherm and increases the stability of the layer by 0.5 mN m −1 .
At the same time, as the general tendency in behavior of studied SSP, we note the determinative influence of PDMS-arms in comparison with the features of the branchingout center structure. It seems advisable to continue the assessment of influence of various elements of the SSP structure with an even bigger reduction in arms length that would allow the influence of branching-out center features on SSP properties to be more accurately revealed.