Laboratory synthetic methods are generally divided into two categories, step-growth polymerization and chain-growth polymerization.[13] The essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only,[14] such as in polyethylene, whereas in step-growth polymerization chains of monomers may combine with one another directly,[15] such as in polyester. However, some newer methods such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a catalyst. Laboratory synthesis of biopolymers, especially of proteins, is an area of intensive research.