In 1901, F Kipping synthesized stable silane ketones for the first time; In 1943, Dow Corning achieved the industrial production of silicone for the first time. Due to its special physical and chemical properties, different forms of silicone materials (oil, rubber, gel and resin) have been widely used.
However, there are still the following issues with organic silicon synthesis:

1. The bottleneck issue in organic silicon synthesis is susceptibility to cyclic oligosiloxane contamination

Although significant progress has been made in the synthesis of organosilicon, a well-known bottleneck problem is the presence of systemic contamination of cyclic oligosiloxanes (approximately 10 to 15%) regardless of the method used.

2. Oligomeric siloxanes have potential toxicity, and solving their pollution problem is imperative

Currently, nearly 50% of new skincare products contain at least one type of organic silicon, and low molecular weight cyclic siloxanes can overcome the human skin barrier and have potential toxicity. Therefore, solving this pollution problem is crucial.

3. There is inevitable competition between the anti biting process at the end of polymer chains and the ring opening of cyclic monomers

Cyclic oligosiloxane contamination is usually caused by a reverse biting reaction at the end of the polymer chain, which competes with chain growth through ring opening polymerization (ROP) of cyclic monomers, is considered inevitable, and typically results in a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxane=85/15).

New ideas

In view of this, Tsuyoshi Kato et al. from the University of Toulouse found that the use of simple coordination between alcohols and anionic chain ends can prevent the back biting process of siloxane chains and avoid the generation of cyclic impurities. By carefully designed quaternary phosphine cations acting as self quenching systems, they respond to the loss of coordination alcohols, thereby preventing the reaction from proceeding before the ring back biting reaction begins. The combination of these two effects has been utilized to achieve thermodynamic control of ROP for octamembered siloxane ring D4, avoiding the generation of cyclic oligosiloxane byproducts.

Technical solution:

Summarized the research ideas of this work

The author elucidates the reasons for the formation of polymer/cyclic oligomer mixtures and proposes that the production of byproducts can be suppressed by inhibiting the anti biting process and changing the thermodynamic polymer/cyclic oligomer ratio.

2. The initiator was synthesized and its stability was evaluated

The author proposes to construct an automatic quenching system that responds to the loss of coordination alcohols to enhance the inhibition process of the anti biting reaction. An alcohol free and stable phosphoalkoxide 1-xBnOH was synthesized, and it was demonstrated that this substance has thermal instability and can be used as an intelligent self quenching system automatically triggered by coordination alcohol loss.

3. Explored the complete ROP process of eight membered siloxane rings

The author suggests that the use of a disiloxane blocking agent, 1,3-divinyltetramethylsiloxane (M2Vi), can achieve more precise polymer size control. The developed initiator based on polydimethylsiloxane (PDMS) and D4 can achieve an organic silicon yield of>99%.

Technical advantages:

Effective inhibition of the reverse biting process of siloxane chains was achieved through simple alcohols

The author reported that a simple coordination between alcohol and active silanol ester chain ends can prevent the biting process, and the use of quaternary phosphine cations can serve as an intelligent self quenching system to suppress the reaction before it begins.

2. Achieved nearly 100% ring opening polymerization of octamembered siloxane ring D4 for the first time

The author combined the effects of benzyl alcohol inhibiting the backbiting of siloxane chains and quaternary phosphine cations inhibiting the occurrence of backbiting reactions to achieve complete ring opening polymerization of eight membered siloxane rings.

Technical details

Research ideas

The author elucidates that the generation of thermodynamic controlled polymer/cyclic oligomer mixtures is mainly due to a competitive balance between the reverse biting rate at the end of the polymer chain and the growth through ring opening of cyclic monomers. Therefore, by suppressing the anti biting process and changing the thermodynamic polymer/cyclic oligomer ratio, the ROP of D4 can be achieved without producing cyclic oligomers. The author used a simple coordination between benzyl alcohol and active silanol ester chain ends to prevent the backbiting process, and carefully designed quaternary phosphine cations to prevent the occurrence of backbiting reactions. Ultimately, the thermodynamic control of anionic ROP in D4 was achieved, eliminating the generation of cyclic oligosiloxane pollutants.

Synthesis and stability evaluation of initiators

The author innovatively proposes to construct an automatic quenching system that responds to the loss of coordination alcohols to enhance the inhibition process of anti biting reactions. Using (isopropyl) tri (dialkylamino) phosphine alkoxy compound 1-xROH, this alkoxide forms alcohol coordinated stable alkoxide anions through weak hydrogen bonding interactions. Once the coordination alcohol is lost, the ion pairs at the end of the chain begin to react to form a neutral chain end, thereby quenching the polymerization process. The author synthesized 1-xBnOH, an alcohol-free and stable phosphoalkoxide salt, and the characterization results showed that the substance was thermally unstable. The stability of 1-xBnOH is closely related to the steric hindrance around the P atom, and the decomposition of single coordination quaternary phosphine alkoxides is intensified. It can be used as an intelligent self quenching system automatically triggered by coordination alcohol loss.

The Complete ROP Process of Eight membered Siloxane Rings

The author found that the size of the resulting polymer can be roughly adjusted by the amount of alcohol added. The author calculated through density functional theory of model reactions, indicating that the use of a disiloxane blocking agent 1,3-divinyltetramethylsiloxane (M2Vi) can achieve more precise polymer size control without losing the complete conversion characteristics of ROP. By changing the amount of M2 Vi relative to 1b-4BnOH, organosilicon polymers can be obtained with excellent yields (>99%). Furthermore, the author prepared an initiator 1b-4PDMSOH based on PDMS. In the presence of M2 Vi, D4 achieved a yield of>99% organic silicon with the ROP of the initiator, and had the expected polymer size. The results indicate that the silanol functional group (Si OH) in PDMS can also serve as a stabilizer for the anionic chain end to prevent the formation of cyclic oligomers. Therefore, the author believes that the complete ROP of D4 can actually be achieved.

expectation

In summary, this work avoids the backbiting process of siloxane chains through the coordination of alcohols with anionic chain ends, and utilizes quaternary phosphine cations to suppress the occurrence of ring backbiting reactions, achieving thermodynamic control of ROP for octamembered siloxane ring D4 and avoiding the generation of cyclic oligosiloxane by-products.

Source: Tongxin Weimin (Xueyanhui Technology Center)

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Original title: "Science: Breaking the Organosilicon Neck Problem