Vinyl-Terminated PDMS (α,ω-Divinyl-PDMS, CAS 68083-19-2): The Reactive Base Polymer for Addition-Cure Silicone
SEMITECH vinyl-terminated PDMS is the reactive base polymer for addition-cure silicone — α,ω-divinyl-polydimethylsiloxane with vinyl groups on both chain ends, providing the unsaturation site that platinum catalysts use to crosslink with Si-H crosslinkers in LSR injection moulding, RTV-2 cast moulding, food-contact silicone, and medical-device elastomer. Multiple viscosity grades from 500 cP (low-modulus) to 60,000 cP (HCR-bridge gum) at ≥98% active vinyl. 200 kg drum / 1 t IBC.
Contents
| 68083-19-2 | 500–60,000 cP | 0.10–0.30% |
|---|---|---|
| CAS number | Viscosity range | Vinyl content |
Chemistry & Specifications
α,ω-divinyl-PDMS chain; clear colourless to pale-yellow liquid; vinyl content scaled with chain length.
Vinyl-terminated PDMS — α,ω-divinyl-polydimethylsiloxane, CAS 68083-19-2 — is a linear polysiloxane with the structure CH₂=CH-Si(CH₃)₂-O-[Si(CH₃)₂-O]ₙ-Si(CH₃)₂-CH=CH₂. The vinyl groups (CH=CH₂) on each chain end provide the reactive unsaturation site that Pt-catalysed hydrosilylation uses to form crosslinks with Si-H crosslinkers (methyl hydrogen silicone, polymethylhydrosiloxane). Standard commercial grades span 500 cP to 60,000 cP at 25°C, corresponding to chain lengths n = 100 to n = 1,500 silicon-oxygen repeat units, MW 7,000 to 100,000 g/mol, and vinyl content 0.30% (low-MW) to 0.10% (high-MW) — vinyl content scales inversely with chain length because the two end groups represent a smaller fraction of the total mass at higher MW.
SEMITECH stocks vinyl-terminated PDMS in five viscosity tiers: 500 cP (vinyl ~0.28%), 1,000 cP (~0.20%), 5,000 cP (~0.13%), 20,000 cP (~0.10%), 60,000 cP (~0.07%). All grades are supplied at ≥98% active polymer (residual cyclic siloxane and linear oligomer ≤2%), water content ≤500 ppm, and APHA colour ≤30. Manufacturing pathway: vinyl-terminated PDMS is produced by anionic ring-opening polymerisation of D4 (or DMC mixture) with a divinyl-disiloxane (1,3-divinyl-1,1,3,3-tetramethyldisiloxane, CAS 2627-95-4) chain-transfer agent — the divinyl chain-transfer agent caps the polymer at both ends with vinyl groups and controls molecular weight by the polymer:CTA ratio. Different viscosity grades come from different polymer:CTA ratios in the polymerisation reactor.
Pt-Catalysed Hydrosilylation Cure: Vinyl + Si-H Crosslinking
Vinyl group reacts with Si-H from crosslinker under Pt catalysis at 80–200°C; produces Si-CH₂-CH₂-Si network.
Vinyl-terminated PDMS forms the A-side of two-component addition-cure silicone systems. The vinyl groups are unreactive at room temperature without catalyst — the polymer is shelf-stable for years sealed. Cure begins when the A-side (vinyl-PDMS + filler + Pt catalyst) is mixed with the B-side (vinyl-PDMS + Si-H crosslinker like methyl hydrogen silicone + inhibitor) and heated. The Pt catalyst (Karstedt or Speier complex) coordinates to the vinyl group and the Si-H bond, then catalyses oxidative addition + migratory insertion + reductive elimination to form a new Si-CH₂-CH₂-Si bond between the polymer chain and the crosslinker. The net cure is a 3D network where each polymer chain is crosslinked at both ends through the crosslinker, with crosslink density determined by the Si-H:vinyl ratio (typically 1.0–1.5).
- 500 cP grade — low-modulus elastomer, optical encapsulation, skin-adhesive medical, soft-tissue prosthetics
- 1,000–5,000 cP — LSR injection moulding general-purpose; RTV-2 mould rubber soft-grade
- 5,000–20,000 cP — high-precision LSR moulded parts (medical, baby-care); RTV-2 medium-grade
- 20,000–60,000 cP — HCR-bridge gum, calendered sheet, high-modulus LSR for industrial sealing
Cure inhibitors (1-ethynyl-1-cyclohexanol, methylbutynol, 2-phenyl-3-butyn-2-ol) at 100–500 ppm are added to provide pot-life control — the inhibitor reversibly binds to the Pt catalyst at room temperature, preventing premature cure during mixing and metering, and dissociates at 80°C+ to allow normal cure on heating. Without inhibitor, a freshly-mixed Pt + vinyl-PDMS + Si-H system gels in 5–15 minutes at 23°C; with 200 ppm inhibitor, pot-life extends to 2–4 hours at 23°C with full cure at 150°C in 30 seconds. Pt poisoning is the dominant operational issue — sulphur, amines, organotin, phosphines, and certain heavy metals at ppm levels permanently deactivate the Pt catalyst and prevent cure. Production lines for addition-cure silicone must be fully segregated from any tin- or condensation-cure silicone production.
Applications: LSR Injection Moulding, RTV-2 Medical & Optical, Food-Contact, Skin Adhesive
LSR injection moulding (largest), RTV-2 medical/optical, food-contact silicone, optical encapsulation, skin-adhesive medical.
LSR (liquid silicone rubber) injection moulding is the largest single application — automotive sealing components, baby-care nipples and pacifiers, medical-device elastomers, kitchenware, electrical connectors. Vinyl-terminated PDMS at 60–80% on total formulation paired with reinforcing fumed silica, methyl hydrogen silicone crosslinker, Karstedt platinum, and inhibitor produces a 2-component LSR (A-side + B-side metered 1:1 by weight at the injection moulder) that cures in 10–30 seconds at 150–200°C tool temperature.
RTV-2 cast moulding silicone for prototype tooling, decorative casting, and food-contact moulds uses vinyl-PDMS at 70–85% with the same crosslinker and Pt catalyst — chosen over condensation-cure tin systems when faster cure, lower shrinkage, and food-contact compliance are required. Optical and electronic encapsulation uses high-clarity vinyl-PDMS (typically the 1,000–5,000 cP grade) at 80–90% to seal LED packages, photovoltaic cell encapsulants, and PCB conformal coatings — chloride-free Karstedt-cured chemistry is required for reliability under DC bias and humidity stress. Skin-adhesive silicone (medical wound dressings, scar treatment sheets, transdermal patches) uses low-MW vinyl-PDMS (500 cP) at 60–75% with partial Si-H loading to give a soft, tacky cured surface. Food-contact silicone (bakeware, baby bottle nipples, beverage tubing) uses vinyl-PDMS with FDA-compliant fumed silica and Karstedt — the absence of organotin and chloride byproducts makes Pt-cured silicone the only chemistry meeting most national food-contact regulations.
Procurement, Storage and Quality Control
CoA per shipment with vinyl content verification; 200 kg drum / 1 t IBC; 24-month shelf life; non-flammable.
SEMITECH issues a CoA on every batch with: vinyl content (FT-IR or H-NMR, target 0.07–0.30% per grade), viscosity at 25°C (Brookfield, target ±10% of grade specification), volatile content (D4/D5 cyclic siloxane residue, GC, target ≤2%), water content (Karl Fischer, target ≤500 ppm), and APHA colour (target ≤30). Standard packing 200 kg HDPE-lined steel drums; 1 t IBC totes for high-volume LSR producers; 25 t ISO-tank for bulk industrial customers (rare). MOQ 200 kg per grade. Lead time 1–2 weeks ex-Zhejiang to Asia ports for stocked grades, 4–6 weeks to Europe and North America after sea freight.
Storage: stable under ambient conditions; non-flammable (flash point >250°C for higher-MW grades, ~150°C for 500 cP grade); no nitrogen blanket required. Recommended: store sealed below 30°C in original packaging. Shelf life 24 months sealed; opened drums consumable within 12 months without significant vinyl-content drift. Pt-poisoning prevention is the critical handling discipline for all vinyl-PDMS used in addition-cure manufacturing: dedicate transfer equipment, hoses, and storage tanks exclusively to addition-cure silicone production; never use any equipment that has contacted condensation-cure (tin-catalysed) silicone, sulphur-vulcanised rubber, or organotin compounds. Cross-contamination at ppm levels is sufficient to deactivate downstream Pt catalysts and produce cure-failure on production batches. Health and regulatory: REACH-registered for industrial use; FDA 21 CFR 175.300 listed for indirect food-contact in adhesive components; USP Class VI compliance available on specific food-contact grade; mild skin and eye irritant; standard PPE — nitrile gloves, splash goggles. Vinyl-PDMS contains residual D4/D5 cyclic siloxane at 1–2% — see DMC and D4/D5 SVHC documentation for downstream regulatory implications.
Vinyl-terminated PDMS is the reactive base polymer for addition-cure silicone — α,ω-divinyl-PDMS at 500–60,000 cP grades. Pairs with methyl hydrogen silicone crosslinker + Karstedt Pt for LSR injection moulding (10–30 sec cycle), RTV-2 medical/optical (5–30 min cure), food-contact silicone, and skin-adhesive applications. Pt-poisoning prevention is mandatory — dedicated equipment, no cross-contamination with tin-cure systems.
Vinyl-Terminated PDMS Specification Sheet
SEMITECH stocked grades (500 / 1,000 / 5,000 / 20,000 / 60,000 cP); CoA per batch with vinyl-content verification.
| Grade | Viscosity (cP) | Vinyl % (target) | MW (typical) | Primary Application |
|---|---|---|---|---|
| VPDMS-500 | 500 ± 50 | 0.28 ± 0.02% | 7,000 g/mol | Skin-adhesive, optical, soft elastomer |
| VPDMS-1k | 1,000 ± 100 | 0.20 ± 0.02% | 15,000 g/mol | LSR general-purpose, RTV-2 soft |
| VPDMS-5k | 5,000 ± 500 | 0.13 ± 0.02% | 35,000 g/mol | LSR medical, RTV-2 medium |
| VPDMS-20k | 20,000 ± 2,000 | 0.10 ± 0.01% | 60,000 g/mol | LSR precision, RTV-2 high-modulus |
| VPDMS-60k | 60,000 ± 6,000 | 0.07 ± 0.01% | 100,000 g/mol | HCR-bridge gum, calendered sheet |
FAQ
+Which viscosity grade do I order for general-purpose LSR injection moulding?
Standard general-purpose LSR formulations use 1,000 cP or 5,000 cP vinyl-PDMS as the primary A-side polymer. The 1,000 cP grade gives easier dispensing through the LSR injection-moulder static-mixer (lower viscosity = lower pressure drop), suitable for thin-section parts (1–3 mm wall) and high-throughput production lines. The 5,000 cP grade gives better dimensional stability in thicker-section moulded parts (3–10 mm wall) and slightly higher cured-rubber tear strength. Many commercial LSR formulations blend the two grades (typically 60% 1,000 cP + 40% 5,000 cP) to balance processability and finished-part properties. For specialty applications (skin-adhesive, optical encapsulation), use lower viscosity (500 cP); for high-modulus or HCR-bridge applications, use higher viscosity (20,000 or 60,000 cP).
+How do I prevent Pt poisoning in my production line?
Pt-poisoning prevention is a complete production-line segregation discipline, not a single safeguard: (1) dedicated incoming-receipt area for addition-cure raw materials separate from tin-cure raw materials; (2) dedicated mixing tanks, hoses, transfer pumps, and metering equipment for vinyl-PDMS — never share with tin-catalysed silicone production; (3) dedicated personnel and PPE protocols — operators handling tin-cure silicone must change gloves and lab coats before transitioning to addition-cure work; (4) substrate-compatibility screening — any moulded part destined for over-mould assembly with PSA, EPDM, or sulphur-vulcanised rubber must be assessed for migration risk; (5) regular gel-time testing of incoming vinyl-PDMS batches with reference Pt catalyst — drift in gel time is the early warning of cross-contamination during transit or storage. Most production-scale Pt cure failures trace back to incomplete production-line segregation; investing in dedicated equipment is the durable solution.
+Can I substitute α,ω-divinyl-PDMS for vinyl-PDMS-with-vinyl-side-groups in my formulation?
Different mechanical properties result. α,ω-divinyl-PDMS (vinyl on chain ends only) gives a network where each polymer chain is crosslinked at both ends only — the chain itself acts as a flexible link between crosslink points, producing high elongation (300–600%) and lower modulus (1–5 MPa) elastomer typical of LSR and RTV-2 silicone. Vinyl-side-group PDMS (vinyl groups distributed along the chain at 1–5 mol% of the methyl positions) gives a network where the polymer is crosslinked at multiple points along the chain — produces higher modulus (5–15 MPa), lower elongation (100–300%), and stiffer elastomer typical of HCR silicone rubber. Both are valid commercial chemistries; the choice depends on target finished-part properties. SEMITECH stocks α,ω-divinyl-PDMS as the standard grade; vinyl-side-group PDMS is available as a make-to-order grade with 4–6 week lead time.