13enigma - 16-12-2017 at 12:59
Why is it that cationic polymerization requires cations as an initiator bit the chain it forms is not a metal but a hydrocarbon? Why can't metals form
long chains with other metals?
aga - 16-12-2017 at 13:47
Mmmmm.
Now that is an interesting idea.
I suppose you could argue that metal crystals are a kind of polymer, in that the atoms link together in never-ending cross-linked chains.
(a.k.a. i don't know the answer)
[Edited on 16-12-2017 by aga]
LearnedAmateur - 17-12-2017 at 03:52
It's all to do with the bonding. Metals obviously bond metallically, forming giant lattices of cations in a sea of delocalised electrons - these are
layers in a pure metal and a kind of random conglomeration in alloys (where different sized metal cations disrupt the continuity, which usually leads
to the increased hardness compared with elemental metals).
All polymers bond covalently in chains as you pointed out, cationic polymerisation is so called because the reactive intermediate is a cation, which
reacts with other monomer units to form the chains. The monomer donates electrons to an ionic species by addition of a hydrogen atom, forming a
carbocation salt which ultimately results in chain growth as there isn't much to terminate the process.
Rather than viewing metals as a type of polymer (which I can understand since the metal atoms are technically monomers), they are in fact more akin to
crystals like NaCl - instead of an anion, free electron(s) provide the negative charge in the lattice, holding it together and preventing the cations
from flying apart in a Coulombic type explosion. Of course there are differences, the electrons are mobile thus metals can conduct electricity and
solid ionic crystals cannot, but it's a very similar principle - especially when you look at very high resistance alloys, the structures of which
severely inhibit movement of the electrons.
Here's an image from Wikipedia which shows cationic polymerisation initiation via a Lewis acid, which forms a cationic species to react with the
monomer (coinitiator and initiator, respectively).
Σldritch - 17-12-2017 at 12:55
Depends on how you define metal but there is stuff like this: http://slideplayer.com/slide/9245212/
I do not know very much about them but ive read that polythiazyl is bronze colored and metallic material.
[Edited on 17-12-2017 by Σldritch]
LearnedAmateur - 17-12-2017 at 16:09
Polythiazyl would be considered a metal under some definitions, mainly due to its conductivity, hardness, and lustre. However it is constituted by
polymerised chains, whereas metals tend to be in a lattice of repeating M(+n) + (n)e- units, like how you would describe a crystalline solid. A bit of
an oddball from what I've read, and the chemistry behind it is out of my reach, I'll certainly have to look into it more since it's not something I've
come across before.