# Chemical Forums

## Specialty Chemistry Forums => Materials and Nanochemistry forum => Topic started by: madscientist on July 11, 2007, 05:15:12 AM

Title: contour length
Post by: madscientist on July 11, 2007, 05:15:12 AM
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
Title: Re: contour length
Post by: AWK on July 11, 2007, 05:42:35 AM
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.
Title: Re: contour length
Post by: madscientist on July 12, 2007, 03:32:14 AM
So if the repeating unit in poly(n-butyl methacrylate) is C8H14O2

(http://www.polymerprocessing.com/polymers/PBMA.jpg)

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?

Cheers
Title: Re: contour length
Post by: AWK on July 12, 2007, 06:30:25 AM
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
Title: Re: contour length
Post by: madscientist on July 13, 2007, 04:09:20 AM
Great sites, thanks AWK. Its helped me find out how to calc. Mn, Mw. 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.

Title: Re: contour length
Post by: AWK on July 13, 2007, 05:06:22 AM
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
Title: Re: contour length
Post by: DrCMS on July 13, 2007, 06:24:48 AM
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.
Title: Re: contour length
Post by: AWK on July 13, 2007, 06:53:36 AM
exactly
Title: Re: contour length
Post by: Matt Lacey on July 13, 2007, 07:06:02 PM
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
Title: Re: contour length
Post by: madscientist on July 18, 2007, 09:45:15 AM
I think I may have figured it out, please let me know if you think I am right.

Poly(n-butyl methacrylate)
(http://www.polymerprocessing.com/polymers/PBMA.jpg)

• Average molar mass (polymer) Mn = 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 = 110o

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

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

How does that look?
Title: Re: contour length
Post by: AWK on July 19, 2007, 01:10:48 AM
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,
Title: Re: contour length
Post by: madscientist on July 23, 2007, 09:37:37 PM
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,

Title: Re: contour length
Post by: AWK on July 24, 2007, 01:08:19 AM
Terminal groups in polymer depend on the method of  polymerization initiation. They "newe" are H-...-H.

sorry
Shoud be "never"
Title: Re: contour length
Post by: madscientist on July 26, 2007, 03:57:46 PM
Terminal groups in polymer depend on the method of  polymerization initiation. They newe are H-...-H.

What do you mean by newe?
Title: Re: contour length
Post by: Borek on July 26, 2007, 04:22:12 PM
never
Title: Re: contour length
Post by: madscientist on July 26, 2007, 04:50:25 PM
So would the contour length in this case be measured from the first carbon in the chain to the last?
Title: Re: contour length
Post by: AWK on July 27, 2007, 01:20:09 AM
In this case: from hydrogen to hydrogen.
Note, one hydrogen atom is wrong, as I said a terminal group depends on initiator of polymerization.
Hydrogen (H-...-H) is never used for this purpose.
Of course, for 552 units, an error will be much less than 1 %
Title: Re: contour length
Post by: madscientist on July 28, 2007, 03:39:48 AM
Thankyou very much for all your help AWK (and others), much appreciated.