Dear CHE 530 students,

My first comment is that in the on-line solutions I forgot to include the part of problem 4 that calculated the number of growing chains. The revised answer to that problem will be in the on-line copy of the problem 1/4/5/6 pdf file. (See page 4.)

My main comment about Hwk 2 is that there was a tendency to miscalculate the molecular weight on the CSTR problem.

It was tempting to calculate X_n via 1/(1-p), since i had just had you calculate p. The problem is that that formula applies to step growth polymerization, not to chain growth. The reason is that in step growth it makes sense to count the number of chain ends, since all molecules are reacting simultaneously, while in chain growth it makes sense to focus only on the single chain that is growing.

The calculation of X_n instead is done using the kinetic chain length, which leads to X_n = ( [M] / [I]^{1/2} ) * (combination of rate consts)

The given rate constants happen to lead to a very slow reaction rate. I forgot that when I gave the problem in the past I had then repeated it using rate constants that correspond to a faster reaction rate. The plots for both cases are in the solutions.

My final comment is that taking the full particle growth into account in problem 2 (emulsion polymerization) was quite tedious, as you may have seen in my solution. One tricky part (in my view) was that [R.] = (# particles/2) needs to be divided by the volume of the particle, since that concentration of growing chains falls over time. There may be some way around the units headaches that I was running into, but I haven't yet figured out what it is. I'll let you know if I figure it out.

If you have questions before the midterm, please contact me to make an appointment, or just try dropping by.

See you in class on Wednesday (if not before).

Prof. Greenfield

P.S. Homeworks will be in your mailboxes/desks/my mailbox in the morning, if they aren't there already.

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