Polymerisation (A2)
A-Level Chemistry Topic 35 7:58 English narration · English + 中文 subtitles burned in
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Look at three very different things. A nylon rope. A plastic drinks bottle. The protein in your own hair. They seem to have nothing in common. Yet all three are built the same way: two molecules meet, join at their ends, and throw out one tiny molecule — usually water. Do that ten thousand times and you have a fibre, a bottle, or living tissue.
看三样非常不同的东西：一根尼龙绳、一个塑料饮料瓶，还有你头发里的蛋白质。 它们看起来毫无共同之处，但三者的形成方式完全一样：两个分子相遇，在末端连接起来， 同时抛出一个很小的分子——通常是水。 重复一万次，你就得到一根纤维、一个瓶子，或者活的组织。
Welcome to condensation polymerisation.
欢迎来到缩合聚合。
We will build the two links the exam cares about, deduce a repeat unit, work backwards from a chain to its monomers, and see why these polymers can rot away while a plastic bag cannot.
我们将搭出考试最看重的两种键，推导重复单元， 再从一段长链反推出它的单体，最后看看为什么这些聚合物会腐烂降解，而塑料袋却不会。
Let's begin.
让我们开始吧。
Start with the definition — often a mark on its own.
先从定义开始——它本身常常就是一分。
In condensation polymerisation many monomers join into one long chain, and every new bond forms with loss of a small molecule.
在缩合聚合中，许多单体连接成一条长链， 而每形成一个新的键，都会脱去一个小分子。
Two conditions matter.
有两个条件很重要。
First, each monomer must carry two reactive groups, so the chain can grow at both ends.
第一，每个单体必须带有 两个反应基团，这样链才能向两端生长。
Second, something really is thrown away: usually water, sometimes hydrogen chloride.
第二，确实有东西被脱去：通常是水，有时是氯化氢。
That is the whole difference from addition.
这一点正是它与加成聚合的根本区别。
Here is the first link.
先看第一种键。
A carboxyl group meets a hydroxyl group.
一个羧基遇上一个羟基。
The carboxyl group gives up an oxygen and a hydrogen; the hydroxyl group gives up just its hydrogen; those pieces leave together as one molecule of water.
羧基交出一个氧和一个氢，羟基只交出它的氢， 这些部分一起以一个水分子的形式离开。
What is left behind is an ester link, and a polymer full of them is a polyester.
剩下来的就是酯键，而充满酯键的聚合物就是聚酯。
Use a dioyl chloride instead and the molecule lost is hydrogen chloride.
如果改用二酰氯，脱去的分子就是氯化氢。
The second link works the same way, but with an amine group.
第二种键的方式完全相同，只是把羟基换成氨基。
The carboxyl group again gives up an oxygen and a hydrogen, the amine group gives up one of its hydrogens, and water leaves again.
羧基同样交出一个氧和一个氢， 氨基交出它的一个氢，水又一次离开。
What remains is an amide link — a carbonyl carbon bonded straight to a nitrogen.
剩下的就是酰胺键——一个羰基碳直接连着一个氮。
Nylon is a polyamide. So is silk, and so is every protein in your body.
尼龙是聚酰胺，蚕丝也是，你体内的每一种蛋白质也都是。
Now the condition students forget. Why must each monomer have two reactive groups?
接下来是学生最容易忘的条件：为什么每个单体必须有两个反应基团？
Take ethanol and ethanoic acid: one hydroxyl, one carboxyl.
拿乙醇和乙酸来说：一个羟基，一个羧基。
They react, they make one ester, and then they stop — a dead end, not a polymer.
它们反应，生成一个酯，然后就停下了—— 这是死路一条，不是聚合物。
Give each molecule a second group at the other end, and every join leaves a free group at each end, so the chain can grow forever.
给每个分子的另一端再加一个基团，一切就不同了： 每次连接之后，两端仍然各留下一个空闲的基团，链就能一直生长下去。
So how do you make a polyester?
那么聚酯到底怎么做出来？
Three routes, and the exam names them precisely.
有三条路线，考试会精确地考它们的名称。
Route one: a diol, with two hydroxyl groups, plus a dicarboxylic acid, with two carboxyl groups; water is lost.
路线一：二醇（有两个羟基）加二羧酸（有两个羧基），脱去水。
Route two: the same diol with a dioyl chloride in place of the acid; now hydrogen chloride is lost.
路线二：同样的二醇，把酸换成二酰氯，这时脱去的是氯化氢。
Route three: a single hydroxycarboxylic acid, which reacts with itself.
路线三：单一的羟基羧酸，它能和自己反应。
Polyamides follow the same pattern, with amine groups doing the work.
聚酰胺遵循同样的模式，只是由氨基来完成连接。
Route one: a diamine, with two amine groups, plus a dicarboxylic acid or a dioyl chloride — that is how nylon is made.
路线一：二胺（有两个氨基）加二羧酸或二酰氯——尼龙就是这样做出来的。
Route two: a single aminocarboxylic acid, which reacts with itself.
路线二：单一的氨基羧酸，它能和自己反应。
Route three, the beautiful one: amino acids joining to each other.
路线三，也是最美的一条：氨基酸彼此连接。
Every protein alive is a natural polyamide.
所有活着的蛋白质都是天然的聚酰胺。
Now the repeat unit, the skill they test most.
接下来是考得最多的技能：推导重复单元。
An addition repeat unit came from one monomer; here it comes from two, so it is longer.
加成聚合的重复单元来自一种单体； 这里它来自两种，所以更长。
Three steps.
三个步骤。
One: line the two monomers up head to tail, with their groups facing.
第一：把两种单体首尾排好，让它们的基团相对。
Two: take away the small molecule at every new link.
第二：在每一个新形成的键处脱去那个小分子。
Three: join what is left, bracket it, write the letter n outside, and let a bond stick out through each bracket.
第三：把剩下的连起来，加上括号， 在括号外写上 n，并让一条键穿出每个括号。
Those are continuation bonds — the mark scheme looks for them.
那两条键叫做延伸键，评分标准会专门找它们。
Let's do one properly.
我们来认真做一道。
Ethanediol reacts with benzene dicarboxylic acid — the full names are on the screen.
乙二醇与对苯二甲酸反应——完整的名称写在屏幕上。
Draw the repeat unit and name the molecule lost.
画出重复单元，并说出脱去的分子。
Step one: line them up, hydroxyl facing carboxyl at both ends.
第一步：把它们排好，两端都让羟基对着羧基。
Step two: at each link the acid loses an oxygen and a hydrogen, the alcohol loses a hydrogen, so one water goes.
第二步：在每一个连接处， 酸失去一个氧和一个氢，醇失去一个氢，于是脱去一个水。
Step three: join what is left, bracket it, n outside, a bond crossing each bracket.
第三步：把剩下的连起来， 加括号，n 写在外面，并让一条键穿出每个括号。
That is the repeat unit — and it is PET, the plastic in every drinks bottle.
这就是重复单元——它就是 PET， 每个饮料瓶用的那种塑料。
Two waters are lost per unit.
每个重复单元脱去两个水。
Here is a real exam question.
这是一道真题。
You are given an amino acid — an amine group at one end, a carboxyl group at the other.
题目给你一个氨基酸——一端是氨基，另一端是羧基。
Draw three monomer residues of the polymer it forms, and mark one repeat unit.
要求画出该聚合物的三个单体残基，并标出一个重复单元。
The mark scheme wants four things.
评分标准要四样东西。
One: three residues, not two.
第一：三个残基，不是两个。
Two: every linkage correct, nitrogen bonded straight to the carbonyl carbon.
第二：每个连接都正确——氮直接连在羰基碳上。
Three: trailing bonds at both ends, showing the chain carries on.
第三：两端都有延伸键，表示链还在继续。
Four: one repeat unit marked, with the right atoms.
第四：清楚地标出一个重复单元，且原子正确。
Now go backwards — the commonest question of all.
现在反过来做——这是最常见的一类题。
You get a section of chain and must find the monomers.
给你一段链，要你找出单体。
First, find the links: an ester link joins a carbonyl carbon to an oxygen, an amide link to a nitrogen.
第一，先找出键：酯键把羰基碳连到一个氧上，酰胺键把它连到一个氮上。
Second, cut the chain at every link, not just the first.
第二，在每一个键处把链切开，不能只切第一个。
Third, hand the cut ends their atoms back: the carbon side takes a hydroxyl group, the other side a hydrogen.
第三，把原子还给切开的两端： 碳的一侧补上羟基，另一侧补上一个氢。
Out come a dicarboxylic acid and a diamine.
这样就得到一个二羧酸和一个二胺。
Then check: every monomer must have two functional groups.
然后检查：每个单体都必须有两个官能团。
Some questions just ask you to predict the type of polymerisation that happened, so learn the two tests.
有些题目只要求你预测发生的是哪一类聚合，所以要记住两个判断法。
Given the monomers: a carbon to carbon double bond means addition; two functional groups on each monomer means condensation.
给你单体时：有碳碳双键就是加成；每个单体有两个官能团就是缩合。
Given a piece of the polymer, look along the backbone.
如果给的是一段聚合物，就沿着主链看。
All carbon, with only side groups hanging off?
全是碳，只有侧基挂在旁边？
Addition.
那是加成。
Oxygen or nitrogen sitting inside the backbone?
主链里面夹着氧或氮？
Condensation.
那是缩合。
Now the environment question.
再来看环境问题。
Polyalkenes — poly(alkene)s — are chemically inert: the backbone is nothing but strong, non-polar carbon to carbon and carbon to hydrogen bonds, with no partly charged atom for water to attack.
聚烯烃——poly(alkene)s——在化学上是惰性的：它的主链全是很强的、 非极性的碳碳键和碳氢键，没有带部分电荷的原子可以让水去进攻。
So they are very hard to biodegrade and last for centuries.
所以它们很难被生物降解， 能存留几百年。
Polyesters and polyamides are different: their links carry a carbonyl carbon that is partly positive, so water can attack it.
聚酯和聚酰胺则不同：它们的键含有带部分正电的羰基碳， 水可以进攻它。
That makes them hydrolysable — more degradable.
这就使它们可水解——更易降解。
Some polymers even carry groups that light itself can break.
有些聚合物甚至带有能被光打断的基团。
Polyesters and polyamides break down by acid and alkaline hydrolysis — and the exam wants the right products.
聚酯和聚酰胺可以被酸性水解和碱性水解分解，而考试要的是正确的产物。
Hot acid splits a polyester back into the dicarboxylic acid and the diol.
热的酸把聚酯拆回二羧酸和二醇。
Hot alkali gives the diol and the salt of the acid, because any acid formed is neutralised at once.
热的碱给出二醇和酸的盐，因为生成的酸会立刻被中和。
Now the polyamide, where students slip.
再看聚酰胺，学生常在这里出错。
Hot acid gives the dicarboxylic acid, but the amine is protonated, so it leaves as an ammonium salt.
热的酸给出二羧酸，但氨基会被质子化， 所以它以铵盐的形式出来。
Hot alkali gives the free amine.
热的碱给出游离的胺。
Three marks students throw away.
三个学生常丢的分。
First, draw the continuation bonds.
第一，一定要画出延伸键。
The mark scheme asks for correct trailing bonds, and they must cross the brackets, with the letter n outside.
评分标准要求延伸键正确， 而且它们必须穿出括号，n 写在括号外面。
Second, always name the small molecule lost: water from the acid, hydrogen chloride from the dioyl chloride.
第二，一定要写出脱去的小分子： 用酸时是水，用二酰氯时是氯化氢。
Third, count the functional groups on every monomer you write.
第三，数一数你写的每个单体上的官能团。
Two groups, always.
永远是两个。
Get those right and this topic is yours.
做对这三点，这个专题就是你的了。