[jokin]

One of the question is:

Styrene is polimerized with AIBN as initiator.By hydrolysis of the resulting polymer are released 6 g of ammonia per 10 kg of polymer. If all terminal groups derived from the radical initiator (a-cyanoisopropile),what is the average styrene units in each chain of PS?

Original text(
Una muestra de estireno se polimeriza con ayuda de trazas de azoisobutironitrilo como iniciador . Por hidrólisis del polímero resultante se liberan 6 g de amoníaco por cada 10 kg de polímero . Si todos los grupos terminales derivan del radical iniciador (a-cianoisopropilo), ¿qué promedio de unidades de estireno existe en cada cadena?)

[Dan]

Please show your attempt, as requested in the Forum Rules.

Start with a balanced chemical equation for the hydrolysis the polymer.

[jokin]

Hi to all

My answer to the posted problem will be somethin like this:

When AIBN initiator breaks, and starts polymer chain polymerization, we get a nitrile group r-CN.
We can then hydrolyze the polymer to obtain r-COOH and NH3. 

There we can use the NH3 released 6g

(6g)/(17 g/mol)= 0,3529 mol of NH3     Each of these molecules came from the fist chain link, ant it comes from initiator (AIBN).

The neutralized polymer are 10Kg so, usin the molecular weight of the styrene repetitive unit (104g/mol)
(10000g)/(104g/mol)= 96.154 mol of repetitive units

We can see then the proportion between repetitive units of styrene and the N. It is 96.154/0.3529 = 272.5 units of styrene per each unit of initiator.

[Dan]

You're on the right track, but remember there is a nitrile at both ends of the polymer:

NCC(CH₃)₂-[CH₂CHPh]_n-C(CH₃)₂CN + 2H₂O -> HOOCC(CH₃)₂-[CH₂CHPh]_n-C(CH₃)₂COOH + 2NH₃

[jokin]

The chains may finish polymerization by two main ways.
One gets a chain with two terminal initiator units (combination of macro radical chains)  
I-R* + *R-I = I-R-R-I
Other way is by disproportiontion  (Building a double link in one side)
I-R* + *R-I = I-R + R-I

That is the reason of not knowing if there is nitrile in both sides.


Poly(styrene) is used to finish polymerization by combination. But if we don’t know we would not be able to say what are the average styrene units in each chain of PS.

Would we need to answer by the two ways? With two different options?

[jokin]

The real problem what I`m trying to solve is a little bit different:

Styrene is polymerized with AIBN as initiator. By hydrolysis of the resulting polymer are released 6mg of ammonia per 10g of polymer. If all terminal groups derived from the radical initiator (a-cyanoisopropile), what are the average styrene units in each chain of PS?
If the kinetic length of the chain is v=136 units of styrene. What is the main mechanism of termination?

All is the same till we see the 136 units per living chain...

If we take in care the previous relation between styrene and initiator 272.5 units. Are these two assessments compatible?

[Dan]

That is true - but the question states:

If all terminal groups derived from the radical initiator (a-cyanoisopropile),what is the average styrene units in each chain of PS?

I read this to mean that it is assumed that there is a nitrile at both ends of each polymer chain.

I calculated an average chain length of 543 units based on the chemical equation I posted previously.

NCC(CH₃)₂-[CH₂CHPh]_n-C(CH₃)₂CN + 2H₂O -> HOOCC(CH₃)₂-[CH₂CHPh]_n-C(CH₃)₂COOH + 2NH₃

mol of ammonia: 0.353 mol
mol of polymer: 0.1765 mol
MW of polymer: 136+104n g/mol
mass of polymer: 10000 g

So, 10000/0.1765 = 136+104n
and                 n = 543

This would correspond to a kinetic chain length of 271.5 (because two initiator radicals are consumed by each polymer chain).

I don't see how it is possible that the kinetic chain length is 136 units if there is 6g of ammonia per kg of polymer on hydrolysis, but I only have a basic understanding of polymer chemistry - hopefully someone more qualified can step in to help?