**1. Introduction**

Polydimethylsiloxane (PDMS) oils are the parent commercial silicone polymers from which almost all other silicones are derived. PDMS possesses unique properties including low Tg, 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 D4 (D = ~Me2SiO~) [3,4] (Figure 1B). Cyclooligosiloxanes, particularly D4, have attracted different levels of concerns by regulatory agencies because of their purported environmental behaviors [5,6]; D4 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 [10–12] (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 D3 is utilized as a starting material instead of D4.

**Citation:** Liao, M.; Chen, Y.; Brook, M.A. When Attempting Chain Extension, Even Without Solvent, It Is Not Possible to Avoid Chojnowski Metathesis Giving D3. *Molecules* **2021**, *26*, 231. https://doi.org/10.3390/ molecules26010231

Academic Editors: Sławomir Rubinsztajn, Marek Cypry and Wlodzimierz Stanczyk Received: 11 December 2020

Accepted: 31 December 2020 Published: 5 January 2021

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**Figure 1.** Traditional routes to silicone polymers: (**A**) silanol condensation; (**B**) redistribution; (**C**) ringopening polymerization.

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) (MHMH) 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 MHMH 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 [26–28]. 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).

**Figure 2.** (**A**) Polymerization of telechelic HSi-silicones and water to give block copolymers. (**B**) Formation of highly branched silicones using the PCRK (Piers–Chojnowski–Rubinsztajn–Kawakami) reaction.

Tetramethyldisiloxane (MHMH) 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, MHMH is slightly more expensive that non-functional D3 and much more expensive than D4 monomers. Attempts to produce high-molecular-weight PDMS oil from MHMH 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 MHMH (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 MHDDMH; secondary copolymerization of D3 and MHMH with activation with B(C6F5)3 could be used to lead to higher molecular weight polymers [33].

Neither the Ganachaud nor Chojnowski outcomes with MHMH 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 MHMH 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.

**Figure 3.** Proposed reactions for cyclooligosiloxane and polymer formation from MHMH. Hydrolysis of SiH compounds to give hydroxy-capped (**A**) dimer; (**C**) tetramer; (**D**,**E**) polymers. Chain extension to give HSi-terminated (**B**) tetramer; (**F**) polymer; (**K**) hexamer. Chojnowski metathesis to give (**H**) D3; (**K**) D5; (**L**) Me2SiH2; (**G**) Cyclization to give D4. (**I**,**J**). Metathesis polymerization to higher polymers. (**M**) Chain homologation between silanols and Me2SiH2.
