1.1.3　Dimers，Trimers and Oligomers

The polymerization of a monomer often occurs in a sequential（序列）manner.In other words，two monomer molecules first react together to form a dimer（二聚体）.The dimer may then react with a third monomer to yield a trimer（三聚体），and so on.Dimers are usually linear molecules，but trimers，tetramers（四聚体），pentamers（五聚体），and so on，can be linear or cyclic（环化的）.

The reactions outlined in the above scheme indicate the relationship between monomers，dimers，and trimers.Low molecular weight products obtained by polymerization，for example，dimers，trimers，tetramers，pentamers，and so on，which may be cyclic or linear，are known as oligomers（低聚体或低聚物）.Some care should be taken to avoid use of the term "polymer" to describe materials that are real oligomers because the two types of products have distinctly different properties.