Anionic Ring-opening Polymerization Of Octamethylcyclotetrasiloxane Research Articles

From ultra-high molecular weight polydimethylsiloxane to super-soft elastomer

A polydimethylsiloxane-α,ω-diol (UHMW-PDMS) with unusually high molecular weight (Mw = 1,395,000 g/mol) and narrow polydispersity (PDI = 1.49), free of monomer or catalyst traces, was obtained in high yield by a scalable process, easy to achieve (bulk anionic ring-opening polymerization of octamethylcyclotetrasiloxane catalyzed by tetramethylammonium hydroxide). Spectral and thermogravimetric analysis confirmed the theoretical structure, purity and thermal stability of the polymer. Individual or aggregated macromolecules were assessed by dynamic light scattering (DLS) and visualized by microscopic methods (TEM, AFM). The mechanical, dielectric, and thermal performances of application interest for this polymer were evaluated on samples of chemically defined elastomer cUHMW-PDMS prepared by condensation crosslinking through the polymer chain ends with tetraethylorthosilicate. The results were compared with those obtained on a lower molecular weight (Mw = 720,100 g/mol, PDI = 1.87) homologue (PDMS) prepared and processed in identical conditions, but also with a commercial silicone elastomer. UHMW-PDMS-based elastomer shows superior results on elastic recovery and stress decay at high elongation, as compared with reference samples.

Polymerization of octamethylcyclotetrasiloxane between montmorillonite nanoplatelets initiated by surface anions

Research deals with the assessment of grafting of cyclosiloxanes onto organically modified montmorillonite (MMT) layers through anionic ring-opening polymerization (AROP) without additional catalyst or initiator. Pristine Na+-MMT was organically modified by systematically varying the hexadecyltrimethylammonium bromide (HDTMABr) loading level. X-ray diffraction and thermogravimetric analysis of differently modified MMTs (OMMTs) showed that HDTMABr cations that exchanged sodium cations were strongly attracted to the silicate platelet surfaces. Part of HDTMABr formed a sandwiched HDTMABr layer between organically modified silicate platelets. The obtained OMMTs were tested as initiator material for AROP of octamethylcyclotetrasiloxane (D4) in the bulk, since the D4 polymerization may be initiated by oxygen anions located on the silicate surfaces. The highest monomer conversion (91 % after 24 h at 80 °C) was observed when OMMT with HDTMABr amount equal to cation exchange capacity was used. At lower amounts of HDTMABr, the MMT was less organically modified and thus less compatible with D4. On the other hand, when the HDTMABr sandwiched layer was thicker, lower rates of polymerization were observed due to the hindered transport of monomer to silicate platelet surfaces. The active function of surface anions in MMT platelets in the initiation process was confirmed experimentally by varying the OMMT/D4 ratio. It was observed that monomer conversion increased with increasing OMMT/D4 ratio. The obtained composites were exfoliated. In the last part of the research, OMMT was added to the formulation for D4 emulsion polymerization, where HDTMABr was also used as an emulsifier. The obtained composite material had a significantly improved thermal stability.

Synthesis Optimization of Novel Cross-Linked Polysiloxane with End-capped Epoxy Groups as Waterproofing Agent for Fabric

Novel cross-linked polysiloxane with end-capped epoxy groups as waterproofing agent for fabric was synthesized by hydrosilylation of polymethylhydrosiloxane and an epoxy groups-terminated polyvinylmethylsiloxane, which was previously prepared via anionic ring-opening polymerization of octamethylcyclotetrasiloxane, 2, 4, 6, 8-Tetramethyl-2, 4, 6, 8-tetravinylcyclotetrasiloxane, and epoxy group-terminated polydimethylsiloxane using tetramethylammonium hydroxide as a catalyst. Synthesis parameters were discussed in detail. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectra confirmed structure of the resultants. X-ray photoelectron spectroscopy analysis further verifies that the cross-linked polysiloxane film was indeed covered on cotton surface. Thermal stability of those polymers was improved with augment of the cross-linking degree.

Tailor-Made Fibrous Hydroxyapatite/Polydimethylsiloxane Composites: Insight into the Kinetics of Polymerization in the Presence of Filler and Structure–Property Relationship

A scrupulously manipulated rate acceleration of anionic ring-opening polymerization of octamethylcyclotetrasiloxane (D4), a precursor for polydimethylsiloxane (PDMS) was designed and executed using hydroxyapatite (HA) fiber surface coated with polyethyleneglycol (PEG) as the rate accelerator. Kinetics of polymerization was investigated through programmed FTIR analysis. In this paper, the beneficial role of HA as a polymerization rate promoter was investigated and hence established. In addition, PEG molecules on the filler surface acted as the activator in the polymerization reaction. Hence, a synergism of these two independent effects resulted in a momentous increase in the rate of polymerization. The rate of reaction was found to increase from 6.35 × 10–2 to 10.32 × 10–2 s–1 in the presence of 4 wt % of HA filler. A morphology–property relationship was established for these tailor-made composites to explore the diverse fields of application of these advanced materials. Although the in situ prepared sampl...

Semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion

The semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion was studied under the condition of no seed particles used in polymerization system. Concentration of emulsifier was set above critical micelle concentration. Effect of monomer feed rate on the chemical kinetics, average particle size and distribution and on the polymer molar mass and distribution was investigated. During monomer addition, polymerization rate was constant and controlled by the monomer feed rate. According to the proposed mechanism for anionic polymerization in emulsion, it was assumed that also the number of active polymerization sites and monomer concentration at the particle surface, where propagation occurs, remain constant. A continuous increase of average particle size and absence of extensive condensation reactions at high conversion suggested that the freshly added monomer diffuses toward hydrophobic polymer particles, where it is consumed in propagation reaction and/or accumulated in the particle core.

Anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion above critical micelle concentration

Batch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion using nonionic and cationic emulsifiers was studied. The concentration of emulsifiers was set above their critical micelle concentration. Effects of emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/initiator (KOH) ratio on the kinetics, average particle size and distribution and on the average molecular weight and distribution were investigated and discussed. At the beginning of the polymerization, empty micelles, active micelles (polymer particles) and monomer droplets co-exist in emulsion. The transport of monomer from monomer droplets toward empty micelles was confirmed by monomer droplets and empty micelles disappearance and by formation of smaller particles. The transport of monomer from monomer droplets toward polymer particles was not confirmed, since the average polymer particle size did not increase during polymerization. It was proposed, that at lower conversions, monomer diffuses from polymer particle interior to particle surface, while at higher conversions, the monomer diffuses from larger to smaller polymer particles. Emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/KOH ratio have an evident effect on the kinetics and on the average molecular weight, thus demonstrating that cationic emulsifier participates to the initiation reaction.

Synthesis of well-defined difunctional polydimethylsiloxane with an efficient dianionic initiator for ABA triblock copolymer

A new efficient dianionic initiator dipotassium diphenylsilanediolate (Ph2Si(OK)2) was synthesized by reacting diphenylsilanediol with potassium in the mixture of THF and benzene. The anionic ring-opening polymerization of octamethylcyclotetrasiloxane (D4) or tetramethyltetravinylcyclotetrasiloxane (D) initiated by Ph2Si(OK)2 was systematically investigated. A number of factors including the nature of initiators, promoters, and monomers, the molar ratio of promoter to initiator (Cp/Ci ratio) and monomer to initiator (Cm/Ci ratio) affect the polymerization remarkably. Using N-methyl-2-pyrrolidinone as the promoter, a series of well-defined difunctional HSi-terminated polydimethylsiloxanes (PDMSs) were prepared with high monomer conversion (> 90%) and relatively low polydispersity (< 1.6) at 30°C. By hydrosilation of the synthesized PDMS with ally end-capped poly(ethylene oxide) (PEO), amphiphilic PEO-b-PDMS-b-PEO triblock copolymers with low polydispersities and high block purities were obtained. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 153–159, 2007