Askeleittopolymerointi

Askeleittopolymerointi, known in English as step-growth polymerization, is a polymer formation mechanism in which polymer chains grow predominantly by reactions between functional groups on monomers, dimers, trimers, and larger oligomers. Unlike chain-growth polymerization, where growth proceeds mainly through successive addition at reactive chain ends, step-growth polymerization allows any two molecules with reactive groups to combine at any stage, leading to a gradual increase in molecular weight as polymerization proceeds. The most common forms are condensation (polycondensation) polymerizations, in which a small molecule such as water, methanol, or hydrogen chloride is released as the polymer chains join.

Typical monomers are difunctional or multifunctional; the average functionality determines the possibility of network formation and gelation. The Carothers equation relates the degree of polymerization to the extent of reaction in ideal conditions, indicating that achieving high molecular weight requires near-complete conversion. When functionality exceeds two or when multifunctional monomers promote crosslinking, gelation can occur, producing networked, insoluble polymers.

Common examples include polyesters and polyamides formed by condensation of diols with diacids or their derivatives, and polyurethanes from diols and diisocyanates. Conditions often require elevated temperatures and removal of the by-product to drive the reaction, and catalysts or catalytic systems can accelerate the process. The resulting polymers typically exhibit broader molecular weight distributions and can form thermoplastic polymers or thermosetting networks depending on functionality and stoichiometry.

Historically, step-growth theory was developed in the early 20th century, contrasting with chain-growth polymerization. Applications range from high-strength fibers and coatings to resins and networks, where controlled molecular weight and crosslinking are advantageous. Limitations include the need for careful stoichiometric balance and potential gelation that limits achievable molecular weight.