You are asking about polymers made by radical polymerization.
But in the monomers at the very ends of a polymer chain, there is an unfilled bond, a carbon atom with only three bonds filled.
During polymerization this is true, the radical will continue to react with monomer and propagate the polymerization, but the radical does not sit in the bulk. There are a number of ways which an additional atom amy bond with the carbon radical in a process called termination. Typically this is done by combination or inhibition.
Combination is the process where two radicals come together to form a bond leaving no free radicals to continue the reaction. This may occur when two radical chains combine or when a selected radical is added to cease the reaction. This radical could be the other half of the initiator which began the radical polymerization.
Combination of two polymerizing chains.
Combination of polymerizing chain with radical
For inhibition, a radical combines with a molecule that can form a stable radical such as oxygen or butylated hydroxytoluene (BHT).
Radical chain combining with oxygen to form stable, non-ractive radical.
In these cases the radical will eventually find a stray radical or atom to bond with (like water), the unstable carbon radical will not exist permanently.
I say this because I don't know if there are any polymers made where the bond inside the original monomer is a triple bond. In all the commonly used plastics (those that have recycling symbols) the original monomers have double bonds between the carbon atoms.
Polyacetylene uses a triply bonded monomer in its production.