Polymerisation
A-Level Chemistry Topic 20 7:24 English narration · English + 中文 subtitles burned in
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Transcript
Look around you. A drinks cup. A shopping bag.
看看你的周围：一个塑料杯、一个购物袋、一张塑料包装。
A plastic wrapper. All three are made of the same kind of thing: one enormous molecule, a chain that is tens of thousands of carbon atoms long.
这三样东西都由同一类物质构成—— 一个巨大的分子，一条长达几万个碳原子的链。
And here is the surprise. Every one of those giant chains was built from one small gas molecule, joined to itself again and again and again.
而令人惊讶的是：每一条这样的巨链， 都是由一种很小的气体分子，一次又一次地不断连接自己而形成的。
Learn how that joining works, and polymer questions become free marks.
学会这种连接方式，聚合物题目就是白送的分。
Welcome to polymerisation.
欢迎来到聚合反应。
We will build a polymer from its monomer, find the repeat unit, work backwards from a chain to the monomers that made it, and then face the problem of throwing plastics away.
我们将从单体出发搭出聚合物，找出重复单元， 再从一段长链反推出制造它的单体，最后面对塑料废弃物的处理难题。
Let's begin.
让我们开始吧。
Start with the definition, because the exam asks for it directly.
先从定义开始，因为考试会直接考它。
In addition polymerisation, many small molecules join together to make one very long chain.
在加成聚合中，许多小分子连接起来，形成一条很长的链。
Three parts matter.
有三点很重要。
First, the small molecule is called the monomer, and the long chain is the polymer.
第一，这种小分子叫做单体，形成的长链叫做聚合物。
Second, the monomer must be unsaturated: it has to contain a carbon to carbon double bond.
第二，单体必须是不饱和的：它必须含有碳碳双键。
Third, and this is the part students forget, no other product is made.
第三，也是学生最容易忘的一点—— 不生成任何其他产物。
Nothing is lost and nothing is given off. Every atom ends up in the chain.
没有东西损失，也没有东西放出，每个原子都进入了链中。
Watch it happen.
来看它是怎么发生的。
Each ethene molecule has a carbon to carbon double bond.
每个乙烯分子都有一个碳碳双键。
One of those two bonds opens, which leaves every carbon with a spare bond, free to grab a neighbour.
两条键中的一条打开， 于是每个碳原子都多出一个空余的键，可以去抓住旁边的分子。
The molecules link hand to hand, and the chain grows and grows.
分子手拉手连接起来， 链不断增长。
Thousands of ethene molecules later you have one molecule of polyethene, and nothing else has been produced.
几千个乙烯分子之后，你就得到一个聚乙烯分子，而且没有生成任何别的东西。
So why must the monomer be unsaturated?
那么，单体为什么必须是不饱和的？
Compare ethane and ethene.
把乙烷和乙烯比一比。
Ethane is saturated: its two carbons are held together by a single bond, and every other place is already taken by a hydrogen atom. It has nothing spare, so it cannot join up.
乙烷是饱和的： 两个碳原子之间只有一条单键，其余位置都已经被氢原子占满，没有任何空余， 所以它无法连接起来。
Ethene is different. Its double bond is really two bonds, and the weaker of the two breaks easily.
乙烯不一样：它的双键其实是两条键，其中较弱的那一条很容易断开。
When it opens, each carbon gains one free bond.
它一打开，每个碳原子就获得一个空余的键。
That is why every addition polymer starts from a monomer with a carbon to carbon double bond.
这就是为什么每一种加成聚合物， 都必须从含有碳碳双键的单体开始。
Now the skill the exam tests most: the repeat unit.
接下来是考试考得最多的技能：重复单元。
The repeat unit is the small part that is copied again and again along the chain. Three steps build it.
重复单元就是沿着链一次又一次被复制的那一小部分。
Take the monomer. Change the double bond into a single bond. Then draw one bond going out at each end, put brackets round it, and write the letter n outside.
三步就能画出来：先写出单体；把双键改成单键；然后在两端各画出一条键，加上括号， 并在括号外写上字母 n。
For ethene that gives polyethene — poly(ethene).
对乙烯来说，得到的就是聚乙烯——poly(ethene)。
And notice the atoms: the repeat unit has exactly the same atoms as the monomer.
注意原子：重复单元和单体含有完全相同的原子。
The same rule still works when the monomer carries a side group.
当单体带有侧基时，这条规则同样适用。
Chloroethene is ethene with one hydrogen swapped for a chlorine atom.
氯乙烯就是把乙烯的一个氢换成一个氯原子。
Open the double bond, draw a bond out at each end, and you get polychloroethene — poly(chloroethene), the plastic we call PVC.
打开双键，在两端各画一条键，就得到聚氯乙烯——poly(chloroethene)，也就是我们常说的 PVC。
The chlorine takes no part in the reaction. It simply comes along and hangs off the chain.
氯原子并不参与反应，它只是跟着过来，挂在链的旁边。
So only the two backbone carbons change, every single time.
所以每一次都只有主链上的那两个碳原子发生变化。
Let's do one properly.
我们来认真做一道。
Propene is ethene with a methyl group in place of one hydrogen. Find the repeat unit of the polymer it makes.
丙烯就是把乙烯的一个氢换成一个甲基，求它形成的聚合物的重复单元。
Step one: open the double bond, so those two carbons are now joined by a single bond.
第一步：打开双键，这两个碳原子现在用一条单键相连。
Step two: draw a bond out at each end and close the brackets.
第二步：在两端各画出一条键， 并把括号补上。
That gives you the repeat unit of polypropene — poly(propene).
这样就得到了聚丙烯——poly(propene)——的重复单元。
Now check it: three carbons and six hydrogens on both sides.
再检查一遍：两边都是三个碳、六个氢。
If the atoms do not match, you have made a mistake somewhere.
如果原子对不上，说明中间某一步做错了。
Now go the other way.
现在反过来做。
The exam gives you a piece of a polymer chain and asks for the monomer.
考试会给你一段聚合物链，让你写出单体。
Take one repeat unit only.
只取一个重复单元。
Rub out the two bonds that stick out at the ends.
把两端伸出去的两条键擦掉。
Then put the double bond back, and this is exactly where marks are lost: put it between the two carbons of the backbone, never inside a side group.
然后把双键放回去——失分往往就在这里： 双键要放在主链的两个碳之间，绝不能放进侧基里。
Two checks keep you safe. The monomer and the repeat unit must have the same molecular formula. And the monomer must be an alkene, with its double bond in the main chain.
两个检查能保你安全： 单体和重复单元的分子式必须相同；而且单体必须是烯烃，双键在主链上。
Here is a real exam question.
这是一道真题。
This section of chain was made from two different monomers, and you have to find them.
这一段链由两种不同的单体形成，你要把它们找出来。
First, ignore the side groups and count the backbone carbons: there are four of them.
第一，先不看侧基，数一数主链上的碳：一共四个。
Second, cut the backbone into pairs, because every monomer gives exactly two carbons to the chain. That leaves two pieces.
第二，把主链按两个一组切开， 因为每个单体正好给链贡献两个碳，这样就得到两段。
Third, put a double bond back inside each pair.
第三，在每一组里把双键放回去。
The first piece carries two methyl groups, so it came from methylpropene. The second carries only one, so it came from propene.
第一段带两个甲基，所以它来自甲基丙烯；第二段只带一个，所以它来自丙烯。
One chain, two monomers.
一条链，两种单体。
Polymers are useful. The trouble starts when we throw them away.
聚合物很有用，麻烦出在丢弃的时候。
Polyalkenes are very hard to get rid of, for two reasons, and the exam wants both of them.
聚烯烃很难处理，原因有两个，考试两个都要写。
First, they are inert and non-biodegradable.
第一，它们惰性且不可生物降解。
The chain is a long line of strong carbon to carbon bonds, and microbes have no enzyme that can break it, so the plastic stays in the ground for hundreds of years.
链是一长串很强的碳碳键，微生物没有能分解它的酶， 所以塑料会在地下留存几百年。
Second, burning them gives harmful products.
第二，燃烧它们会产生有害产物。
Burning PVC releases hydrogen chloride, a toxic and acidic gas.
燃烧 PVC 会放出氯化氢，一种有毒的酸性气体。
So what do we actually do with the waste?
那么这些废塑料到底怎么办？
There are three routes, and each one has a cost.
有三条路，每一条都有代价。
Landfill is cheap and simple, but the plastic takes up space and sits there for centuries.
填埋便宜又简单， 但塑料占地方，而且会在那里待上几百年。
Incineration destroys the waste and releases useful energy, but the gases must be cleaned first, or toxic products escape into the air.
焚烧能销毁废物并放出有用的能量， 但必须先把烟气净化，否则有毒产物就会排到空气里。
Recycling is the best of the three, because it saves crude oil, but the waste has to be collected, sorted by type and cleaned before it can be melted again.
回收是三者中最好的， 因为它能节省石油，但废塑料必须先收集、按种类分拣并清洗，才能重新熔化使用。
One last distinction, and it is a favourite trap.
最后一个区分点，也是常见的陷阱。
Addition is not the only kind of polymerisation.
加成并不是唯一的聚合方式。
In addition polymerisation the monomer has a carbon to carbon double bond, and the polymer chain is the only product.
在加成聚合中，单体含有碳碳双键，而聚合物链是唯一的产物。
In condensation polymerisation, which you meet later, each new join throws out a small molecule, usually water.
在缩合聚合中——这部分你以后会学到——每形成一个连接，就会脱去一个小分子，通常是水。
So here is the quick test: if a small molecule is lost, it is condensation. If nothing is lost at all, it is addition.
所以有一个快速判断法：如果脱去了小分子，就是缩合；如果什么都没有失去，就是加成。
Three marks students throw away.
三个学生常丢的分。
First, draw the repeat unit properly: the two end bonds must cross the brackets, and the letter n sits outside them.
第一，重复单元要画规范：两端的键必须穿出括号，字母 n 写在括号外面。
If the question asks for two repeat units, draw the backbone twice.
如果题目要求画两个重复单元，主链就要画两遍。
Second, check the formula.
第二，检查分子式。
The monomer and the repeat unit always contain the same atoms, so if yours differ, the double bond has gone back in the wrong place.
单体和重复单元含有的原子总是相同的，如果对不上，说明双键放错了位置。
Third, for disposal, use the marking words: non-biodegradable, and harmful combustion products.
第三，处理废弃物时要用评分标准的原话：不可生物降解，以及有害的燃烧产物。
A vague answer about pollution earns nothing.
只泛泛地说污染是拿不到分的。
Get those right, and this topic is yours.
做对这三点，这个专题就是你的了。