Activated chain end mechanism

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The active chain ends mechanism (ACE) ( English active chain end mechanism ) and the activated-monomer mechanism (AM) ( English activated monomer mechanism ) are two possibilities ( reaction mechanisms ), according to which the cationic polymerization of epoxides can proceed. While the cationic polymerization proceeds regularly according to the ACE mechanism, it can also proceed according to the AM mechanism if hydroxyl groups are present. Hydroxy groups act as chain transfer agents through the AM mechanism ; if the hydroxyl group is a functional group of an alcohol, this can thereby be incorporated into the polymer network. In this way, both the speed of the reaction and the network density and structure are influenced (and thereby also the mechanical properties of the polymer).

Overview

The following figure briefly shows the ACE and AM mechanisms:

ACE AM Mechanisms general.svg

The ACE mechanism is similar to other polyaddition mechanisms in that monomers react with the active chain end of the polymer (the cation ) and are thus added to the growing polymer chain. As can be seen in the figure, there is an activated three-membered ring with a tertiary oxonium cation at the end of the chain. The three-membered ring is attacked on one of the carbon atoms by the oxygen atom of a monomer. Both carbon atoms of the activated three-membered ring can be attacked, which leads to the (quasi) identical products a and b.

The AM mechanism can take place in the presence of hydroxyl groups (e.g. alcohols or water ). In this case, a hydroxyl group (e.g., the hydroxyl end group of the polymer) reacts with a monomer in a nucleophilic attack, forming a protonated ether and a hydroxyl end group. The proton is given to the next monomer and the reaction can proceed in the same way. The polymer is thus formed by the successive addition of temporarily protonated monomer to its hydroxyl end group. If the hydroxyl group is not the end of a polymer but rather an alcohol present in the reaction mixture, it can act as a chain transfer agent; this has the effect on the reaction rate. The alcohol is then incorporated into the polymer network and the network density is reduced, resulting in a change in the mechanical properties.

ACE and AM mechanisms are in competition with one another. The greater the [HO group] / [monomer] ratio, the more the AM mechanism is favored. At the same time, however, the degree of polymerization is related to the [monomer] / [HO group] ratio, the less monomer is present in the ratio, the shorter the polymers become (as is to be expected, since hydroxyl groups act as chain transfer agents). If the polymerization is to proceed according to the AM mechanism, but at the same time polymers with a high degree of polymerization are desired, constant small amounts of the monomer can be added to the reaction mixture, which overall results in a high but low ratio of [monomer] / [HO group] causes. Forcing the reaction through the AM mechanism in this way may be desirable in order to minimize the level of cyclic by-product (formed by backbiting) or in order to produce polymers with hydroxyl end groups.

In general, it has been found that the rate of reproduction of the AM mechanism k AM is 5 times greater.

history

Polymerization of glycidol according to the ACE and AM mechanisms.

In the early 1980s, the cationic polymerization of cyclic ethers in the presence of low molecular weight alcohols as chain transfer agents was investigated. The intention was to make diol-terminated polyethers in this manner. Indeed, it was found that the level of cyclic by-product formed by "back-biting" decreased in the presence of alcohols. The activated monomer mechanism (AM) was introduced to explain this phenomenon.

In the following investigations it was shown, using the example of glycidol , that the product of the ACE mechanism exclusively carries primary hydroxyl groups, but that of the AM mechanism also carries secondary hydroxyl groups, see figure above.

Individual evidence

  1. ↑ a b Przemyslaw Kubisa, S. Penczek: Cationic activated monomer polymerization of heterocyclic monomers . In: Progress in Polymer Science . 24, No. 10, 1999, pp. 1409-1437. doi : 10.1016 / S0079-6700 (99) 00028-3 .
  2. ^ Brian Dillman, Julie LP Jessop: Chain transfer agents in cationic photopolymerization of a bis-cycloaliphatic epoxide monomer: Kinetic and physical property effects . In: Journal of Polymer Science Part A: Polymer Chemistry . 51, No. 9, 2013, pp. 2058-2067. doi : 10.1002 / pola.26595 .
  3. ↑ Kubisa, Przemyslaw: Hyperbranched polyether of by ring-opening polymerization: Contribution of activated monomer mechanism . In: Journal of Polymer Science Part A: Polymer Chemistry . 41, No. 4, 2002, pp. 457-468. doi : 10.1002 / pola.10605 .
  4. ↑ Philippe Dubois, Olivier Coulembier, Jean-Marie Raquez: Handbook of Ring-Opening Polymerization . 1st edition. Wiley, 2009, ISBN 978-3-527-31953-4 , pp. 39 ( limited preview in Google Book search).
  5. ↑ Tadeusz Biedron, Krystyna Brzezinska, Przemyslaw Kubisa and Stanislaw Penczek: macromonomer by activated polymerization of oxiranes. Synthesis and polymerization . In: Polymer International . 36, No. 1, January 1995, pp. 73-80. doi : 10.1002 / pi.1995.210360110 .
  6. ↑ Philippe Dubois, Olivier Coulembier, Jean-Marie Raquez: Handbook of Ring-Opening Polymerization . 1st edition. Wiley, 2009, ISBN 978-3-527-31953-4 , pp. 40 ( limited preview in Google Book search).
  7. ↑ R. Tokar, Przemyslaw Kubisa, S Penczek, A Dworak: Cationic polymerization of glycidol: coexistence of the activated monomer and active chain end mechanism . In: Macromolecules . 27, No. 2, 1994, pp. 320-322. doi : 10.1021 / ma00080a002 .