Behind a liquid-silicone-rubber keypad, a medical seal, a baby bottle nipple, and a precision molded gasket is the same two-part chemistry, and the polymer that does the crosslinking is vinyl-terminated PDMS. It is the reactive backbone of addition-cure silicone rubber, the system that cures cleanly with no byproducts and is molded into millions of parts a day.
The short version: vinyl-terminated PDMS, or vinyl PDMS, is polydimethylsiloxane with reactive vinyl groups at the ends of the chain. It is the base polymer for addition-cure (platinum-catalyzed) silicone rubber, including liquid silicone rubber (LSR), addition-cure RTV-2, and high-consistency rubber. The vinyl groups react with a methylhydrogen siloxane crosslinker under a platinum catalyst to build the cured elastomer network, and because that reaction produces no byproducts, the rubber cures fast, deep, and dimensionally stable. Its vinyl content and viscosity set the properties of the cured rubber.
What vinyl-terminated PDMS is
Vinyl-terminated PDMS is a polydimethylsiloxane chain capped at each end with a vinyl group (a reactive carbon-carbon double bond). On its own it is a clear, flowable fluid; its job is to be the crosslinkable backbone that turns into a rubber when it reacts. It is supplied across a range of viscosities and vinyl contents, which is how the cured rubber’s hardness and properties are dialed in.
The addition cure: how it becomes rubber
Addition-cure silicone uses a reaction called hydrosilylation. The vinyl groups on the vinyl-terminated PDMS react with silicon-hydrogen (Si-H) groups on a methylhydrogen siloxane crosslinker, catalyzed by a platinum complex. Each reaction links two chains, and as it repeats the fluid builds into a crosslinked elastomer network.
The defining feature is what it does not make: the reaction produces no byproducts. Condensation-cure silicones, by contrast, release a small molecule such as an alcohol or acetic acid as they cure. No byproduct means addition-cure silicone cures quickly even in thick sections, does not shrink from outgassing, and holds tight dimensional tolerances, which is exactly what high-volume molding needs.
Why vinyl content and viscosity matter
The cured rubber’s properties are set before cure, by the base polymer. Higher vinyl content and shorter chains give more crosslink sites and a harder, stiffer rubber; lower vinyl content and longer chains give a softer, more extensible one. Selecting the vinyl-terminated PDMS grade, its viscosity and vinyl level, is how a formulator targets a durometer and a property profile.
The system around it
Vinyl-terminated PDMS is the backbone of a formulated system, not a standalone product in use. A typical addition-cure silicone combines the vinyl-PDMS base, a methylhydrogen siloxane crosslinker, a platinum catalyst, a cure inhibitor to control pot life, and a reinforcing filler such as fumed silica for strength. The base polymer determines the network; the other components control cure and reinforcement.
Where it is used
| Product type | Use |
|---|---|
| Liquid silicone rubber (LSR) | High-volume injection molding of precision elastomer parts |
| Addition-cure RTV-2 | Mold-making, encapsulation, and casting |
| Medical and food-contact silicone | Qualified grades for devices, seals, and bakeware |
| Seals, gaskets, and keypads | Durable, stable elastomer components |
| Encapsulants and potting | Clean-curing protection for electronics |
A note on cure types
If a silicone rubber crosslinks through vinyl-terminated PDMS and a Si-H crosslinker under platinum, it is addition cure. If it crosslinks through silanol-terminated PDMS and releases a byproduct, it is condensation cure. The two are not interchangeable, and the base polymer is what distinguishes them. The PDMS fundamentals are in what is silicone oil.
Buying vinyl-terminated PDMS in bulk
RawSource supplies Vinyl-Terminated Polydimethylsiloxane (Vinyl PDMS) across viscosity and vinyl-content grades for plastics and polymer addition-cure silicone rubber, LSR, RTV-2, and encapsulant formulation, in drums, IBC totes, and pallets, with CoA documentation. Tell us your target durometer, cure system, and application, and request a sample to validate cure and properties on your own formulation.
Frequently asked questions
What is vinyl-terminated PDMS used for?
It is the base polymer for addition-cure (platinum-catalyzed) silicone rubber: liquid silicone rubber (LSR), addition-cure RTV-2, and high-consistency rubber. Its vinyl groups crosslink with a methylhydrogen siloxane under platinum to form the cured elastomer.
What is the difference between addition cure and condensation cure silicone?
Addition cure uses vinyl-terminated PDMS plus a Si-H crosslinker and a platinum catalyst and produces no byproducts, so it cures fast and deep with tight dimensional control. Condensation cure uses silanol-terminated PDMS and releases a small-molecule byproduct as it cures.
Why does addition-cure silicone produce no byproducts?
Because the cure is a hydrosilylation addition reaction: the vinyl group and the Si-H group simply join, with nothing released. That is why addition-cure silicone does not shrink from outgassing and cures well in thick sections.
How does vinyl content affect the cured rubber?
More vinyl content and shorter chains mean more crosslink sites and a harder, stiffer rubber; less vinyl and longer chains give a softer, more extensible rubber. The grade is chosen to target the durometer and properties.
Is vinyl-terminated PDMS the same as liquid silicone rubber?
No. Vinyl-terminated PDMS is the reactive base polymer; liquid silicone rubber (LSR) is the complete formulated, two-part system built on it, including the crosslinker, platinum catalyst, inhibitor, and filler.
Editorial note. This article is general technical guidance for silicone-rubber and polymer professionals. Vinyl-terminated PDMS here is an industrial base polymer; medical, food-contact, and other regulated uses require appropriately qualified grades and are the buyer’s responsibility, and nothing here is a medical or efficacy claim. Cure chemistry and property guidance is general and must be validated on your own formulation; the Certificate of Analysis governs the material you buy. The material is non-toxic but environmentally persistent, not a biodegradable or “green” product. Always consult the current Safety Data Sheet (SDS) before handling. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
