Popcorn polymerization
The popcorn polymerization (also proliferating polymerization ) is a radically extending polymerization , wherein from monomers foamy, crusty polymer grains having an irregular surface formed. Their popcorn- like appearance is eponymous for this type of reaction. Popcorn polymers are characterized by their insolubility in common solvents and limited swelling capacity.
Reaction course
Popcorn polymers result from the growth of polymer chains from a “popcorn seed” (“popcorn core”, “popcorn center”). The polymerization is based on a free radical mechanism, but often does not require an initiator that generates free radicals . The radicals can also develop by breaking polymer chains under the combined action of polymerization and swelling, whereby a large number of reactive sites are formed. As a result, growing chains are initiated at fixed positions at different points on the polymer chains, which independently form further growing centers that cannot react with one another. The newly formed chains intertwine with those already formed. The formation of the first nuclei requires a longer induction period, after which the polymerization proceeds quickly.
Styrene / divinylbenzene , methyl acrylate , butadiene / styrene and N- vinyl-2-pyrrolidone are listed as monomers for popcorn polymerization in the literature .
There are several considerations for the formation of popcorn sprouts:
- A substance that functions as a popcorn seed must be insoluble in the monomer, but swellable, and have a sufficient number of groups that can form free radicals after activation.
- Popcorn formation is possible as soon as the chain length of the molecules is large enough to intertwine and form a "framework" that can act as a popcorn germ.
- Some chemical crosslinks are necessary to allow popcorn germs to develop.
The characteristic shape of the polymer grains arises from the fact that the polymerization is spatially hindered by the entanglement of the polymer chains, which leads to tensions in the chain structure, which result in deformations of the particle shape.
Properties of popcorn polymers
Popcorn polymers are almost insoluble in common solvents. Due to the synthesis process, the crosslinking density is high, which is why a relatively low swellability results.
Popcorn polymers can undesirably arise as by-products in synthesis processes or from improper storage of monomer substances. A compound produced specifically through popcorn polymerization is polyvinylpolypyrrolidone (crospovidone).
Individual evidence
- ^ A b Harry G. Brittain: Analytical Profiles of Drug Substances and Excipients , Academic Press, 1996. P. 92 restricted preview in the Google book search.
- ↑ Bengt Ranby, Jan F. Rabek: ESR Spectroscopy in Polymer Research , Springer Science & Business Media, 2012. P. 115 limited preview in the Google book search.
- ↑ W. Strohmeyer, P. Hartmann: Targeted “Popcorn” polymerization of acrylic acid ethyl ester with metal carbonyl derivatives. In: Z. Naturforsch. 19b (1964), p. 655 ( PDF ).
- ↑ M. Alger: Polymer Science Dictionary , Springer Science & Business Media (1997), p. 467 restricted preview in the Google book search.
- ↑ a b EH Immergut: Popcorn Polymerization . In: Macromolecular Chemistry and Physics, Volume 10, Edition 1 (1953), pp. 93-106 doi : 10.1002 / macp.1953.020100108 .
- ↑ a b Broja M. Mandal: Fundamentals of Polymerization , World Scientific, 2013. p. 144 ( available online )
- ^ A b E. Müller (Ed.): Methods of Organic Chemistry (Houben-Weyl) , Volume 14/1, 4th Edition (Georg Thieme Verlag, 1961), p. 98 restricted preview in the Google book search.