In chemistry, vinylidene means the divalent functional group =C=CH2 (ethenylidene) or >C=CH2 (1,1-ethenediyl), namely two carbon atoms connected to each other by a double bond, one of them bearing two hydrogens, and the other one connected to the rest of the molecule by a double bond or two single bonds.
Compounds containing this group (namely, with the general formula R=C=CH2 or RR'C=CH2) are called "vinylidenes". An example is 1,1-dichloroethene (CCl2=CH2) commonly called vinylidene chloride.
Vinylidene derivatives RR'C=CH2 can be polymerized through the double C=C bond, yielding linear polymers (−CRR'−CH2−)n with single-bonded backbones. For example, vinylidene chloride yields the plastic polyvinylidene chloride (PVDC) (−CCl2−CH2−)n. Likewise, vinylidene fluoride gives polyvinylidene fluoride (PVDF). Note that these vinylidene polymers are structurally different from those afforded by the vinylene derivatives, like polyvinylene fluoride from vinylene fluoride HFC=CHF. These polymers can be seen to include the vinyl derivatives as a special case where R' is hydrogen.
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