Hydride Transfer Ring-Opening Polymerization of a Cyclic

Mar 7, 2012 - Center of Molecular and Macromolecular Studies, Polish Academy ... ABSTRACT: The ring-opening polymerization of 2,4,6,8-tetramethyltetra...
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Hydride Transfer Ring-Opening Polymerization of a Cyclic Oligomethylhydrosiloxane. Route to a Polymer of Closed Multicyclic Structure Julian Chojnowski,*,† Jan Kurjata,† Witold Fortuniak,† Slawomir Rubinsztajn,‡ and Barbara Trzebicka§ †

Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 94-011 Łódź Poland Global Research Center, General Electric Company, 1 Research Circle, Niskayuna, Schenectady, New York 12309, United States § Center of Polymers and Carbon Materials, Polish Academy of Sciences, M. Skłodowskiej Curie 34, 41-819 Zabrze, Poland ‡

ABSTRACT: The ring-opening polymerization of 2,4,6,8-tetramethyltetrahydro-cyclotetrasiloxane, DH4, in the presence of tris(pentafluorophenyl)borane, B(C6F5)3, has been studied in dilute toluene solution. The mechanism of this polymerization was investigated. The initiation and propagation steps are the B(C6F5)3 catalyzed ring-opening of DH4 by the SiH group resulting in its addition to a growing chain, which occurs with hydride ion transfer to the chain end. Since there are multiple SiH groups present in the monomer, the formed polymer has extensive branching. Although the bulk polymerization leads to gel formation at an early stage of the process, no gelation is observed when the reaction is performed in 10 wt % solution in toluene. This unexpected observation is due to two reaction features. First, an extensive cyclization occurs leading to a closed multicyclic polymer structure. Second, the initiation leads to the formation of very reactive −OSiMeH2 chain end, which is the real propagation center. Branching does not occur in the propagation but in the initiation and termination steps. Gaseous MeSiH3 is released in the latter step.

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INTRODUCTION Tris(pentafluorophenyl)borane, B(C6F5)3, readily catalyzes the transfer of hydride ions from silicon to organic compounds. Thus, it is often used as a catalyst in the reduction of organic compounds by hydrosilanes.1−3 This borane is also a very active catalyst for hydride transfer reactions in organosilicon chemistry, which lead to the formation of siloxane or silylether bonds.4−7 Kawakami used B(C6F5)3 to promote the polycondensation of hydrosilane with silanols.8,9 It is also a very effective catalyst for the dehydrocarbon polycondensation of hydrosilanes with alkoxysilanes,4,10−17 as well as a promoter of metathesis reactions between hydrosilane groups and alkoxysilane4 and metathetic dismutation reactions of oligohydrosiloxanes.18 The hydride transfer from silicon promoted by B(C6F5)3 is also the main step in a new class of ring-opening polymerization reactions of cyclic siloxanes,19 which is proposed here to be named the hydride transfer polymerization reaction. In the hydride transfer ring-opening polymerization (ROP) of cyclic siloxanes the monomer ring is opened by SiH with the assistance of B(C6F5)3. Concurrently the new siloxane bond is formed, which links the opened monomer to the polymer chain while the hydride ion is transferred to the polymer chain end, eq 1.19

protic or Lewis acids, which occurs according to a quite different mechanism. B(C6F5)3 is known as a fairly strong Lewis acid, which when activated by a protic compound, is used as an initiator for the cationic polymerization of olefin monomers such as methoxystyrene.20−22 Being activated by a protic molecule, it is also able to open a siloxane bond.23 The hydride transfer polymerization occurs in the absence of protic compounds but it requires the presence of a silyl hydride, which initiates the polymerization. Tris(pentafluorophenyl)borane plays the role of the catalyst in this polymerization.19 The hydride transfer polymerization of cyclic siloxanes is accompanied by hydrosiloxane metathetic processes and is limited to strained ring monomers such as hexamethylcyclotrisiloxane, (D3).19 If the siloxane bond is in an unstrained cyclic molecule, such as octamethylcyclotetrasiloxane (D4), it is not cleaved by the SiH-B(C6F5)3 system at room temperature. However, the opening of siloxane bonds may occur when hydrogen is bonded to a silicon atom of the SiOSi group.18 An even more active system is one in which both silicon atoms bear hydrogen. This was observed in rapid metathetic dismutation of 1,1,3,3-tetramethyldisiloxane in the presence of B(C6F5)3.18 It was thus interesting to investigate the behavior of an unstrained cyclosiloxane having SiH groups exclusively. Such a compound could also be the monomer in the hydride transfer polymerization. It would not require the Received: December 12, 2011 Revised: February 22, 2012 Published: March 7, 2012

It should be noted that the hydride transfer polymerization is different than the classical cationic polymerization initiated by © 2012 American Chemical Society

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a toluene solution was introduced by a Hamilton precision syringe. The subsequent samples were withdrawn at timed intervals and introduced to an Eppendorfer vessel containing an excess of 4-ethylpyridine. The time of the introduction of the sample to the quencher solution was considered as the zero time of the reaction. The samples were analyzed by gas chromatography. Trapping of Volatile Product. DH4 (2 mL, 1.98 g) was placed in a glass reactor equipped with a bubbler through which nitrogen gas was flowing and an outlet connected with an NMR tube by means of a Teflon pipe. The outlet of the pipe was placed deep under the surface of deuterated chloroform, which was in the tube. B(C6F5)3 (40 μL of a 0.05 × 10−2 M solution in toluene) was introduced into the reactor. The evolution of a gaseous product was observed, immediately, which together with vapors of toluene and monomer was carried out by the stream of nitrogen to the NMR tube where they were partly absorbed by chloroform. After 5 min of reaction time, the formation of gel was observed. The evolution of the volatile product continued after the gelation of the reaction mixture. After 15 min of reaction time, tube was taken out, closed tightly with a Teflon stopper and inserted into the NMR spectrometer. 29Si NMR spectrum of products trapped in CDCl3 showed a singlet signal at δ = −63.90 ppm. A triplet of DH4 was also observed. There was no other silicon resonance seen in the spectrum. This new signal corresponded to the resonance of methyltrihydrosilane MeSiH3 for which −64.86 ppm had been found by Fritz and Arnason.28 This structure was fully confirmed by 1H NMR spectrum which yields a quartet assigned to CH3SiH3 centered at δ = 3.52 ppm, JH−H = 4.60 Hz and a quartet of CH3SiH3 centered at δ = 0.18 ppm, which were both exactly the same as those obtained in the simulated spectrum of methyltrihydrosilane by ACD/ChemSketch software.

Si−H initiator since the polymerization would be initiated by the monomer itself. This thought prompted an investigation of the polymerization of 2,4,6,8-tetramethylcyclotetrasiloxane (DH4) initiated by B(C6F5)3. The cationic polymerization of DH4 initiated by a strong protic acid was the subject of previous research.24−27 This polymerization leads to linear poly(hydromethylsiloxane). However, DH4 is a polyfunctional monomer in the hydride transfer polymerization so branching and crosslinking is expected when its polymerization is promoted by B(C6F5)3.

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EXPERIMENTAL SECTION

Materials. 2,4,6,8-Tetramethylcyclotetrasiloxane (DH4) reagent grade product of ABCR (Germany) was kept over calcium hydride and distilled. Purity was checked by gas chromatography and was above 98%. Toluene (POCH, Polskie Odczynnki Chemiczne) was shaken with H2SO4, washed with water, then with a sodium carbonate solution, again with water, dried over MgSO4, distilled over P2O5 and distilled again from sodium. The catalyst was reagent grade tris(pentafluorophenyl)borane (Aldrich) with a reported purity of 95% and was use without further purification. Analytical Methods. Gas chromatography analyses were performed using a Hewlett-Packard HP 6890 chromatograph equipped with a thermal conductivity detector and a HP1 capillary column l = 30 m, d = 0.53 mm. Typical conditions were as follows: helium carrier gas flow rate 5 mL/min, detector temperature 250 °C, injector temperature 250 °C, and column programmed for 3 min at 40 °C isotherm, from 40 to 240 °C at a rate of 10 °C/min and 10−15 min at 240 °C isotherm; n-dodecane was used as the internal standard. MALDI TOF spectra were recorded on a Voyager Elite spectrometer equipped with a N2 laser operating at 337 nm in linear mode. Dithronal was used as the matrix and NaI as the cationizing agent. SEC measurements were performed in THF at 35 °C using a differential refractive index detector Δn-2010 RI WGE Dr. Bures. The column set from Polymer Standard Service (PSS) contained four SDV columns (5 μm): 1 × 105 Å, 1 × 103 Å and two 1 × 102 Å. Apparent molar masses were calculated based on polystyrene calibration using the WinGPC Unity software from PSS. The molar masses of the samples taken at 50% monomer conversion were measured with an LDC analytical RefractoMonitor equipped with two Phenogel columns covering the molar mass range 102−105 and a refractive index detector. Toluene was used as the solvent with polystyrene as the standard. 1 H NMR spectra were recorded on a Bruker DRX 500 operating system at 500 MHz. 29Si NMR were recorded on the same instrument working at 99.6 MHz in the inverse gate pulse sequence mode with a relaxation delay of 3 s, scan number 10 K, time domain 65 K and a 90 deg pulse 15.00. The Cr(acac)3 complex was added as a relaxation reagent. Polymerization of DH4 and Kinetic Studies. Polymerizations were performed in a 50 mL Schlenk flask equipped with a magnetic stirrer and a three-way stopcock through which argon was flowed. Substrate, catalyst, GC standard and solvent were introduced through this stopcock by means of a precision Hamilton syringe with a long needle. Samples were withdrawn at various time intervals and neutralized upon the addition of 4-ethylpyridine and subjected to GC analysis. n-Dodecane was used as the GC standard. The polymerizations were performed at room temperature (23 °C). The polymeric product was precipitated three times from toluene solution upon addition of methanol. The polymer was subjected to NMR, SEC and MALDI TOF analysis. 1H NMR, δ in ppm: 0.15−0.18, 0.18−0.22, br s, SiCH3; 4.74, br s SiH. 29Si NMR, δ in ppm: −33.0 − (−36.5), br m, HCH3SiO; −53 − (−58), −62 − (−67), br m CH3SiO1.5. An example of experiments used for kinetic studies is given as follows: The reactor was purged with nitrogen and charged with DH4 (1.23 g, 5.1 × 10−3 mol), toluene (10.6 g, 12.3 mL) and n-dodecane (0.43 g, 2.53 × 10−3 mol). The zero-time sample was withdrawn by means of a Hamilton syringe. Catalyst B(C6F5)3 (3.1 × 10−3 g, 6.1 × 10−6 mol) in

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RESULTS AND DISCUSSION Reaction Features. Tris(pentafluorophenyl)borane (TPFPB) is considered to be a fairly strong Lewis acid.29 In the presence of this Lewis acid the classical cationic ringopening polymerization of tetramethylcyclotetrasiloxane (D4) did not proceed with noticeable rate at room temperature. The behavior of 2,4,6,8-tetrahydro-tetramethylcyclotetrasiloxane (DH4) was found to be quite different. The addition of the 10−3 mol/L of (C6F5)3B to neat DH4, i.e. 0.005 g of the borane to 10 g of the monomer, resulted in relatively rapid formation of gel. Such behavior could be expected as DH4 has four potential reactive centers for the initiation of the hydride transfer polymerization. In addition, the open polymer chain has multiple potential centers for subsequent hydride transfer reactions which could lead to branching and cross-linking of the polymer product. In contrast to the polymerization in bulk, when the reaction was carried out in a dilute solution in inert solvent such as 10 wt % of DH4 solution in toluene, and when a similar concentration of (C6F5)3B, i.e., 10−3 mol/L was used, the reaction proceeded at room temperature with a moderate rate and gel was not formed even after a longer reaction time. Higher cyclic oligomers DH5 and DH6 were detected by GC− MS and the isolated polymer had a molar mass of Mn = 3 × 103 at about 50% monomer conversion. The 29Si NMR analysis revealed that its chain was mostly composed of MeHSiO units. These observations were convincing proof that the hydride transfer ring-opening polymerization dominated over processes leading to branching and cross-linking, at least in the early stage of this reaction. The conversion of monomer was monitored by sampling and gas chromatography analysis. The monomer conversion−time dependencies at various concentrations of the borane catalyst are shown in Figure 1. The striking feature of this polymerization is that it proceeded to the full conversion of monomer. Its cyclic homologues, which were transiently formed as a result of backbiting, see Figure 2A, were fully consumed in the late 2655

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polymer occurred as shown by 29Si NMR, see below, which was accompanied by an increase in its molar mass and the formation of a volatile product. However, when the initial monomer concentration was 10 wt %, the gel was not formed even though the reaction was allowed to proceed for a long time. The polymerization in its initial period is well approximated by the first order kinetic law, and the external order in catalyst is first order as well. The apparent catalytic constant was 0.46 dm3 mol−1 s−1 at 23 °C.31 Analysis of the Volatile Product. Elucidation of a rational polymerization mechanism first requires knowing the structure of the volatile product formed. The polymerization was performed in a concentrated toluene solution of DH4 under conditions close to that in the bulk. Nitrogen gas was bubbled through the reactor sweeping away the gaseous product formed together with vapors of the solvent and monomer. The volatile products were partially trapped in an NMR tube filled with CDCl3. The 29Si NMR and 1H NMR spectra of the trapped compounds are shown in Figure 3, parts A and B. Inspection of

Figure 1. Monomer conversion−time dependences of DH4 polymerization in 10 wt % solution in toluene at 23 °C at various concentrations of B(C6F5)3.

stages of this polymerization (Figure 2B). The ring-opening polymerization (ROP) of unstrained cyclic siloxanes, such as

Figure 2. Gas chromatogram of the polymerization system of DH4 in 10 wt % solution in toluene at 23 °C: (A) quenched at the moment when DH4 was at its equilibrium concentration in classical ROP and (B) quenched after a longer reaction time.

DH4, is an entropy driven process which is reversible. In classical cationic or anionic ROP of DH4 the concentration of the monomer and cyclic oligomers cannot drop below their equilibrium values, which for DH4 is known to be about 0.1 mol/L30 and is independent of temperature and initial monomer concentration. The decrease of the monomer concentration below this limit means that the reaction takes a course which is different from that of classical ROP of cyclic siloxanes. Another feature of this polymerization is that the polymer formation is accompanied by the evolution of a volatile gaseous product. It was produced even after all monomer had been consumed, thus indicating that the formation of the volatile product was related to a process leading to a transformation of the formed polymer. A slow increase in the viscosity of the reaction mixture was also observed in this late period of the reaction. A branching process of the

Figure 3. (A) 29Si NMR spectra and (B) 1H NMR spectra of products trapped in CDCl3 from N2 gas stream bubbled through the polymerization mixture of DH4.

the spectra revealed that the mixture, besides containing toluene and monomer, also contained the volatile methyltrihydrosilane CH3SiH3. Characterization of Polymer. The isolated polymer product was analyzed by 1H and 29Si NMR and by SEC. The NMR spectra are shown in Figures 4 and 5. The analysis proved that the polymer chain is composed of two kinds of units: 2656

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of the polymer product in the subsequently drawn samples. Results are listed in Table 1. The degree of branching was characterized by comparison of the contribution of linear and branched units to the structure of the polymer. The method for calculating the degree of branching used for classical hyperbranched polymers,32 was not used as it does not fit this type of polymerization mechanism. The results indicate that in the initial stage of polymerization, the linear units dominate the polymer structure. At 50% of the monomer conversion, one branching point corresponds to about 10 linear MeHSiO units. The branching became more prominent at a higher monomer conversion. When the concentration of the monomer fell to 8% of its initial concentration, which occurred after 1 h, the number of linear units was only twice as large as that of the branched units. This point may be recognized as the end of the monomer polymerization since its concentration was well below its cyclic monomer−linear chain equilibrium concentration. Further reaction led mostly to a conversion of the linear MeHSiO units into the branched MeSiO1.5 units, which caused a further decrease in the monomer concentration. The reaction proceeds according to a simplified eq 2 and must lead to a strong restructuring of the siloxane chain.

Figure 4. 29Si NMR spectrum of the polymer product isolated from the polymerization system of DH4 in 10 w% solution catalyzed by (C6F5)3B, 1.00 × 10−3 mol/L, at 23 °C, quenched after 2 h.

The analysis of the polymer, when the reaction was quenched after 24 h, showed that the three-way-bonded branching unit strongly prevails over the two bonded linear unit as the respective number ratio is 3:1. It could be predicted that polymer gelation would occur at a much earlier stage in reaction 2. However, the polymer after 24 h of reaction time was still soluble even though its molar mass has grown as shown in Table 1. The only explanation for the unusual behavior of this system is that the formation of cyclic structures dominates the pattern of the polymerization in its later stage. Confirmation of the validity of proposed reaction 2 and important information about the mechanism of the polymerization of DH4 initiated by B(C6F5)3 are provided in the results of a MALDI TOF study. The spectrum of the polymer isolated from the reaction system after the monomer concentration decreased below its chain-ring equilibrium value is shown in Figure 7. The peaks correspond well to the molar mass of the combination of numbers of MeHSiO and MeSiO1.5 units, which are labeled above the signals. This result is not only consistent with a process that proceeds according to eq 2, but it also suggests that other structural units are not present in the macromolecule chain nor do they appear at its termini. The only explanation for this observation is that cyclic structures occupy chain ends or the whole molecule has a cyclic structure. The mechanism of this polymerization must also explain another strange feature of the MALDI TOF spectrum of the polymer. It shows that the number of MeSiO1.5 units in the macromolecule is always even, i.e., 2, 4, 6, 8, .... Signals corresponding to macromolecules having odd number of branching units are not detected. Reaction Mechanism. The classical cationic polymerization of octamethylcyclotetrasiloxane (D4) initiated by B(C6F5)3 does not proceed at a measurable rate under the

Figure 5. 1H NMR spectrum of the polymer product isolated from the polymerization system of DH4 in 10 wt % solution in toluene catalyzed by (C6F5)3B, 1.00 × 10−3 mol/L, at 23 °C, quenched after 1 h.

The former is a chain extending unit while the latter is a branching unit. The spectra did not show any pendant chainend units. In a separate experiment, the reaction was carried out under the same conditions, as used in the kinetic experiment described above represented by the curve marked with squares in Figure 1. Samples were withdrawn after various times and the polymer product was isolated and subjected to NMR and SEC analysis. Results are listed in Table 1. The process was followed over a period much longer than was required to reach the equilibrium concentration of DH4 was achieved. Although only molar masses of polymers are apparent, determined with the use of a polystyrene calibration curve, their increase with the reaction time accompanied by significant increase in dispersities is indubitable (see also Figure 6). The percentages of both units, MeHSiO, and MeSiO1.5, were calculated by 1H and 29Si NMR, which permitted determination of the branching 2657

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Table 1. Characteristics of Polymer Products Isolated from the Polymerization System of DH4 in 10 wt % Solution in Toluene, Catalyzed by (C6F5)3B, 1.00 × 10−3 mol/L, at 23 °C, after Various Reaction Times reaction time, h 0.25 1 2 6 24

monomer conversion % 50 92 99 100 100

molar massa Mn × 10−3 2.5 3.0 2.9 5.7 9.3

molar massa Mw × 10−3

dispersity Mw/Mn

MeHSiO units, %

MeSiO1.5 units, %

4.6b 7.7 17.1 64.0 95.3

1.8b 2.6 6.0 11.2 10.3

90.3 72.2 51.6 42.9 25.2

9.7 27.8 48.4 57.1 74.8

b

a

These are apparent molar masses measured by SEC with RI detection and calculated with polystyrene calibration. Since the polymers have branched structures their absolute molar masses could be higher than the values obtained basing on calibration with linear polystyrene. bMeasured on an LDC Analytical Refractomonitor with RI detector.

research,18,19 is shown in Scheme 1 and all of the steps of this process are discussed below: Initiation. It was shown elsewhere that the hydride transfer mediated by B(C6F5)3 may lead to the opening and reformation of the siloxane bond if the silicon atom is also bonded to hydrogen.18 It is worth recalling that this process occurs particularly readily if both silicon atoms of the disiloxane unit bear hydrogen. It was shown that the rate of HMe2SiOSiMe2H disproportionation catalyzed by B(C6F5)3 is higher than that of Me3SiOSiMe2H by about 2 orders of magnitude.19 The ringopening of D3 occurs also much faster by HMe2SiOSiMe2H than by Me3SiOSiMe2H. It is thus conceivable that the DH4 ring is opened by another DH4 molecule in the presence of B(C6F5)3 and, in this way, the hydride transfer polymerization of DH4 is initiated according to eq 3.

Figure 6. SEC curves of the polymer product isolated from the polymerization system of DH4 in 10 wt % toluene solution catalyzed by (C6F5)3B, 1.00 × 10−3 mol/L, at 23 °C, after 1, 2, 6, and 24 h.

The mechanism of this reaction is analogous to those of the metathesis of 1,1,3,3-tetramethydisiloxane18 and condensation of alkoxysilane with hydrosilane.4 The hydride ion is transferred from the monomer to the borane with the formation of a bicyclic oxonium ion intermediate by the addition of another monomer molecule to a vacant silicon. Then an H− is transferred to the silicon atom of the added monomer leading to the opening of its ring. The formed OSiMeH2 end unit is very reactive toward hydride transfer and may be considered as an active propagation center, eq 3. It was demonstrated in our earlier studies19 that the ring-opening of the D3 in the presence of B(C6F5)3 proceeded more rapidly with PhMeSiH2 than with PhMe2SiH by a factor of 300. This suggests that linear propagation dominates over branching and cross-linking. A silicon atom bearing two hydrogen atoms reacts with the siloxane bonds under catalysis by B(C6F5)3 much more rapidly than a silicon atom having only one hydrogen bonded to it. The initiation reaction leads to the transformation of one MeHSiO unit into a branching MeSiO1.5 unit. It also leaves the eight membered ring structure at the beginning of the initiated chain, which is connected to the chain by the branching unit as illustrated in eq 3. Propagation. The active propagation center −OSiMeH2 forms an oxonium salt with the monomer and the borane in the propagation step. The oxonium salt decomposes upon monomer ring-opening. The ring-opened monomer is added to

Figure 7. MALDI TOF spectrogram of the polymer product isolated from the polymerization system of DH4 in 10 wt % toluene solution catalyzed by (C6F5)3B, 1.00 × 10−3 mol/L, at 23 °C, quenched after 1 h. Peaks are marked by the sum of the number of branching units xMeSiO1.5 and chain extending units yMeHSiO which corresponds to the m/z value at the peak maximum.

conditions used in this study. The observed relatively fast polymerization of DH4, which occurred in the presence of B(C6F5)3, must be explained by a nonclassical cationic mechanism related to the presence of the SiH groups in the cyclic monomer. The mechanism must also explain the formation of MeSiO1.5 units and the evolution of MeSiH3 as well as the extensive cyclization of the polymer formed. A general scheme of the polymerization, which is in agreement with the experimental results and with results of earlier 2658

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Scheme 1. Ring-Opening Polymerization of 2,4,6,8-Tetramethylcyclotetrasiloxane, DH4, in the Presence of B(C6F5)3

a growing polymer end while the hydride ion is transferred to the chain end thus regenerating the active propagation center, eq 4.

then the monomer ring is added to the chain end through the formation of a MeSiO1.5 unit, as shown in eq 5 and reaction 5a in Scheme 1. When the chain is terminated by the reaction with the MeHSiO unit within its own chain, then a ring is formed (Scheme 1 reaction 5b). An intermolecular reaction of this type leads to branching or cross-linking (Scheme 1 reaction 5c). The low initial concentration of monomer is crucial to obtaining soluble polymer product. Most probably, the methylsilane byproduct, MeSiH3 could reversibly reinitiate a new chain forming two active propagation centers (OSiMeH2), but it is volatile and escapes from the reaction system. Reinitiation. The formation of the propagating center OSiMeH2 also occurs as a result of reactions between monomer and polymer chain, as shown in reaction 6a of Scheme 1. In this case, either a new chain is grafted from the polymer chain or the chain is cleaved into two parts. One of them closes the monomer ring through its MeSiO1.5 unit and the other gains the propagating OSiMeH2 end. Still other reinitiation reactions may occur either intra- or intermolecularly between two MeHSiO polymer chain units as shown in eq 6b of Scheme 1. These reactions dominate in the

The propagation is reversible since the propagation center may form the oxonium ion intermediate with a siloxane unit in its polymer chain. This backbiting process may lead to a series of cyclics homologues of DH4 (reaction 3 in Scheme 1). If the analogous reaction occurs intermolecularly it leads to a chain transfer, which causes chain scrambling as shown in reaction 4 in Scheme 1. Termination. The important feature of this reaction is the chain termination in which the OSiMeH2 active propagation center is transformed to volatile MeSiH3. This reaction occurs when the oxonium ion intermediate is formed by hydride transfer from an OSiMeH unit in the monomer or polymer to the active propagating center OSiMeH2, reactions 5abc in Scheme 1. The termination which occurs as a result of the reaction of OSiMeH2 with monomer is shown in eq 5. MeSiH3 is cleaved from the active center and the remaining oxygen is used to form a bond with vacant silicon. The termination on polymer takes an analogous path. If the chain termination takes place with the participation of monomer, 2659

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Figure 8. Schematic representation of the macromolecule of the polymer formed in the hydride transfer polymerization of DH4 catalyzed by (C6F5)3B in a dilute solution of an inert solvent at an early stage in the reaction.

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CONCLUSIONS DH4 undergoes ring-opening polymerization initiated by B(C6F5)3. The polymerization proceeds according to a nonclassical mechanism, which may be classified as hydride transfer ring-opening polymerization. The −OSiMeH2 chain end is the active propagating center, but its stationary concentration in the polymerization system is very small. The polymerization occurs with the transformation of linear MeHSiO units into branching MeSiO1.5 units, which takes place in initiation and termination steps. Volatile MeSiH3 forms as a byproduct in the termination step. Cyclic structure formation dominates in the products of the polymerization. Gel formation may be avoided by performing the polymerization in dilute solution in an inert solvent. The polymerization of 10 wt % solution of DH4 in toluene leads to full conversion of monomer into soluble polymer. The macromolecule formed has a nearly perfect multicyclic structure composed of small rings originating from the monomer and its homologues as well as larger macrocycles of various sizes. This easily synthesized soluble polymer having many reactive SiH groups may be used as a core starting material for the synthesis of more complex multifunctional polymer structures.

second slow stage of the process. They occur much more slowly than the DH4 polymerization. Since the process is performed in a dilute solution, the intramolecular reactions strongly dominate. They lead to the extensive formation of loops, so that the resulting macromolecule has a multicyclic structure schematically presented in Figure 8. The intermolecular reactions are manifested in the increase in molar mass of the polymer. This may gain importance when a formed rigid, multicyclic macromolecule is less susceptible to intramolecular reactions than to intermolecular chain extension. Each of the initiation and termination reactions results in the conversion of one MeHSiO unit into one MeSiO1.5 unit, which decreases the rates of monomer reformation and monomer analogues formation in depolymerization processes (reaction 3 of Scheme 1). This is the reason for the disappearance of the monomer and cyclic oligomers from the reaction system shown in Figure 2B. Reactions 6a and 6b (Scheme 1) also result in the formation of the OSiMeH2 chain ends. As mentioned previously, the OSiMeH2 group is very reactive in both the propagation and termination steps so its stationary concentration in the reaction system, particularly in the second stage of the reaction, should be at a low level. Each of the initiation and reinitiation steps, reactions 1, 6a, and 6b in Scheme 1, lead to the formation of one MeSiO1.5 unit and one OSiMeH2 terminal group. Each of the termination reactions 5a, 5b and 5c in Scheme 1 similarly give rise to the formation of one MeSiO1.5 unit, but lead to the disappearance of one OSiMeH2 terminus. The even number of the MeSiO1.5 units found by MALDI-TOF indicates that the number of initiation and reinitiation turns is equal to the number of the termination acts. It may be further concluded that the OSiMeH2 group is not present in the final polymer product at measurable concentrations which is consistent with NMR spectroscopic analysis. The only possible open chain end-group for the investigated polymer is −OSiMeH2. Lack of the −OSiMeH2 group in the final polymer strongly indicates that the formed macromolecule is built from various size cyclic structures. The polymerization of DH4 in the presence of B(C6F5)3 is a good example of three-dimensional addition polymerization leading to closed multicyclic structures free of pendant chain ends. Such structures have so far been obtained by equifunctional polycondensation of multifunctional monomers under ideal reaction conditions.33,34 A polysiloxane with polycyclic structure was generated by the hydrolytic coupling of a cyclic methylhydrosiloxane, but the polymer was insoluble and contained some quantity of silanol groups.35

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the Center of Molecular and Macromolecular Studies of the Polish Academy of Sciences in Łódź Poland for the financial support of this research. The authors are indebted to Mary Krenceski Ph.D. from Momentive Performance Materials and to David Simone Ph.D. and Matthew Butts Ph.D. from General Electric GRC for the language improvement of this paper.

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REFERENCES

(1) Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1. (2) Gevorgian, V.; Rubin, M.; Benson, S.; Liu, X.; Yamamoto, Y. J. Org. Chem. 2000, 65, 6179. (3) Berkefeld, A.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc. 2010, 132, 10660. (4) Chojnowski, J.; Rubinsztajn, S.; Cella, J.; Fortuniak, W.; Cypryk, M.; Kurjata, J.; Kaźmierski, K. Organometallics 2005, 24, 6077.

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dx.doi.org/10.1021/ma202687u | Macromolecules 2012, 45, 2654−2661