2024年3月15日发(作者：)

As against this well-defined behavior of a simple chemical compound, a

polymer like polyethylene does not melt sharply at one particular temperature into

clean liquid. 与这类简单化合物明确的行为相比，像聚乙烯这样的聚合物不能在某一特定

的温度快速地熔融成纯净的液体。

Also, we can add a very large quantity of the polymer to the same quantity of

water without the saturation point ever being reached. 同样地，我们可以将大量的聚

合物加入到同样量的水中，不存在饱和点。

While linear polymers are important, they are not the only type of molecules

possible. 线状聚合物是很重要的,他们不是唯一可能类型的分子。

Substituent groups such as methyl or phenyl groups on the repeat units are

not considered branches. Branching is generally introduced into a molecule by

intentionally adding some monomer with the capability of serving as a branch. Let

us consider the formation of a polyester. The presence of difunctional acids and

difunctional alcohols allows the polymer chain to grow. These difunctional

molecules are incorporated into the chain with ester linkages at both ends of each.

Trifunctional acids or alcohols, on the other hand, produce a linear molecule by

reacting two of their functional groups. If the third reacts and the resulting chain

continues to grow, a branch has been introduced into the original chain.

Adventitious branching sometimes occurs as a result of an atom being abstracted

from the original linear molecule, with chain growth occurring from the resulting

active site. Molecules with this kind of accidental branching are generally still

called linear, although the presence of significant branching has profound effects

on some properties of the polymer, most notably the tendency to undergo

crystallization.

The polymerization is a chain reaction in two ways: because of the reaction

kinetic and because as a reaction product one obtains a chain molecule. 聚合反应

是链式反应的原因有两种：因为反应动力学和因为作为反应产物它是一种链式分子。

Thus, molecular weight increases slowly even at high levels of monomer

conversion, and it will continue to increase until the viscosity build-up makes it

mechanically too difficult to remove water of esterification or for reactive end

groups to find each other. 因此，聚合反应将以逐步的方式继续进行，其每一步酯化反

应的反应速率和反应机理均与初始单体的酯化作用相同。因此，分子量缓慢增加直至高水

平的单体转化率，而且分子量将继续增加直到粘度的增加使其难以除去酯化反应的水或难

以找到相互反应的端基。

If some monofunctional impurity is present, its reaction will limit the molecular

weight by rendering a chain end inactive. Similarly, high-purity monomers are

necessary in the A-B type of polycondensation and it follows that high-yield

reactions are the only practical ones for polymer formation, since side reactions

will upset the stoichiometric balance. 假如存在一些但官能团杂质，由于链的端基失活，

反应将使分子量减少。同样，在A-B类的缩聚反应中高纯度的单体是必要的，而且可以归

结高收率的反应仅是形成聚合物的实际反应，因为副反应会破坏当量平衡。

Chain termination takes place only through impurities, or through the addition

of certain compounds such as water, alcohols, acids, amines, or oxygen, and in