  We use a mixture of a polymer and its dimer to control dynamics in a manner
inspired by \emph{pinning} a fraction of the system. In our system of
$\alpha$-methyl styrene, where the polymer has a glass transition at higher
temperature than the dimer, at intermediate temperatures, the polymer acts to
"pin" the dimer. Within this temperature range, we use differential scanning
calorimetry to infer a point-to-set length which we find to be profoundly
influenced by the degree of pinning. We determine the dynamics of the system
with dielectric spectroscopy and find that while the dynamics are very
substantially slowed by the "pinning", the fragility exhibits only a small
change relative to the precision of our measurements. This may indicate that in
the approach we have used, fragility has a relatively weak dependence on
quantities such as the point--to--set length. % than one might expect, However,
an alternative explanation is that the dimer may act to \emph{plasticize} the
polymer and thus open routes to relaxation that may be inaccessible to fully
pinned systems.
