This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Sedis was the first with its Sedisport (five element) chain to
introduce swaged bushings, formed into the side plates, to replace
(six element) chains with full width steel bushings on which the
rollers and pins bear. Although stronger and lighter than prior
chains, the five element chain achieves its light weight at the
expense of durability. These chains, now the only derailleur chains
available, have only vestigial sleeves in the form of short collars on
the side plates to support the roller on the outside and the link pin
on the inside. This design is both lighter and stronger because the
side plates need not have the large hole for insertion of sleeves.
Pins inside full bushings of (six element) chains were well protected
against lubricant depletion because both ends were covered by closely
fitting side plates. Some motorcycle chains have O-ring seals at each
end. In the swaged bushing design there is no continuous tube because
the side plates are formed to support the roller and pin on a collar
with a substantial central gap. In the wet, lubricant is quickly
washed out of pin and roller and the smaller bearing area of the
swaged bushing for the pin and roller easily gall and bind when
lubrication fails. Although this is not a problem for this type of
chain when dry it has feet of clay in the wet.