Synthesis, Characterization and Microstructure of New Liquid Poly(methylhydrosiloxanes) Containing Branching Units SiO4/2

Six liquid branched poly(methylhydrosiloxanes) of new random structures (PMHS-Q), containing quadruple branching units SiO4/2 (Q), both MeHSiO (DH) and Me2SiO (D) chain building units (or only mers MeHSiO), and terminal groups Me3SiO0.5 (M) were prepared by a hydrolytic polycondensation method of appropriate organic chlorosilanes and tetraethyl ortosilicate (TEOS), in diethyl ether medium at temperature below 0 °C. Volatile low molecular weight siloxanes were removed by a vacuum distillation at 150–155 °C. Yields of PMHS-Q reached from 55–69 wt%. Their dynamic viscosities were measured in the Brookfield HBDV+IIcP cone-plate viscometer and ranged from 10.7–13.1 cP. Molecular weights (MW) of PMHS-Q (Mn = 2440–6310 g/mol, Mw = 5750–10,350 g/mol) and polydispersities of MW (Mw/Mn = 2.0–2.8) were determined by a size exclusion chromatography (SEC). All polymers were characterized by FTIR, 1H- and 29Si-NMR, and an elemental analysis. A microstructure of siloxane chains was proposed on a basis of 29Si-NMR results and compared with literature data.

Twelve new liquid branched poly(methylhydrosiloxanes) with statistical structures (b-r-PMHS), containing triple branching units MeSiO 1.5 (T), both Me 2 SiO (D) and MeHSiO (D H ) chain building units (or only mers MeHSiO), and two b-r-PMHS containing five different structural units: D, D H , T and T H and trimethylsiloxy end groups Me 3 SiO 0.5 (M) were prepared by the hydrolytic polycondensation method of appropriate chlorosilanes in diethyl ether medium at temperature <0 • C. Yields of b-r-PMHS ranged from 57-84 wt% (after removal of low molecular weight oligosiloxanes by a vacuum distillation at 125-150 • C). All polymeric products were characterized by FTIR, 1 H-and 29 Si-NMR, and elemental analysis. Their dynamic viscosities were very low and usually ranged from 8-30 cP, which presumably resulted from their globular structure [9].
Methyl-substituted silica gels with Si-H functionalities were prepared by hydrolysis and condensation reactions of triethoxysilane and methyldiethoxysilane, used in various molar ratios [62]. They gave higher ceramic residue after pyrolysis than gels based only on MeSiO 1.5 branching units [63].
In the present work, we describe the hydrolytic polycondensation synthetic route to new liquid branched poly(methylhydrosiloxanes) of random structures (PMHS-Q), containing both MeHSiO (D H ) and Me 2 SiO (D) chain building units (or only mers MeHSiO), quadruple branching units SiO 4/2 (Q), and terminal groups Me 3 SiO 0.5 , from appropriate organic chlorosilanes and tetraethoxysilane.
An elementary analysis (% C and % H) was performed at the Centre of Molecular and Macromolecular Studies of the Polish Academy of Sciences in Łódź (CBMM PAN). The content of Si-H groups was calculated from an integration ratio of their signals to CH 3 signals in 1 H-NMR spectra, and compared to theoretical integration ratios of Si-H and CH 3 signals. The content of Si was determined by the gravimetric method with H 2 SO 4 (p.a.) [64].
The molecular masses and molecular mass distribution of polysiloxanes were analyzed by a size exclusion chromatography (SEC) in toluene solution, using LDC analytical chromatograph (Artisan Technology Group, Champaign, IL 61822, USA) equipped with refractoMonitor and a battery of two phenogel columns covering the MW range 10 2 -10 5 g·mol −1 . Calibration was made with polystyrene Ultrastyrogel standards with MWs: 10 2 , 10 3 , and 10 4 g·mol −1 .

Synthesis of Branched Polymethylhydrosiloxanes (PMHS-Q)
Branched polymethylhydrosiloxanes, containing only units D H and Q, terminated with Me 3 SiO 0.5 groups, with structures described by a general formula: (where: y = 1-3, m = n = 49-52, p = 2y + 2), were synthesized by the hydrolytic polycondensation of mixtures of tetraethoxysilane Si(OEt) 4 and appropriate chlorosilanes: dichloromethylsilane MeHSiCl 2 , dichlorodimethylsilane Me 2 SiCl 2 , and chlorotrimethylsilane Me 3 SiCl, in the medium of diethyl ether and water, at temperature ranged from −10-0 • C, within 3-5 h. Molar ratios of chlorosilanes were changed, depending on expected molecular formula of polysiloxane. Amounts of substrates used in syntheses of branched PMHS-Q and times of additions of chlorosilanes are presented in Table 1.
In the hydrolytic polycondensation reactions were used such amounts of distilled water, which were sufficient for a formation of hydrochloric acid with a final concentration about 20 wt%. Reaction mixture was allowed to warm to room temperature within 120-170 min, acid layer was separated, and organosilicon layer was washed with water until neutral, transferred to an Erlenmayer flask, and dried at~4 • C with anhydrous magnesium sulfate overnight. Magnesium sulfate was filtered through Schott funnel G-3 and washed with ether. Alternatively, instead of drying with anhydrous MgSO 4 traces of water were removed from products by cooling their ether solution in a refrigerator overnight, warming up the content of the flask to room temperature, and the ether solution of products was decanted from drops of water. The solvent was distilled off. In order to remove volatile cyclic and linear low molecular weight siloxane oligomers, the prepared products were heated at temperature 150-155 • C under reduced pressure (16-21 mm Hg, 2128-2793 Pa), and subsequently under a vacuum (3-5 mm Hg, 400-665 Pa).
In a second step of syntheses of Q 3 D H 50 M 8 and other poly(dimethyl-co-methylhydro)siloxanes, containing both mers D and D H , with a general formula: (where: y = 1-3, m = n = 49-52, p = 2y + 2), so called "extra blocking" of unreacted silanol groups Si-OH was applied: in reactions with (chloro)trimethylsilane, in the presence of triethylamine, which was used as an acceptor of hydrogen chloride with~5% excess with respect to a stoichiometric amount.
(4-Dimethylamino)pyridine (DMAP) was used as a nucleophilic catalyst in 1:10 mole ratio with respect to Et 3 N. Products untreated with extra amounts of TMCS and DMAP/Et 3 N showed increase of their viscosity after few months and a presence of small drops of water from a homo-condensation reaction of residual Si-OH groups.
The "extra blocking" reactions of silanol groups were carried out after drying step of ether solutions of products of the hydrolytic polycondensation, at room temperature within few hours. Precipitates of amines hydrochlorides were dissolved in diluted solution (5-10 wt%) of hydrochloric acid, a water layers were discarded and washed with distilled water until neutral, dried with anhydrous MgSO 4 , and filtered. Ether was distilled off under atmospheric pressure and final products were evacuated under vacuum at temperature 150-155 • C ( Table 2). A chemical composition of volatile siloxanes was not analyzed.

Synthesis of Branched Polymethylhydrosiloxanes (PMHS-Q)
Syntheses of poly(methylhydrosiloxanes) with statistical and branched structures containing quadruple branching points SiO 4/2 were carried out in the medium of diethyl ether at temperature below 0 • C. Solutions of chlorosilanes and Si(OEt) 4 in dry ether were added dropwise to water. In all syntheses were used such amounts of water which were necessary for hydrolysis reactions and dissolution of HCl, allowing to obtain hydrochloric acid with concentrations approximately 20 wt%.
Applying the hydrolytic polycondensation of mixtures of appropriate amounts of (tetraethoxy)-silane Si(OEt) 4 where y = 1-3, m = n = 49-52, p = 2y + 2. After addition of substrates stirring of obtained reaction mixtures was continued within next 2-3 h, in order to reach full conversion of substrates and full hydrolysis of Si-Cl and Si-OC 2 H 5 groups. In the case of syntheses of Q3, Q1D, Q2D, and Q3D termination reactions (so called "extra blocking" reactions) of unreacted silanol groups Si-OH in reactions with (chloro)trimethylsilane were applied, in the presence of: (1) triethylamine as the acceptor of hydrogen chloride (used with~5-10% excess with respect to stoichiometric amounts); and (2) (4-dimethylamino)pyridine (DMAP) as the nucleophilic catalyst (used in 1:10 mole ratio with respect to Et 3 N).
Products not treated with additional amounts of TMCS and DMAP/Et 3 N showed increase of their viscosity after few months and a presence of traces of water, which could originate from the homocondensation reaction of residual Si-OH groups. However, in the case of syntheses of Q1 and Q2 "extra blocking" was not applied, and no increase of their viscosity was observed during longer storage of these PMHS-Q. Ether solutions of products Q1, Q2, and Q3 were dried with anhydrous MgSO 4 , while polymers Q1D, Q2D, and Q3D were dried by freezing traces of water in the refrigerator overnight. Yields of prepared PMHS-Q ranged from 55-69 wt% ( Table 2). The highest yield was obtained for Q3.
The chemical structures of all PMHS-Q were confirmed by spectroscopic methods: FTIR and NMR ( 1 H and 29 Si) and the elemental analysis (% C, % H, and % Si) (see Table 3).
Dynamic viscosities (η 25 ) of PMHS-Q containing quadruple branching points SiO 4/2 , were very low and ranged from 10.7-13.1 cP. Low viscosities of PMHS-Q in comparison with linear polysiloxanes having similar molecular weights presumably may result from a globular structure of hyperbranched macromolecules. It is commonly known from a literature that dendrimers and hyperbranched polymers in solution and in melt have low viscosities. Their viscosities and molecular weights are much lower than those for linear analogs and depend on a degree of branching, a polarity of a solvent, a kind of functional group on their "surface", and also on pH of a polymer solution. Dendritic and hyperbranched polymers have a variable hydrodynamic radii depending on the property of solvents; they are smaller than those of their linear analogs with the same molar mass.
The values of molecular weights of prepared PMHS-Q determined by SEC method were lower than calculated values for predicted molecular formulas: QD 52  A polydispersity of molecular weights of PMHS-Q ranged from 2.0 to 2.8. The molecular weights of dendrimers and hyperbranched polymers determined by SEC using polystyrene standards are regarded with some scepticism. The hydrodynamic radii were also susceptible to the polarity of functional groups on the periphery [65][66][67]. Values of M n and M w determined by SEC method with polystyrene standards for hyperbranched polysiloxanes were much lower than MW obtained with application of MALLS detectors [68][69][70].
Köhler et al. used the SEC, 1 H-and 29 Si NMR, and MALDI-TOF-MS methods for characterization of a linear poly(dimethylsiloxane)-co-poly(hydromethysiloxane) (PDMS-co-PHMS) copolymer with respect to chain length distribution, heterogeneity of chemical composition, and sequence distribution [71].

Characterization of PMHS-Q by NMR
In 1 H-NMR spectra of copolymers, QD H 48M4, Q2D H 49M6 and Q3D H 50M8 were present signals at δ 0.01 -0.22 ppm, corresponding to hydrogen atoms of Si-CH3 groups and signals at δ about five parts per million, characteristic for hydrosilane groups Si-H. In the 1 H-NMR spectra of copolymers:

Characterization of PMHS-Q by NMR
In 1 H-NMR spectra of copolymers, QD H 48M4, Q2D H 49M6 and Q3D H 50M8 were present signals at δ 0.01 -0.22 ppm, corresponding to hydrogen atoms of Si-CH3 groups and signals at δ about five parts per million, characteristic for hydrosilane groups Si-H. In the 1 H-NMR spectra of copolymers: QD52D H 52M4, Q2D49D H 49M6, and Q3D50D H 50M8 were present signals at δ 0.0 -0.30 ppm, corresponding to hydrogen atoms of Si-CH3 groups and signals at δ about five parts per million, characteristic for Si-H groups. Examples of the 1 H-NMR and 29 Si-NMR spectra of branched poly(methylhydrosiloxanes) are presented in Figures 4-7.     [8,9,52]. It was impossible to observe signals of quadruple silicon atoms of units SiO4/2 in 29 Si-NMR spectra, which were registered by the INEPT technique, so it was necessary to run 29 Si-NMR spectra with application of the INVGATE program. A summary of chemical shifts data in the 1 H-and 29 Si-NMR (INEPT and INVGATE) spectra of all PMHS-Q is presented in Table 5.

Characterization of PMHS-Q by NMR
In   [8,9,52]. It was impossible to observe signals of quadruple silicon atoms of units SiO 4/2 in 29 Si-NMR spectra, which were registered by the INEPT technique, so it was necessary to run 29 Si-NMR spectra with application of the INVGATE program. A summary of chemical shifts data in the 1 H-and 29 Si-NMR (INEPT and INVGATE) spectra of all PMHS-Q is presented in Table 5.
In the 29 Si-NMR INVGATE spectra of branched random PMHS were present signals of silicon atoms corresponding to linear mers:   In the 29 Si-NMR spectra (recorded by INEPT and INVGATE techniques) in the range of δ −33 -−37 ppm exist signals of middle silicon atoms of units D H , which undergo changes in pentades ( Table  5). Signals of silicon atoms in the range of δ −102 to −109 ppm, presumably correspond to Si atoms in the central units Q, in the following sequences of siloxane structures: Chemical shifts in the range of 7-11 ppm in the 29 Si-NMR spectra (INEPT and INVGATE) correspond to silicon atoms of the end groups M and change in tetrads (Table 5) [8,9,74].
Signals at δ −64 ppm of a very low intensity, registered both in INVGATE and INEPT 29 Si-NMR spectra of these three copolymers, probably come from Si atoms of units MeSiO1.5 (T), which were formed during syntheses of PMHS-Q from trace hydrolysis of Si-H bonds [74].
Signals at δ −64 ppm of a very low intensity, registered both in INVGATE and INEPT 29 Si-NMR spectra of these three copolymers, probably come from Si atoms of units MeSiO 1.5 (T), which were formed during syntheses of PMHS-Q from trace hydrolysis of Si-H bonds [74].
Assignments of all 29 Si-NMR signals resulting from the microstructure of siloxane chain of branched polymethylhydrosiloxanes are summarized in Table 6.