When Attempting Chain Extension, Even Without Solvent, It Is Not Possible to Avoid Chojnowski Metathesis Giving D3

A simple, mild and efficient method to prepare HSi- or HOSi-telechelic, high-molecular-weight polydimethylsiloxane polymers (to 41,600 g·mol−1) using the one-shot hydrolysis of MHMH is reported; titration of the water allowed for higher molecular weights (to 153,900 g·mol−1). The “living” character of the chain extension processes was demonstrated by adding a small portion of MHMH and B(C6F5)3 (BCF) to a first formed polymer, which led to a ~2-fold, second growth in molecular weight. The heterogeneous reaction reached completion in less than 30 min, much less in some cases, regardless of whether it was performed neat or 50 wt% in dry toluene; homogeneous reactions in toluene were much slower. The process does not involve traditional redistribution, as judged by the low quantities (<3%) of D4 produced. However, it is not possible to avoid Chojnowski metathesis from MHDDMH giving D3, which occurs competitively with chain extension.


Introduction
Polydimethylsiloxane (PDMS) oils are the parent commercial silicone polymers from which almost all other silicones are derived. PDMS possesses unique properties including low T g , good thermal stability, high optical transparency, excellent dielectric properties and excellent biocompatibility. Conventionally, two methods dominate the commercial preparation of high-molecular-weight PDMS oil: dehydration of silanol-terminated oligomers [1] and acid-or, particularly, base-catalyzed equilibration of cyclic monomers [2] (Figure 1A,B). However, each of these approaches suffers from inherent shortcomings. The former process, as with all condensation processes, slows down with increasing conversion, which makes the synthesis of high-molecular-weight polymers challenging; both processes typically lead to polymers with high dispersities Ð M . In the latter case, with base-catalyzed equilibration that has an equilibrium constant near 1, the desired polymer is accompanied by the formation of large quantities of cyclic monomers, e.g., <15% for D 4 (D =~Me 2 SiO~) [3,4] ( Figure 1B). Cyclooligosiloxanes, particularly D 4 , have attracted different levels of concerns by regulatory agencies because of their purported environmental behaviors [5,6]; D 4 concentrations are regulated in Canada and the UK [7,8]. Hence, there is an increasing consensus that the value of silicone polymers would be increased if they contained lower cyclooligosiloxane concentrations. Note that the removal of cyclic monomers from silicone oils becomes more difficult as the MW (molecular weight) and viscosities of both cyclics and oils increase.
The competing commercial process for synthesis of high-MW silicone oils is ringopening polymerization [9], typically initiated by anions [10][11][12] (Figure 1C). Polymers with high MW can result, but the process is challenged by the need to be scrupulously dry to avoid premature termination; higher MW PDMS polymers with narrow dispersity Ð M are more easily achieved when the more expensive, ring-strained monomer D 3 is utilized as a starting material instead of D 4 . are more easily achieved when the more expensive, ring-strained monomer D3 is utilized as a starting material instead of D4. Hydrosilane monomers, particularly HMeSiCl2 and HMe2SiCl, are produced in a direct process [13] in concentrations that typically exceed commercial needs. As a consequence, oligomers HMe2SiOSiMe2H) (M H M H ) and polymers Me3Si(OSiMeH)nOSiMe3 (PHMS) based on these monomers are readily available. The compounds are potent reducing agents and, as Larson has noted, are both efficacious and inexpensive [14,15]. In organic synthesis, a benefit is the ease with which silicone products, after reduction, are readily separated from the desired organic product(s).
Tris(pentafluorophenyl)borane B(C6F5)3 (BCF), a metal-free, water-tolerant [16] and thermally stable (up to 270 °C) compound [17], is renowned for its effectiveness as a coinitiator for industrial olefin polymerization [18]. In his extensive and elegant studies of reduction of carbonyl groups, Piers showed that it was also a potent catalyst for hydrosilane reductions [17]. We used M H M H in the presence of BCF to reduce the sulfur crosslinks in automobile rubber tires, permitting reuse of the organic constituents [19]. When using HSiEt3 in the presence of BCF, Piers et al. originally reported that over-reduction of carbonyl groups led first to silyl ethers and then to alkanes plus disiloxanes. Rubinsztajn and Cella recognized that the Piers reduction was a new route to silicones, and that was first patented and then published in the open literature [20]. Chojnowski et al. have made several seminal contributions to our understanding of the mechanism of this process [21,22]. It is worth noting that growing optically active siloxanes from silanols, rather than alkoxysilanes, using B(C6F5)3 was pioneered by Kawakami [23].
In retrospect, perhaps we should have named the reaction the Piers-Chojnowski-Rubinsztajn-Kawakami (PCRK) reaction, rather than the PR reaction [24], and will do so for this paper.
The PCRK reaction is a particularly convenient route to synthesize silicone polymers. One simply chooses the number of alkoxysilane or silanol substituents, or water, required for the synthesis of a given linear or branched monomer, and then adds the appropriate mono-, di-or oligofunctional HSi-containing molecules in the presence of BCF [25]. It is thus possible to create linear polymers, including block-copolymers, simply by combining telechelic HSi + HOSi silicones or HSi silicones + water [26][27][28]. We have previously exploited this method to reliably introduce branches along linear silicone backbones [29] including, in the limit, highly branched dendron-like structures [30], including MDTQ resins (D = Me2SiO2/2) [31] (Figure 2). Hydrosilane monomers, particularly HMeSiCl 2 and HMe 2 SiCl, are produced in a direct process [13] in concentrations that typically exceed commercial needs. As a consequence, oligomers HMe 2 SiOSiMe 2 H) (M H M H ) and polymers Me 3 Si(OSiMeH) n OSiMe 3 (PHMS) based on these monomers are readily available. The compounds are potent reducing agents and, as Larson has noted, are both efficacious and inexpensive [14,15]. In organic synthesis, a benefit is the ease with which silicone products, after reduction, are readily separated from the desired organic product(s).
Tris(pentafluorophenyl)borane B(C 6 F 5 ) 3 (BCF), a metal-free, water-tolerant [16] and thermally stable (up to 270 • C) compound [17], is renowned for its effectiveness as a coinitiator for industrial olefin polymerization [18]. In his extensive and elegant studies of reduction of carbonyl groups, Piers showed that it was also a potent catalyst for hydrosilane reductions [17]. We used M H M H in the presence of BCF to reduce the sulfur crosslinks in automobile rubber tires, permitting reuse of the organic constituents [19]. When using HSiEt 3 in the presence of BCF, Piers et al. originally reported that over-reduction of carbonyl groups led first to silyl ethers and then to alkanes plus disiloxanes.
Rubinsztajn and Cella recognized that the Piers reduction was a new route to silicones, and that was first patented and then published in the open literature [20]. Chojnowski et al. have made several seminal contributions to our understanding of the mechanism of this process [21,22]. It is worth noting that growing optically active siloxanes from silanols, rather than alkoxysilanes, using B(C 6 F 5 ) 3 was pioneered by Kawakami [23]. In retrospect, perhaps we should have named the reaction the Piers-Chojnowski-Rubinsztajn-Kawakami (PCRK) reaction, rather than the PR reaction [24], and will do so for this paper.
The PCRK reaction is a particularly convenient route to synthesize silicone polymers. One simply chooses the number of alkoxysilane or silanol substituents, or water, required for the synthesis of a given linear or branched monomer, and then adds the appropriate mono-, di-or oligofunctional HSi-containing molecules in the presence of BCF [25]. It is thus possible to create linear polymers, including block-copolymers, simply by combining telechelic HSi + HOSi silicones or HSi silicones + water [26][27][28]. We have previously exploited this method to reliably introduce branches along linear silicone backbones [29] including, in the limit, highly branched dendron-like structures [30] Tetramethyldisiloxane (M H M H ) is an inexpensive, atom-efficient hydrosilane that has been selected as a silane source for a number of reactions [14,32,33]. We note that, at the time of writing, M H M H is slightly more expensive that non-functional D3 and much more expensive than D4 monomers. Attempts to produce high-molecular-weight PDMS oil from M H M H and H2O in aqueous media, a traditional PCRK reaction (mole ratio, [OH]/[SiH] = 56) was made by the group of Ganachaud who reported formation of an elastomer; it was concluded that the reaction was not readily controllable [32]. Chojnowski et al. showed under anhydrous Schlenk line conditions that the oligomerization of M H M H ( Figure 3H-J) in the presence of a BCF catalyst led to HSi-terminated oligomers and D3-Chojnowski metathesis [22], however, with large quantities of D3 that were produced M H DDM H ; secondary copolymerization of D3 and M H M H with activation with B(C6F5)3 could be used to lead to higher molecular weight polymers [33].
Neither the Ganachaud nor Chojnowski outcomes with M H M H matched our experience of HSi-terminated silicones in the presence of water and B(C6F5)3, which led smoothly to high-molecular-weight PDMS oils [26] (Figure 2). We hypothesized that differences arose because of the quantity of available water and BCF and, perhaps, other experimental conditions. Herein, we report a simple and mild process for the formation of HOSi-or HSi-terminated high-molecular-weight PDMS oil by the hydrolysis of M H M H in a kinetic process that generates relatively low quantities of D4 (<3%). However, dilution with good solvents for silicone enhances the fraction of D3 produced alongside the polymer. Tetramethyldisiloxane (M H M H ) is an inexpensive, atom-efficient hydrosilane that has been selected as a silane source for a number of reactions [14,32,33]. We note that, at the time of writing, M H M H is slightly more expensive that non-functional D 3 Figure 3H-J) in the presence of a BCF catalyst led to HSi-terminated oligomers and D 3 -Chojnowski metathesis [22], however, with large quantities of D 3 that were produced M H DDM H ; secondary copolymerization of D 3 and M H M H with activation with B(C 6 F 5 ) 3 could be used to lead to higher molecular weight polymers [33].
Neither the Ganachaud nor Chojnowski outcomes with M H M H matched our experience of HSi-terminated silicones in the presence of water and B(C 6 F 5 ) 3 , which led smoothly to high-molecular-weight PDMS oils [26] (Figure 2). We hypothesized that differences arose because of the quantity of available water and BCF and, perhaps, other experimental conditions. Herein, we report a simple and mild process for the formation of HOSi-or HSi-terminated high-molecular-weight PDMS oil by the hydrolysis of M H M H in a kinetic process that generates relatively low quantities of D 4 (<3%). However, dilution with good solvents for silicone enhances the fraction of D 3 produced alongside the polymer.

Results
The reaction between M H M H and water leads first to disiloxanol 1 and then to the tetrasiloxane formation 2 ( Figure 3A,B); both reactions are rapid and lead to the concomitant formation of hydrogen at the water droplet interface. Note: caution must be taken, as this can lead to a pressure spike of this flammable gas. Repetition of these processes lead to PDMS polymer initially terminated with SiH and then SiOH groups ( Figure 3C-E). The processes were followed using 1 H-NMR (particularly the relationship between integrated peak areas of Si-H (~4.7 ppm vs. Si-CH3 ~0.1-0.2 ppm [34,35])) and 29 Si-NMR spectra that allow clear differentiation between cyclic silicone monomers, linear silicone polymers and HSi-terminated linear materials [35], as well as gel permeation chromatography (GPC) and Fourier-transform infrared spectroscopy (FTIR; Supplementary Material, SM).
The objective of the research was to identify simple, efficient processes that would lead to linear PDMS, optionally terminated with SiH groups. Preliminary experiments

Results
The reaction between M H M H and water leads first to disiloxanol 1 and then to the tetrasiloxane formation 2 ( Figure 3A,B); both reactions are rapid and lead to the concomitant formation of hydrogen at the water droplet interface. Note: caution must be taken, as this can lead to a pressure spike of this flammable gas. Repetition of these processes lead to PDMS polymer initially terminated with SiH and then SiOH groups ( Figure 3C-E). The processes were followed using 1 H-NMR (particularly the relationship between integrated peak areas of Si-H (~4.7 ppm vs. Si-CH 3~0 .1-0.2 ppm [34,35])) and 29 Si-NMR spectra that allow clear differentiation between cyclic silicone monomers, linear silicone polymers and HSiterminated linear materials [35], as well as gel permeation chromatography (GPC) and Fourier-transform infrared spectroscopy (FTIR; Supplementary Material, SM).
The objective of the research was to identify simple, efficient processes that would lead to linear PDMS, optionally terminated with SiH groups. Preliminary experiments examined the impact of reaction parameters, including catalyst concentration, solvent and, in particular, the effect of homogeneity between water and silicone phases during the course of hydrolysis. In addition, the impact of "one-shot" addition of reagents was compared to a titration in which water and/or additional BCF were added when reaction ceased.
Simply mixing M H M H , B(C 6 F 5 ) 3 stock solution in dry toluene with bulk water led to linear silicones (entries 1-3, Table 1). Unsurprisingly, reactions were faster with more catalyst, but satisfactory rates were already achieved with only 0.02 mol% of this not inexpensive catalyst (<30 min, see below). Adding water in excess to the stoichiometric requirement did not lead to an improved outcome: cyclics were formed as a byproduct (see below, Supplementary Materials). Dilution with solvent, initially dichloromethane (containing 72.5 ppm water), demonstrated that much higher molecular weight polymers were accessible with even lower levels of catalyst (entries 4-6, Table 1); however, D 4 was also a byproduct of these reactions (Supplementary Materials). Thus, if high molecular weight is most desired, using small amounts of organic solvents is beneficial (entries 5, 6, Table 1); if suppression of cyclics is key, use slightly more BCF (entry 1, Table 1). Note that the use of a completely homogeneous reaction was disadvantageous for practical and kinetic reasons. For example, the reaction of 1 g of M H M H would require 14.3 mL of "wet" toluene (saturated with sufficient water to complete SiH hydrolysis), so scale up would not be practicable. When we did attempt the reaction on a small scale under these conditions, it proceeded for only a small extent over 5 h and then remained unchanged even after 24 h (see Supplementary Materials). examined the impact of reaction parameters, including catalyst concentration, solvent and, in particular, the effect of homogeneity between water and silicone phases during the course of hydrolysis. In addition, the impact of "one-shot" addition of reagents was compared to a titration in which water and/or additional BCF were added when reaction ceased.
Simply mixing M H M H , B(C6F5)3 stock solution in dry toluene with bulk water led to linear silicones (entries 1-3, Table 1). Unsurprisingly, reactions were faster with more catalyst, but satisfactory rates were already achieved with only 0.02 mol% of this not inexpensive catalyst (<30 min, see below). Adding water in excess to the stoichiometric requirement did not lead to an improved outcome: cyclics were formed as a byproduct (see below, SM). Dilution with solvent, initially dichloromethane (containing 72.5 ppm water), demonstrated that much higher molecular weight polymers were accessible with even lower levels of catalyst (entries 4-6, Table 1); however, D4 was also a byproduct of these reactions (SM). Thus, if high molecular weight is most desired, using small amounts of organic solvents is beneficial (entries 5, 6, Table 1); if suppression of cyclics is key, use slightly more BCF (entry 1, Table 1). Note that the use of a completely homogeneous reaction was disadvantageous for practical and kinetic reasons. For example, the reaction of 1g of M H M H would require 14.3 mL of "wet" toluene (saturated with sufficient water to complete SiH hydrolysis), so scale up would not be practicable. When we did attempt the reaction on a small scale under these conditions, it proceeded for only a small extent over 5 h and then remained unchanged even after 24 h (see SM).

High-Molecular-Weight PDMS Preparation Using Hydrolysis
The preparation of telechelic PDMS polymers from M H M H terminated with either SiOH or SiH groups was straightforward by adding water in a one-shot process in the presence of 0.02% B(C6F5)3. The two-phase reaction of water/silicone was surprisingly rapid; reaction times of less than 30 min led to polymers and concomitant formation of H2. The hydrolysis/condensation reactions in 50 wt% toluene were yet more rapid, as judged by the rate of build of the polymer molecular weight (entries 1-9,

High-Molecular-Weight PDMS Preparation Using Hydrolysis
The preparation of telechelic PDMS polymers from M H M H terminated with either SiOH or SiH groups was straightforward by adding water in a one-shot process in the presence of 0.02% B(C 6 F 5 ) 3 . The two-phase reaction of water/silicone was surprisingly rapid; reaction times of less than 30 min led to polymers and concomitant formation of H 2 . The hydrolysis/condensation reactions in 50 wt% toluene were yet more rapid, as judged by the rate of build of the polymer molecular weight (entries 1-9, Table 2). We have not determined if the hydrolytic processes in toluene/water involve only homogeneous or a mixture of homogeneous/heterogeneous steps. The efficiency of chain extension will decrease as the living polymer increases in size, particularly above the entanglement limit of about 29,000 g·mol −1 (note: in the literature, reported entanglement limits ranges from about 15,000-35,000 g·mol −1 . Here we use data from the seminal study of Mrozek et al. [36]). This was clearly observed here, as the final M n were 31-45 kg·mol −1 (entry 9, Table 2); complete consumption of SiH groups at higher conversion terminates polymerization by forming HOSi-terminated polymers. Re-initiation of such "dead" polymers is straightforward; however, addition of small quantities of M H M H caps the SiOH groups leading to dimerization or higher homologues of the existing polymer (entry 10, Table 2, Figure 3F). That is, a beneficial consequence of this process is that if HSi-terminated polymers are desired, one need only add excess M H M H to cap (Silicone-Me 2 SiOH → Silicone-(Me 2 SiO) 2 Me 2 SiH) and, if desired, grow the polymers before quenching the catalyst. This observation demonstrates that the process is living (entry 10, Table 2) [26]. Note that any residual water will compete for the silanol chain ends such that, if high molecular weights are desired, it is advantageous before capping to remove using distillation the small quantities of low molecular weight materials present, including residual water. determined if the hydrolytic processes in toluene/water involve only homogeneous or a mixture of homogeneous/heterogeneous steps. The efficiency of chain extension will decrease as the living polymer increases in size, particularly above the entanglement limit of about 29,000 g·mol −1 (note: in the literature, reported entanglement limits ranges from about 15,000-35,000 g·mol −1 . Here we use data from the seminal study of Mrozek et al. [36]). This was clearly observed here, as the final Mn were 31-45k g·mol −1 (entry 9, Table 2); complete consumption of SiH groups at higher conversion terminates polymerization by forming HOSi-terminated polymers. Re-initiation of such "dead" polymers is straightforward; however, addition of small quantities of M H M H caps the SiOH groups leading to dimerization or higher homologues of the existing polymer (entry 10, Table 2, Figure 3F). That is, a beneficial consequence of this process is that if HSi-terminated polymers are desired, one need only add excess M H M H to cap (Silicone-Me2SiOH → Silicone-(Me2SiO)2Me2SiH) and, if desired, grow the polymers before quenching the catalyst. This observation demonstrates that the process is living (entry 10, Table 2) [26]. Note that any residual water will compete for the silanol chain ends such that, if high molecular weights are desired, it is advantageous before capping to remove using distillation the small quantities of low molecular weight materials present, including residual water.  1  0  ---134  ---134   2  2  ---240  ---250   3  4  ---300  ---310  4  6  ---520 c  ---330  5  8  ---310  ---440  6  10  ---330  ---1030   7

Managing Cyclics
Our objective was to develop simple, practicable polymer syntheses that avoided the need for inert gas blankets; a septum with a bubbler was used to control pressure. One of the challenges presented by M H M H is its high volatility, which was problematic with or without solvents. The evolution of cyclics was followed by a combination of gravimetric analysis for volatile products (trapped in a cold trap that permitted cogenerated H 2 to escape) and, for the polymerization mixture, 29 Si-NMR, which is particularly sensitive to subtle differences in the chemical environment of D units; it is straightforward to distinguish D 3 Table 1).
The hydrolysis/condensation of M H M H in dilute, homogenous toluene solution was very slow. However, two-phase reactions (water/silicone or water/silicone+toluene) were very rapid reactions and completed in <30 min. Reactions can occur at the interface or within the organic fluid. In the case of the water/silicone mixture, the polymer yield was near 80%, with competing growth in D 3 and small amounts of D 5 ; D 4 was only inefficiently formed ( Figure 4A). By contrast, even with the small dilution provided in the 50% water/silicone+toluene system, much less polymer was formed at the expense of D 3 and D 5 production, again with little D 4 ( Figure 4B).  Table 1).
The hydrolysis/condensation of M H M H in dilute, homogenous toluene solution was very slow. However, two-phase reactions (water/silicone or water/silicone+toluene) were very rapid reactions and completed in <30 min. Reactions can occur at the interface or within the organic fluid. In the case of the water/silicone mixture, the polymer yield was near 80%, with competing growth in D3 and small amounts of D5; D4 was only inefficiently formed ( Figure 4A). By contrast, even with the small dilution provided in the 50% water/silicone+toluene system, much less polymer was formed at the expense of D3 and D5 production, again with little D4 ( Figure 4B).  Cyclic monomers are important starting materials for silicone synthesis. As noted above, the ring strain in D3 makes it an attractive starting material for ring-opening polymerizations. On the other hand, there is an interest for a variety of reasons in making cyclic-free silicones, particularly from an inexpensive starting material like M H M H that polymerizes so rapidly. We had hoped to be able to fine tune the polymerization to create high molecular polymers in the absence of cyclics.
After dimerization, M H M H → M H DDM H 2, the tetramer can undergo hydrolysis to give 3 then further chain extension, or cyclization to give D4 ( Figure 3D vs. G). The generation of D4 during polymerization under equilibrating conditions is favored both enthalpically and entropically, possessing virtually zero ring strain, and the SiO bond strength in cyclics is similar to those in linear chains and larger number of molecules than in linear polymers [1]. In neither of the cases examined was D4 a significant product ( Figure 4, Table 1). Thus, under these conditions of chain extension, 3 outcompetes cyclization ( Figure 3D,K vs. G). Adding small amounts of toluene led to an increase in D4 product only from 1% to 3%.
More problematic, with respect to cyclics, was the formation of D3 and, to a lesser extent, D5. The most concentrated (neat) solution produced 13% D3, but in the diluted sample 33% of the product mixture was D3, in addition to linear polymer (entries 7,8, Table 1). These data show that, even in the presence of water, the Chojnowski metathesis to give D3 and Me2SiH2 from 2 is highly competitive with hydrolysis ( Figure 3H). The presence of D5 in such high quantities is consistent with chain extension from 3 to 4 and then a different  Cyclic monomers are important starting materials for silicone synthesis. As noted above, the ring strain in D 3 makes it an attractive starting material for ring-opening polymerizations. On the other hand, there is an interest for a variety of reasons in making cyclic-free silicones, particularly from an inexpensive starting material like M H M H that polymerizes so rapidly. We had hoped to be able to fine tune the polymerization to create high molecular polymers in the absence of cyclics.
After dimerization, M H M H → M H DDM H 2, the tetramer can undergo hydrolysis to give 3 then further chain extension, or cyclization to give D 4 ( Figure 3D vs. Figure 3G). The generation of D 4 during polymerization under equilibrating conditions is favored both enthalpically and entropically, possessing virtually zero ring strain, and the SiO bond strength in cyclics is similar to those in linear chains and larger number of molecules than in linear polymers [1]. In neither of the cases examined was D 4 a significant product (Figure 4, Table 1). Thus, under these conditions of chain extension, 3 outcompetes cyclization ( Figure 3D,K vs. Figure 3G). Adding small amounts of toluene led to an increase in D 4 product only from 1% to 3%.
More problematic, with respect to cyclics, was the formation of D 3 and, to a lesser extent, D 5 . The most concentrated (neat) solution produced 13% D 3 , but in the diluted sample 33% of the product mixture was D 3 , in addition to linear polymer (entries 7,8, Table 1). These data show that, even in the presence of water, the Chojnowski metathesis to give D 3 and Me 2 SiH 2 from 2 is highly competitive with hydrolysis ( Figure 3H). The presence of D 5 in such high quantities is consistent with chain extension from 3 to 4 and then a different Chojnowski metathesis leading to D 5 and Me 2 SiH 2 (e.g., Figure 3K,L. We thank a referee for this suggestion). It is believed that the formation of M H DM H , found in the cold trap, can be ascribed to reactions with Me 2 SiH 2 with disiloxanes in the presence of B(C 6 F 5 ) 3 ( Figure 3M).
Previous experience with polymer chain extension of HSi-telechelic polymers with water did not lead to the formation of cyclics. Under these conditions with low catalyst concentrations (0.02%) (BCF/H 2 O and B(C 6 F 5 ) 3 ·OH 2 [38]), redistribution reactions are similarly not efficient. While the reactions here were complete in 30 min, there was no change in the cyclics profile between 30-180 min (SM); redistribution would favor the formation of D 4 which, of the cyclics characterized, was formed in the lowest concentration. Under equilibration in the absence of solvents, the normal concentration of D 4 is~15% [4]. Similarly, redistribution cannot explain the higher concentrations of D 3 and D 5 .
The formation of cyclics vs. linears is therefore a consequence of the kinetics of intravs. intermolecular reactions within a silicone/organic solvent, or at the water interface. When done in 50wt% toluene, more cyclics, particularly D 3 , were produced than when the only solvent for the silicone was M H M H and silicone products themselves, i.e., than when the system was more concentrated (Figure 4B vs. Figure 4A). This shows the power of Chojnowski metathesis. The conversion of 2 → D 3 (and 4 → D 5 ) effectively competes with hydrolysis of 2 → 3 when the reaction is slightly diluted, and is an important reaction even when done neat. The formation and then disappearance in the 29 Si-NMR of a maximum of 1.4% of Me 2 SiH 2 was observed based on D-units (for full details of all intermediates from Figure 4, see Supplementary Materials). M H DM H , observed in the cold trap, could arise from reaction of Me 2 SiH 2 with 3 and there may be additional homologation reactions that consume this reactive material. We cannot distinguish between polymerization involving hydrolysis/condensation ( Figure 3B-E) vs. the reaction between D 3 and hydrosiloxanes, as shown by Chojnowski et al. (Figure 3I,J) [33]. However, the rapid rate and high molecular weight support the hydrolytic/condensation process as dominant.
Molecular weight is, of course, an important determinant of polymer properties. One advantage of this process is the relatively narrow dispersities achieved in the higher molecular weight polymers (Table 2). They do not match dispersities of polymers formed from ring-opening polymerization of D 3 but, apart from the challenge of managing hydrogen generation, are easier to perform. In part, the objectives of the work were met. It was possible to obtain rapid polymerization of M H M H with water to get medium-molecular-weight linear silicones terminated, if desired, with SiH groups. High concentrations facilitated the rates of polymerization. The process is living and any premature suppression of polymerization by complete hydrolysis to SiOH groups can be overcome by the addition of small amounts of M H M H , preferably after removing any residual water, a process that allows much longer polymers to form. However, unlike the hydrolytic process with starting materials that have a higher MW (e.g., DP > 6) [26], polymerization competed with undesired cyclic formation. Distinctions between these in previous studies are mostly related to concentrations. The important precedent work of Ganachaud showed that with very high water and higher catalyst concentrations, complex equilibration reactions occur under less controlled conditions [32]. On the other hand, with exceptionally low concentrations of water and catalysts, Chojnowski et al. showed metathesis of 2 dominates the process; one can elect to capture and then use D 3 for polymerization [21,22]. This work demonstrates the power of Chojnowski metathesis. Even without a solvent, efficient formation of D 3 is observed, up to 33%, made worse even by small amounts of solvent.

Methods
1 H-and 29 Si-NMR spectra were recorded on a Bruker Advance 600 MHz nuclear magnetic resonance spectrometer using deuterated solvent chloroform-d (Milton, ON, Canada). Chromium(III) acetylacetonate (3 × 10 −3 mol·L −1 ) was added as a relaxation reagent for some of these measurements.
Gel permeation chromatography was carried out using a Viscotek GPC Max (VE 2001 GPC Solvent/Sample Module) (no longer manufactured). The system was equipped with a Viscotek VE 3580 RI Detector, a Viscotek 270 Dual Detector, and a PolyAnalytik SupeRes PAS-101 (8 mm Å~30 cm) column with a single pore, styrene-divinylbenzene gel, 6 µm particle size (London, Canada). It was additionally calibrated using a single narrow polydispersity polystyrene standard (93 kDa) from Polymer Laboratories. Toluene was used as the eluent at a flow rate of 1.0 mL·min −1 .
FTIR data were collected on a Nicolet 6700 FTIR using Thermo Electron's OMNIC software (version 8.0).
GC-MS analyses were performed using an Agilent 6890 N gas chromatograph (Santa Clara, CA, USA) equipped with a DB-17ht column (30 m × 0.25 mm i.d. × 0.15 µm film, J & W Scientific), and coupled to an Agilent 5973 MSD single quadruple mass spectrometer (Santa Clara, CA, USA). One microliter of sample was injected using Agilent 7683 autosampler (Santa Clara, CA, USA) using an injector temperature was 250 • C and a carrier gas (helium) flow of 0.8 mL/min. The transfer line was set to 280 • C and the MS source temperature was 230 • C. The column temperature started at 40 • C, raised to 70 • C at 5 • C/min, raised to 95 • C at 10 • C/min, raised to 300 • C at 40 • C/min, and was then held at 300 • C for 8 min for a total run time of 21.73 min. Full scan mass spectra between m/z 50 and 800 were acquired, with the MS detector turned off between 2.0-2.8 min for solvent. Please note that the sample is dissolved in propyl formate.
Water concentrations were determined using Karl Fischer titrations (Mettler Toledo DL39 Coulometer) (Columbus, OH, USA) with a one-component system containing a Hydranal Composite solution.

Experimental Procedures for High-Molecular-Weight PDMS Preparation Using Hydrolysis
The following paragraphs refer to entries in Table 1 (Table 1): To a pre-dried 100.0 mL round-bottomed flask we added tetramethyldisiloxane (M H M H ) (134 g·mol −1 , 4.12 g, 0.031 mol) and "wet" DCM (3 mL, 72.5 ppm water, 0.016 mmol of water present) which was then capped with a septum with a needle with a bubbler open to atmosphere to balance the pressure. The mixture was stirred for 5-10 min prior to the addition of B(C 6 F 5 ) 3 Table 1) was added. To the mixture B(C 6 F 5 ) 3 stock solution in dry toluene (0.1 M, 0.298 mL, 0.0298 mmol) was added. The cold trap condenser was submerged in dry ice/acetone with the addition of acetone and was directly connected to the round-bottomed flask through 6 mm O.D. Pyrex tubing. The mixture in the round bottle flask was stirred at room temperature for 3h and the reaction was quenched by alumina and the residual water droplet was removed by adding sodium sulfate with 5h quenching time. Before the work up procedures, the mixtures in the round bottle flask were weighed and characterized by 1 H-, 29 Si-NMR (SR) followed by filtration through Celite under reduced pressure. The same process was repeated for the experiment that also included toluene (entry 8, Table 1): M H M H (134 g·mol −1 , 10.32 g, 77.0 mmol) with distilled water (18 g·mol −1 , 1.34 mL, 74.4 mmol), dry toluene (6.73 mL, 7.76 g) and B(C 6 F 5 ) 3 stock solution in dry toluene (0.1 M, 0.298 mL, 0.0298 mmol).

Experimental Procedure for Controlled Growth of Linear PDMS Using M H M H and Water
The following paragraphs refer to Table 2. 3.4.1. In Neat Water, or in Water/Toluene Mixtures To a series of pre-dried 25.0 mL vials we added M H M H (134 g·mol −1 , 1.04 g, 7.8 mmol) with distilled water (18 g·mol −1 , 0.134 mL, 7.4 mmol). To the mixtures, B(C 6 F 5 ) 3 stock solution in dry toluene (0.1 M, 0.03 mL, 0.003 mmol, respectively) was added and stirred at room temperature. At time points from 2-180 min alumina was added to quench the reaction at different time intervals together with~0.5 g of sodium sulfate to remove the residual water. Before the work up procedures, the products were characterized by 1 H-, 29 Si-NMR (SR) followed by filtration through Celite under reduced pressure. The same process was repeated for the experiments that also included toluene: M H M H (134 g·mol −1 , 1.04 g, 7.8 mmol) with distilled water (18 g·mol −1 , 0.134 mL, 7.4 mmol), dry toluene (0.677 mL, 0.585 g) and B(C 6 F 5 ) 3 stock solution in dry toluene (0.1 M, 0.03 mL, 0.003 mmol), respectively, the mixture was then stirred at room temperature (entries 1-9, Table 2).

Chain Extension
To a 10.0 mL round-bottomed flask we added tetramethyldisiloxane (M H M H ) (134 g·mol −1 , 0.015 mL, 0.011 g, 0.085 mmol) with pre-prepared PDMS oil terminated with silanol (21,300 g·mol −1 , 0.90 g, 0.042 mmol, entry 4, Table 1). To be noted here, to the pre-prepared silanol PDMS we performed Kugelrohr distillation at 100 • C under vacuum (635 mmHg) for 60 min before running the GPC to remove the residual water and volatile small molecules. To the mixture, B(C 6 F 5 ) 3 stock solution in dry toluene (0.01 M, 0.0086 mL, 0.086 µmol) was added and stirred at room temperature. After 3 h, alumina was added to the mixture to quench the reaction. The product was collected by filtration through Celite under reduced pressure (Entry 10, Table 2).

Conclusions
The simple, efficient and mild process of hydrolyzing M H M H leads to high-molecularweight PDMS (maximum 153,900 g·mol −1 ) in a controllable and "living" manner. The reactions can be completed in as short a time as 30 min, in two-phase reactions using either bulk water with neat M H M H or M H M H in dry toluene (50 wt%). Under either set of conditions, however, low D 4 (1-3%) content is produced and higher values of D 5 (2-10%) and particularly D 3 (13-33%) are produced. These outcomes are attributed to the particular efficiency of Chojnowski metathesis in organic solvents, even with high water content.
Supplementary Materials: Figures S1 and S2: kinetic plots of reaction progress with and without toluene solvent, 1 H-NMR and 29 Si-NMR and IR characterization of each datapoint on Figure S1, Tables S1 and S2, GC and NMR data for the cold trap with and without toluene solvent (entries 7, 8, Table 1), NMR data for polymer (entry 10, Table 2).