Chemistry Reference
In-Depth Information
The ratio of the fluorescence intensity of an excimer and a normal molecule is a measure of the
probability that the conformational transition takes place during the excited lifetime. A polyamide
with only a small proportion of the following units was used for comparison:
N
N
H
N
H
O
Emission spectra of dilute solutions of the above polyamide and its lowmolecular weight analogwere
measured over a range of temperatures. They showed that the activation energies of the conformational
transitions required for excimer formations are essentially the same for both materials [
5
].
In addition, all bimolecular activation-controlled reactions are independent of the degree of
polymerization [
6
]. Simple S
N
2 reactions between reactive groups attached to chain ends of mono-
disperse macromolecules in a wide range of molecular weights are independent of the DP [
7
,
8
] in the
range of 20-2,000 [
7
]. This was shown on three different reactions. In the first one, the reactivities of
chlorine-terminated low and high molecular weight polystyrenes towards polystyryllithium are equal
in benzene and cyclohexane solvents:
Cl
Li
+
In the second one, the reactivity of primary amine-terminated polyoxyethylenes with sulfonyl
chloride-ended polyoxyethylenes in chloroform is also the same:
NH
2
O
+
OS
2
Adduct
SO
2
Cl
In the third one, chain length dependence of the propagation rates was measured in
polymerizations of methyl methacrylate. In the range of DPs from 130 to 14,200, the propagation
constant was shown to be independent of the chain length [
8
].
On the other hand, unequal reactivity was observed:
1. In bimolecular reactions that are diffusion-controlled.
2. When neighboring group participations become significant.
3. When the properties of the polymers in solution are altered by gelation.
4. When the tacticities of the polymers affect neighboring group interactions.
5. When heterogeneous conditions affect accessibility to the reactive sites.
There are special situations that can occur. For instance, electrostatic charges carried by the
polymers may extend over long distances in solutions and may manifest themselves in reactions
