聚合度Degree of polymerization

加聚物的重复单元就是单体单元，如聚乙烯 Mₙ = 280 000 g/mol 时 Xₙ = 10 000；缩聚物中一个重复单元包含两个单体残基（如尼龙 66 的 –CO(CH₂)₄CO–NH(CH₂)₆NH–），需注明按哪种单元计算。聚合度影响熔点、强度和溶液黏度：低聚物（Xₙ < 约 10–20）常为液体或蜡状，聚合度足够大时性能趋于稳定。

For addition polymers the repeat unit is the monomer unit, so polyethylene with Mₙ = 280,000 g/mol has Xₙ = 10,000. In condensation polymers one repeat unit contains two monomer residues, as in nylon 66 (–CO(CH₂)₄CO–NH(CH₂)₆NH–), so the basis must be stated. DP affects melting point, strength and solution viscosity: oligomers below about 10–20 units are often liquids or waxes, and properties level off once DP is large enough.

逐步聚合中卡罗瑟斯方程 Xₙ = 1/(1−p) 表明，反应程度 p 达到 99% 时 Xₙ 只有 100，因此必须精确等摩尔配比、高纯单体和高转化率。链式聚合中聚合度由动力学链长决定，即链增长速率与链终止、链转移速率之比：引发剂浓度越高、温度越高，聚合度越低；加入链转移剂（如硫醇）可调节分子量。活性聚合中 Xₙ = [单体]₀/[引发剂]₀ × 转化率。

In step-growth polymerisation the Carothers equation Xₙ = 1/(1−p) shows that even 99% conversion gives only Xₙ = 100, so exact stoichiometry, pure monomers and very high conversion are essential. In chain-growth polymerisation DP is set by the kinetic chain length, the ratio of propagation to termination and transfer rates: more initiator or higher temperature lowers DP, and chain-transfer agents such as thiols regulate it. In living polymerisation Xₙ = [monomer]₀/[initiator]₀ × conversion.

通过测 Mₙ（端基分析、渗透压法、凝胶渗透色谱）计算聚合度。纤维素工业常用黏度法测定聚合度，棉纤维约 10 000，再生纤维素约 300–500。

DP is calculated from Mₙ measured by end-group analysis, osmometry or gel permeation chromatography. The cellulose industry often uses viscometry: cotton cellulose has a DP of about 10,000 and regenerated cellulose about 300–500.