Potassium
Polymers of propylene oxide (PPOs) are important polyethers, which have several applications, for example, as impact modifiers, dispersant agents, deemulsifiers, fuel additives, wetting agents, lubricants, adhesives, and biomedical applications [
A disadvantage of anionic polymerization of PO is side reaction, that is, chain transfer to the monomer, which markedly decreases molar mass of polymers (
Mechanism of the chain transfer reaction to the monomer.
The general properties of polyurethanes can be greatly improved by the use of polyether polyols with reduced monol content. It is possible by lowering the temperature and initiator concentration or by using suitable solvents [
One of the methods of unsaturation reduction is using for initiation of polymerization mixtures of potassium alkoxides and alcohols [
Decomposition of the alkoxide active centre/PO complex
Addition of macrocyclic ligand 18-crown-6 (18C6) to the system containing alcohol causes further depression of chain transfer to the monomer resulting in decrease of unsaturation [
An explanation of this effect given by Ionescu [
Decomposition of alkoxide active centre/PO complex
The aim of the present work was to determine the influence of ligand 18C6 on unsaturation and molar mass of polymers obtained in PO polymerization initiated with potassium
Propylene oxide (Aldrich) was dried over CaH2 and finally distilled at 307 K (34°C). Anhydrous tetrahydrofuran (THF) (Across Organics) was kept over CaH2 and distilled at 339 K (66°C). Potassium
All syntheses were performed at room temperature in a 50 cm3 reactor equipped with a magnetic stirrer and a teflon valve enabling substrates delivery and sampling under argon atmosphere. In the first series of polymerizations the initial concentration of the monomer and initiator was equal to 2.0 and 0.1 mol/dm3 but in the second one 10.0 and 0.05 mol/dm3, respectively. For example, tetrahydrofuran (15.2 cm3) and
100 MHz
The results obtained were collected in Table
Characterization of unsaturation of PPOs synthesized in the presence of
Sample | Initiating system | Macromolecules with unsaturated starting groups (mol %) |
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---|---|---|---|---|
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1 |
|
14.0 | 2600 | 1.05 |
2 |
|
26.8 | 2300 | 1.05 |
3 |
|
28.4 | 6400 | 1.07 |
4 |
|
40.1 | 5800 | 1.06 |
|
||||
5 |
|
10.1 | 1700 | 1.07 |
6 |
|
1.3 | 1000 | 1.10 |
7 |
|
22.5 | 6000 | 1.08 |
8 |
|
12.0 | 4700 | 1.12 |
|
||||
9 |
|
0.5 | 900 | 1.06 |
10 |
|
0.3 | 1000 | 1.04 |
11 |
|
0.2 | 1000 | 1.05 |
12 |
|
4.1 | 4800 | 1.02 |
13 |
|
3.5 | 4800 | 1.09 |
14 |
|
3.3 | 4900 | 1.04 |
|
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15 |
|
1.1 | 4100 | 1.03 |
16 |
|
0.3 | 3700 | 1.04 |
Unsaturation of polymers obtained changes in wide range, from 0.2 to 40.1 mol % depending on the presence of additives. Unsaturation is represented by allyloxy starting groups; however, in some polymers also isomeric
Unsaturated region in 13C NMR spectra of polymers: (a) sample 1; (b) sample 2; and (c) sample 9.
Activation of
We assumed that deprotonation of the monomer by ligand activated ion pairs
Mechanism of direct deprotonation of the monomer by ligand activated ion pairs.
It is worth noting that increase of polymer unsaturation in the presence of ligand is accompanied with decrease of molar masses, due to increase of number of polymer chains caused by chain transfer to monomer. Similar effects were observed earlier by us in the polymerization of PO initiated anhydrous potassium hydroxide [
Application of
Formation of hydrogen-bonded ion pair with three alcohol molecules
Diminishing of PPO unsaturation was reported previously for PO polymerization initiated monopotassium salt of glycerol mixtures with glycerol at high concentration of the last one [
Data concerning
Decomposition of hydrogen-bonded ion pair
In our opinion, nucleophilicity and basicity of crowned ion pair
Dispersity (
Cation exchange reaction between polymer chains.
It is also worth noting that the presence of nonpolar solvent, that is,
Summarizing, anionic polymerization of propylene oxide initiated potassium alkoxide is very sensitive on additives such as counterion complexing ligand and/or hydroxylic compound, that is, alcohol. In general, they strongly influence unsaturation and molar mass of the synthesized polyethers.
Potassium In the presence of 18-crown-6 unsaturation of poly(propylene oxide) markedly increases and In this system direct deprotonation of the monomer occurs exclusively and not deprotonation by elimination due to steric hindrance of cation complexed ligand. In the presence of Activation of the system containing alcohol by 18-crown-6 results in further decrease of polymer unsaturation; however, This result differs strongly from that reported earlier for PO polymerization initiated potassium 1-methoxy-2-propoxide/1-methoxy-2-propanol in which in the presence of the same ligand Addition of nonpolar solvent Dispersities of synthesized polymers are relatively low and independent on additives used. In general, the observed effects result from different influence of applied additives, as crown ether and/or alcohol on the rates of propagation and chain transfer to the monomer and alcohol as well as cation exchange reactions.
The authors declare that they have no competing interests.