The preparation of poly(dimethylsiloxane)s having part of the siloxane units substituted with a model organic thioether was investigated. The sulfur group was generated by the ene−thiol addition to the vinyl function bound to silicon. Two synthetic routes to this copolymer were compared (i) one-step kinetically controlled polymerization of organosulfur-substituted hexamethylcyclotrisiloxane and (ii) two-step synthesis including kinetically controlled polymerization of 1,3,3,5,5-pentamethyl-1-vinylcyclotrisiloxane followed by the thiol addition to the vinyl group in the polymer. A cryptand−lithium silanolate complex generated from [(trimethylsilyl)methyl]lithium was selected as the initiator in this polymerization, while tert-butyl mercaptan was used as the hydrosulfidation reagent. It was shown that synthesis routes lead to a high yield of copolymers having a fairly regular chain structure. The arrangement of siloxane units in the polymer chain was controlled by the propagation step. Polymerization of the vinyl-substituted monomer exhibited a significant degree of regioselectivity as one of the three possible modes of the monomer ring opening contributed over 70% to the propagation. Addition of t-BuSH to the vinyl group in the polymer proceeded without rearrangement of the polymer chain and could be performed without any side reaction of the vinyl group. Model copolymers of regular alternating structure were synthesized by the heterofunctional polycondensation of [2-(tert-butylthio)ethyl]methyldichlorosilane with a series of diols, HO(Me2SiO) n H, n = 1−4. They were used for comparison with the copolymers obtained by ring-opening polymerization.

sBuLi and tBuLi have been combined with a new family of ligands, i.e. lithium silanolates, and used as initiators for the ligated anionic polymerization (LAP) of methyl methacrylate (MMA) in toluene at 0 °C. Compared to other known µ-ligands, the accordingly formed Bu(Me 2)SiOLi are more efficient stabilizers for the active centers. Under appropriate reaction conditions, a fast polymerization proceeds quantitatively without self-termination at 0 °C and leads to high molecular weight polymer of narrow molecular weight distribution (M h w/M h n < 1.2) and high isotacticity (90% isotactic triads).

No abstract

The mixed associated species {xsBuLi, (y − x)sBuMe2SiOLi} formed by reaction of sec-Butyllithium (sBuLi) with octamethylcyclotetrasiloxane (Me2SiO)4 in toluene have been used to initiate the anionic polymerization of methyl methacrylate (MMA) in that solvent at 0 °C. The active species generated by this new type of initiator have been characterized by 7Li NMR at 0 °C. The 7Li NMR spectra of sBuLi and sBuMe2SiOLi mixtures in toluene show that only one single type of {sBuLi, 5sBuMe2SiOLi} mixed species prevails at [sBuMe2SiOLi]/[sBuLi] molar ratios higher than ca. 21, as result of a large excess of {6sBuMe2SiOLi} species. The 7Li NMR has also concluded to the formation of {PMMALi, 5sBuMe2SiOLi} species upon the addition of MMA to that initiator solution, supporting the theory that only one type of ionic species is active during the MMA conversion. These active centers are stable for at least 1 h at 0 °C as confirmed by 7Li NMR.

It has been well-known for a very long time that, for anionic homo-polymerization or sequential copolymerization to occur, it is mandatory that the relative nucleophilic reactivity of the initiating species be equal to or larger than the one of the (co)monomer. However, the results described in this paper demonstrate that such a classic rule can be circumvented in some cases. Indeed, it is reported for the first time that potassium trimethylsilanolate, and “living” potassium poly(ethylene oxide), both of which are unable to initiate styrene or methyl methacrylate (MMA) polymerization, can be converted from oxyanionic active ends to silyl anionic ones by reacting with cyclic disila derivatives, so allowing the homopolymerization and sequential (co)polymerization of styrene or MMA to proceed in a controlled manner.