1.3.1　Condensation Reactions

Condensation processes take place when two or more molecules react with each other with the concurrent（同时存在的）loss of water，ammonia，or other small molecules such as methanol.This type of reaction is used as a basis for the synthesis of many important polymers，such as Nylon，polyesters（聚酯），phenol-formaldehyde（酚醛树脂），and urea-formaldehyde resins（脲醛树脂）.

Nylon is a condensation polymer（缩聚物）formed by the reaction of a diamine（二胺）with a dicarboxylic acid（二元羧酸）.In particular，Nylon-66 is made by the condensation of hexamethylenediamine（己二胺）with adipic acid（己二酸）.

Polyesters，such as dacron Terylene，or Mylar，are made by the condensation of a dicarboxylic acid with a diol（二元醇）.

Urea formaldehyde resins（脲醛树脂）are formed by the "methylolation" reaction（甲基化反应）between urea and formaldehyde，and condensation of the methylol units（羟甲基单元）yields the polymer：

Prepolymer by the "methylolation" reaction：

Dimer：

Trimer：

Tetramer：

Pentamer：

Hardening：