Take a look at a simple addition polymerisation reaction:

http://dragonslair.europe.webmatrixhosting.net/Science/Chemistry/Hydrocarbons/polymer.htm

 

Its from my website itself. See the formula that can be used to show the polymers in a row.

 

Now, I have two questions about this:

 

a) Bonds are formed during polymerisation, so, is the reaction exothermic?

 

b)What about the atoms that lie at the end of the chain? What about their side bonds that is supposed to link to other atoms? What happens to them? I don't want an answer like "There are millions, so these are ignored", however, I want to know the facts!

Formation of poly(ethene), when catalysed and heated under pressure, alkenes link together when the double bond opens. The spare bonds are used to join up the molecules.

 

The original small molecule is called the monomer and the long molecule is called the polymer, which is the sort of molecule most plastics consist of. The polymer is now a saturated molecule but has the same C:H ratio as the original alkene.

 

So lots of small molecules join up to form a big long molecule in a process called addition polymerisation and the polymers are named as poly(name of original alkene).

umm... thanx for that wolfson, I already know that What about the two questions I asked first?

Bonds are formed during polymerisation, so, is the reaction exothermic?

 

b)What about the atoms that lie at the end of the chain? What about their side bonds that is supposed to link to other atoms? What happens to them? I don't want an answer like "There are millions, so these are ignored", however, I want to know the facts!

a) Bonds are formed during polymerisation' date=' so, is the reaction exothermic?

 

b)What about the atoms that lie at the end of the chain? What about their side bonds that is supposed to link to other atoms? What happens to them? I don't want an answer like "There are millions, so these are ignored", however, I want to know the facts! [/quote']

It will usually be exothermic, as the breaking of the double bond releases more energy than the formation of the new single bonds. There will be exceptions though, I imagine.

 

I'm not entirely sure of what you mean by side bonds, but I'll assume you mean the bonds at the end of the chain. Often the ends of polymers are reactive during polymerisation, this is how addition polymerisation occurs, the reactive ends are radicals that continue to react with monomers. The radical charge is preserved after each addition of a monomer so the process repeats over and over. The process usually ends when two radicals meet and either bind or simply neutralise each others charge. This isn't the case for condensation polymerisation though, here two monomers simply react, usually releasing a H2O or CO2 molecule. The ends of these polymers won't be especially reactive at any stage, so catalysts are more important.

I'm not entirely sure of what you mean by side bonds, but I'll assume you mean the bonds at the end of the chain

 

Yup, you understood correct! I said it in my first post...

What about the atoms that lie at the end of the chain?

 

 

Thanx for the reply. I kind of get a breif idea on what you are trying to say, but could you please explain this part in simpler language so that I can understand?

 

I'm not entirely sure of what you mean by side bonds, but I'll assume you mean the bonds at the end of the chain. Often the ends of polymers are reactive during polymerisation, this is how addition polymerisation occurs, the reactive ends are radicals that continue to react with monomers. The radical charge is preserved after each addition of a monomer so the process repeats over and over. The process usually ends when two radicals meet and either bind or simply neutralise each others charge. This isn't the case for condensation polymerisation though, here two monomers simply react, usually releasing a H2O or CO2 molecule. The ends of these polymers won't be especially reactive at any stage, so catalysts are more important.

 

Correct me if I am wrong. You mean that as the reaction goes on, the molecules at the end of the chain still have open, reactive bonds. These react with more monomer molecules and make the chain bigger. The part that I didn't get is: What happens when there are no monomers left? WHat will happen to those open bonds?

It depends on the reaction.

 

If it is a ROMP or any other type of olefin metathesis polymerization, then the head groups will consist of whatever carbene fragment was on the catalyst (ie. Grubb's Catalyst, etc.) whenever the polymer chain started growing. If the polymer chain was the first to start growing on the catalyst molecule then it's head group will be what was on the catalyst. In the case of the first generation Grubb's catalyst, it is a styrene-like carbene fragment, one of the ligands already there when you bought the catalyst in a bottle.

 

In the case of something like acrylic acid, it will polymerize if you look at it wrong, and if it's not stabilized. It happens most rapidly in the presence of an acid (itself) or a base. Here the ends are most likely still double bonds (don't take my word for it), or could simply be hydrogenated to methyl groups (not sure).

 

Not all polymerizations are exothermic, in the case of acrylic acid (i think), and are usually endothermic (requiring heat input), in the case of polyethylene:

I think each double bond is ~600 kcal/mol and each single bond is ~350 kcal/mol but there are twice as many, so ~100 kcal/mol must be "put in."

 

The question as to what's at the ends of the polymer chain is a good one because as most polymers go, the ends could be considered a minor component. If each polymer chain consists of only 100 units (a small polymer), the ends make up 2% of the sample, making it's characterization more difficult. I think with a little literature search, this may be cleared up though.

It depends on the reaction.

 

If it is a ROMP or any other type of olefin metathesis polymerization, then the head groups will consist of whatever carbene fragment was on the catalyst (ie. Grubb's Catalyst, etc.) whenever the polymer chain started growing. If the polymer chain was the first to start growing on the catalyst molecule then it's head group will be what was on the catalyst. In the case of the first generation Grubb's catalyst, it is a styrene-like carbene fragment, one of the ligands already there when you bought the catalyst in a bottle.

 

In the case of something like acrylic acid, it will polymerize if you look at it wrong, and if it's not stabilized. It happens most rapidly in the presence of an acid (itself) or a base. Here the ends are most likely still double bonds (don't take my word for it), or could simply be hydrogenated to methyl groups (not sure).

 

Again, the language that has been used is of a very high scientific standard, which I can't understand

 

Could you please explain it in simple language? I have a nightmare with language like this!!!

ah! that sort of thing happens when someone knows a little about a lot sorry

 

if the polymerization reaction was catalyzed then the catalyst will sometimes affect what groups are at the "head" of the growing polymer chain.

 

but there are so many types of polymerization reactions that it would depend on the reaction, is it a polyester? was it a radical polymerization? cationic or anionic? etc

 

i think the most common way to make low-grade polyethylene is by radical polymerization, which needs a radical initiator, this initiator will be sitting at the head of the polymer, then depending on what mechanism brought about the termination of the chain growth, that will determine the end group

 

if the radical was quenched by a combination of two polymer end groups coming together, i guess that would put the initiator at both ends

 

if it was quenched by disproprtionation, there will be some end groups that are single bonded and some that are still double bonded.

 

but that's just for that type of reaction, again, there are many more

Thanx alot for your reply, I understand it now MUCH better!!!

 

I have some questions:

 

if the polymerization reaction was catalyzed then the catalyst will sometimes affect what groups are at the "head" of the growing polymer chain.

How will it affect it?

 

i think the most common way to make low-grade polyethylene is by radical polymerization, which needs a radical initiator, this initiator will be sitting at the head of the polymer, then depending on what mechanism brought about the termination of the chain growth, that will determine the end group

 

 

What is radical polymerisation? IS it the same as addition polymerisation? What chemicals are used for radical polymerisation?

 

Also, if the answer to the question above was "No, it isn't the same as addition polymerisation", then, what about the chains in addition polymerisation? And what about the chains in condensation polymerisation?

 

Thanx alot for your time!

radical polymerization just means that the propagation mechanism proceeded through a radical, and radicals are notorious for kicking off chain reactions (like polymerizations)

 

well, if you mean by "what chemicals are used..." what agent initiated the radical chain reaction then that would be anything that could be classified as a "radical initiator" common ones are usually peroxides or nitriles

 

"addition polymerization" is a more general term that can include radical polymerization, but i think the term refers to what we think of when we say the word "polymerization"

 

really, it seems though, that you are trying to teach yourself a course in polymer chemistry and it would be easier to search for info through google or something, or just go check out some books

 

Cheers!

Thanx alot. That was really useful. I'm only at GCSE level at the moment, but, really curious about these things! Thanx alot apathy!

i have no idea what GCSE means, but, cool...

glad to help!

 

GCSE is a General Certificate of Secondary Education, two-year courses sat by all 15-17 yr olds in the UK.

Its also know as O levels!

GCSE replaced the O-Level.

But, isn't it the same standard and level? When did it replace it?

1984.

 

But you can still sit O-levels if you don't mind moving to Bangladesh.

ok!

OH! I remember hearing the Young Ones discussing their O-levels!

Well, its of a similar level. Year 11 basically!