Question

How can you distinguish polyethylene from polypropylene based on their spectra? Give specific differences both in the C-H region and in the C-C region. What functional group is this attributed to in its structural formula? 7. Ignoring the intensities and considering only the number of peaks in the C-H region, compare the spectra of polyethylene and polypropylene with polyvinyl chloride. Formulate a hypothesis relating the C-H region of the spectrum to the polymer structure (specifically, consider the hydrogen atom environments). Support your hypothesis with evidence from these three polymers.

Answer 1

Solution:

Structures of polyethylene, polypropylene and polyvinylchloride are as follows

Distiguishing features of polyethylene and polypropylene

There is visible difference in the structures of polyethylene and polypropylene. Polyethylene is chain of only -CH2 groups whereas polypropylene contains -CH, -CH2 and -CH3

In 1H NMR

i) In polyethylene each monomer contains [-CH2-CH2] that will show a bunch multiplates at 1.25 - 1.35 ppm only.

ii) In case of polypropylene each monomer contains [-CH2-CH(CH3)-]

-CH2 is attached to two adjacent -CH groups thus this CH2 will show doublet of doulet at 1.25-1.30 ppm

- CH is attached to two -CH2 and one -CH3 group, this will show multiplate at 1.55-1.70 ppm

- CH3 is attached to a -CH thus it will show a doublet at close to 1.0 ppm

In 13C NMR

i) In polyethylene each monomer contains [-CH2-CH2] thus -CH2 will appear at 25-28ppm.

ii) In case of polypropylene each monomer contains [-CH2-CH(CH3)-]

-CH2 will appear at 42-46 ppm

-CH will appear at 25-30 ppm

-CH3 will appear at 20 ppm

Thus, there is significant difference in the C-H region and in the C-C region of polyethylene and polypropylene which are enough to distinguish these two polymers.

Functional group is this attributed to in its structural formula

Ethylene [-CH2-CH2] is functional group for polyethylene in its structural formula.

Isopropylene [-CH2-CH(CH3)-] is functional group for polypropylene in its structural formula.

Comparision of the spectra of polyethylene and polypropylene with polyvinyl chloride

Structura features of polyvinylchloride: It contains -CH, -CH2 and Cl

In 1H NMR

i) In case of polyvinylchloride each monomer contains [-CH2-CH(Cl)-]

-CH2 is attached to two adjacent -CH(Cl) groups thus this CH2 will appear as a triplet at 1.6-1.80 ppm

- CH is attached to two -CH2, this will appear as a multiplate at 3.60-3.70 ppm

In 13C NMR

i) In case of polyvinylchloride each monomer contains [-CH2-CH(Cl)-]

-CH2 will appear at 45-50 ppm

-CH will appear at 55-60 ppm

In case of polyvinylchloride well separated peaks are there due to different environment of each proton in each monomer unit. In case of polypropylene little bit less separated peaks are there especial due to -CH2 group. Whereas polyethylene and not showing well separated peaks due to same environment of each proton in each monomer.