Polymers are formed by polymerization of monomers. Chemists describe a polymer by its degree of polymerization, molar mass distribution, tacticity, copolymer distribution, the degree of branching, by its end-groups, crosslinks, and crystallinity. Chemists also study a polymer's thermal properties such as its glass transition temperature and melting temperature. Polymers in solution have special characteristics for solubility, viscosity and gelation.