Silicone Monomers — Methylchlorosilanes & DMC/D4
title: "Silicone Monomers — Methylchlorosilanes & DMC/D4" description: "How Rochow direct synthesis converts metallurgical silicon and methyl chloride into methylchlorosilanes, and how these are hydrolysed into DMC, D4, and other silicone building blocks." section: "upstream"
The Rochow Direct Synthesis
All silicone products trace back to a single reaction discovered by Eugene Rochow at GE in 1940:
Si + 2CH₃Cl → (CH₃)₂SiCl₂ (dimethyldichlorosilane, M2) + by-products
Metallurgical silicon powder (3303 or 2202 grade, <0.3% Fe to avoid catalyst poisoning) reacts with methyl chloride gas at 250–320°C over a Cu catalyst (with Zn, Al, Sn promoters) in a fluidised-bed reactor. The reaction is exothermic and continuous.
The crude product mix — called "direct synthesis crude" — is separated by fractional distillation into individual chlorosilane fractions:
| Monomer | Formula | Typical share | Role in silicone chain |
|---|---|---|---|
| Dimethyldichlorosilane (M2) | (CH₃)₂SiCl₂ | ~80–85% | Primary building block for PDMS, D4, DMC |
| Methyltrichlorosilane (M1) | CH₃SiCl₃ | ~5–8% | Cross-linking agent, MQ resin precursor |
| Trimethylchlorosilane (M3) | (CH₃)₃SiCl | ~3–5% | End-capper for silicone chains |
| Dimethylchlorosilane | (CH₃)₂SiHCl | ~2–3% | Hydrosilylation reactions |
| Other / residue | — | remainder | Recovered or recycled |
M2 is the pivotal monomer — its selectivity determines the economics of the whole plant.
Hydrolysis to DMC and D4
M2 is hydrolysed with water in a continuous process:
(CH₃)₂SiCl₂ + H₂O → [(CH₃)₂SiO]ₙ + HCl
The initial hydrolysis product is a mixture of linear and cyclic siloxanes called hydrolysate. This is equilibrated (cracked) at 150–160°C with KOH catalyst to yield DMC (dimethyl cyclosiloxane mixture, predominantly D3/D4/D5) and D4 (octamethylcyclotetrasiloxane) as primary intermediates for all downstream silicones.
D4 and DMC are polymerised with acid or base catalysts to give polydimethylsiloxane (PDMS) of any targeted molecular weight — from 1 cSt silicone fluids to high-consistency silicone rubber gum.
Major Producers
The silicone monomer value chain is dominated by a handful of vertically integrated producers:
- Dow (USA) — original Rochow licence; Zhangjiagang, China + US facilities
- Wacker Chemie (Germany) — Burghausen + Zhangjiagang
- Shin-Etsu Chemical (Japan) — Gunma + Naoetsu; world's largest
- Momentive (USA) — Waterford NY
- Hoshine Silicon (China, Xinjiang) — largest Chinese silicon + monomer producer
- Elkem Silicones (Norway/France)