  The work discusses specifics of phase transitions for metastable states of
substances. The objects of condensed media physics are primarily equilibrium
states of substances with metastable phases viewed as an exception, while the
overwhelming majority of organic substances investigated in chemistry are
metastable. It turns out that at normal pressure many of simple molecular
compounds based on light elements (these include: most hydrocarbons; nitrogen
oxides, hydrates, and carbides; carbon oxide (CO); alcohols, glycerin etc) are
metastable substances too, i.e. they do not match the Gibbs' free energy
minimum for a given chemical composition. At moderate temperatures and
pressures, the phase transitions for given metastable phases throughout the
entire experimentally accessible time range are reversible with the equilibrium
thermodynamics laws obeyed. At sufficiently high pressures (1-10 GPa), most of
molecular phases irreversibly transform to more energy efficient polymerized
phases, both stable and metastable. These transformations are not consistent
with the equality of the Gibbs' free energies between the phases before and
after the transition, i.e. they are not phase transitions in "classical"
meaning. The resulting polymeric phases at normal pressure can exist at
temperatures above the melting one for the initial metastable molecular phase.
Striking examples of such polymers are polyethylene and a polymerized
modification of CO. Many of energy-intermediate polymeric phases can apparently
be synthesized by the "classical" chemistry techniques at normal pressure.
