  We provide a perspective on polymer glass formation, with an emphasis on
models in which the fluid entropy and collective particle motion dominate the
theoretical description and data analysis. We first discuss the dynamics of
liquids in the high temperature Arrhenius regime, where transition state theory
is formally applicable. We then summarize the evolution of the entropy theory
from a qualitative framework for organizing and interpreting
temperature-dependent viscosity data by Kauzmann to the formulation of a
hypothetical `ideal thermodynamic glass transition' by Gibbs and DiMarzio,
followed by seminal measurements linking entropy and relaxation by Bestul and
Chang and the Adam-Gibbs (AG) model of glass formation rationalizing the
observations of Bestul and Chang. These developments laid the groundwork for
the generalized entropy theory (GET), which merges an improved lattice model of
polymer thermodynamics accounting for molecular structural details and enabling
the analytic calculation of the configurational entropy with the AG model,
giving rise to a highly predictive model of the segmental structural relaxation
time of polymeric glass-forming liquids. The development of the GET has
occurred in parallel with the string model of glass formation in which concrete
realizations of the cooperatively rearranging regions are identified and
quantified for a wide range of polymeric and other glass-forming materials. The
string model has shown that many of the assumptions of AG are well supported by
simulations, while others are certainly not, giving rise to an entropy theory
of glass formation that is largely in accord with the GET. As the GET and
string models continue to be refined, these models progressively grow into a
more unified framework, and this Perspective reviews the present status of
development of this promising approach to the dynamics of polymeric
glass-forming liquids.
