Reactivity in Chemistry
Substitution at Carboxyloids
CX8b. Ring-Opening Trans-Esterification Polymerisation
Difunctional molecules can sometimes polymerise, provided they have appropriate partners with which they can react on other molecules. For example, hydroxyesters might react with other hydoxyesters, with the hydroxyl group on one molecule reacting with the ester group on another, forming a polyester. Alternatively, there might be two different kinds of molecules that react with each other. For example, a diamine might react with a diacid chloride to form an amide.
In ring-opening polymerisation, the monomer is not difunctional. Instead, it is embedded in a ring. Ideally, there is a little ring strain in the molecule, bumping it up in energy just a little so that it will react more easily. Common examples include caprolactone and lactide, used to make biodegradable yard waste bags and produce containers, respectively (among many other applications). These cyclic esters are sometimes referred to as "lactones".
If an alcohol is added, it can act as an "initiator" in a "chain reaction". The alcohol is a nucleophile, and it donates to the carbonyl, eventually cleaving the carboxyl C-O bond and popping open the ring.
At some point, a proton gets transferred to the oxygen that used to be embedded in the ring. Now we have a new alcohol. What does it do? It reacts with another cyclic ester, popping it open and forming a new alcohol. The cycle repeats itself.
In reality, ring opening polymerisations don't really work if you just add an alcohol to a lactone. Typically, a catalyst is also added. Catalysts most commonly are Lewis acids, such as aluminum, iron or tin compounds. One of the most common catalysts is tin octoate, more properly called tin(II) 2-ethylhexanoate.
Provide a mechanism, with arrows, for the ring-opening polymerisation of caprolactone with tin octoate.
Perform end-group analysis in the following cases to determine
a) the degree of polymerisation (what is the value of "n"?).
b) the molecular weight.
i. The ratio of the integrals for the 1H NMR peaks representing positions b:a is 50:1.
ii. The ratio of the integrals for the 1H NMR peaks representing positions b:a is 80:1.
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This material is based upon work supported by the National Science Foundation under Grant No. 1043566.
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