[madscientist]

Hi all,

Does anyone know any good websites which cover topics involved with materials chemistry, just started 3rd year unit and need some web resources.

How would you calculate the end-to-end length (contour length) of poly(n-butyl methacrylate) given that the average molar mass Mn = 7.85*10⁴g/mol

any hints?

Cheers

[AWK]

Calculate number of monomers, draw linear structure, calculate 1,3 C to C  distance in polymer and multiply it by number of monomers. In this manner you will get lenth of polyethylene skeleton. Then add size of one hydrogen and COOCH3 group projectec on the diection of molecule. Approximate 1,3 C..C distance is 250 pm.

[madscientist]

So if the repeating unit in poly(n-butyl methacrylate) is C₈H₁₄O₂



and this has a molecular weight of 142.2 g/mol than the amount of monomers is:

7.85*10⁴g.mol^-1 / 142.2g.mol^-1 = 552 monomers

I'm confused as to where the monomers join to form the polymer?

any help please,

Cheers

[AWK]

Read something about mechanism of polymerization
http://chem.chem.rochester.edu/~chem424/classes.htm
http://chem.rochester.edu/~chem234/lecture2.pdf
http://chem.chem.rochester.edu/~chem421/cation.htm
http://en.wikipedia.org/wiki/Polymerization

[madscientist]

Great sites, thanks AWK. Its helped me find out how to calc. M_n, M_w. but im still lost with how to calc the end-to-end length of the molecule? couldn't find anything in those websites to work off.  I know that poly(n-butyl methacrylate) is a polyester but am still unsure where its monomers are joined.  All i can think of is that its the amount of monomers multiplied by the length of each monomer.

[AWK]

In ther second link, beginning of page 10, you have a scheme of metacrylate, replace COOMe by COO-nBu. The length of polymer unit can be calculate from a typical C-C bond length ~1.54 Angstroem (154 pm) and valence angle at carbon ~110 degres

[DrCMS]

I know that poly(n-butyl methacrylate) is a polyester but am still unsure where its monomers are joined. 

Look again it is not a polyester. 
It has ester side chains but the back bone is -C-C- only.

[AWK]

exactly

[Matt Lacey]

poly(n-Bu methacrylate) isn't a polyester, it's a polyacrylate (as you might guess from the name)



That's the structure of methyl methacrylate, the monomer used to make Plexiglas. As you can see it's got a C=C bond, and the monomers join up by these bonds by a free radical mechanism... haven't checked the links posted but I'm sure it's probably mentioned in there somewhere.

-Matt

[madscientist]

I think I may have figured it out, please let me know if you think I am right.

Poly(n-butyl methacrylate)

• Average molar mass (polymer) M_n = 7.85*10⁴g/mol

• molar mass (monomer) = 142.2g/mol

• Number of monomers = (7.85*10⁴g/mol) / (142.2g/mol) = 552 monomers

• C-C bond length = 0.153nm

• 1,3 C-C-C bond angle = 110^o

• Using a bit of trigonometry: 1,3 C----C distance = ((0.153nm)*(sin 110^o)) / sin 35^o = 0.251nm

• Contour length = (552 monomers)*(0.251nm/monomer) = 138.6nm

How does that look?

[AWK]

Looks OK. Eventually add terminal groups - depens on method of polymerization, eq for n-BuLi add n-bu + H - about 0.6-0.7 nm,

[madscientist]

Wouldn't both terminal groups just be CH3? So there would be one hydrogen bonded to either end..


Thanks for all the help so far to.

cheers,

Mad

[AWK]

Terminal groups in polymer depend on the method of  polymerization initiation. They "newe" are H-...-H.

sorry
Shoud be "never"

[madscientist]

Terminal groups in polymer depend on the method of  polymerization initiation. They newe are H-...-H.

What do you mean by newe?

[Borek]

never