Octamethylcyclotetrasiloxane (D4)

D4 undergoes ring-opening equilibration polymerization (REP) under KOH or acid catalysis to yield linear PDMS chains. Molecular weight is controlled by the molar ratio of D4 to chain-stopper (hexamethyldisiloxane, MM): MW ≈ 74 × ([D4]/[MM] × 4 + 2). Low [D4]/[MM] gives low-viscosity fluids (5–100 cSt); high ratios give gum-grade PDMS (>10⁶ cSt). Temperature and catalyst level affect equilibration rate but not the equilibrium MW. Residual D4 content in finished PDMS must be ≤0.1% (EU SVHC threshold) for personal care applications, requiring stripping under vacuum at 180–200°C.