User:PolyMERNAJs001/sandbox

= Polyoxetane = Polyoxetane (POX), or poly(oxetane), is synthetic organic heteroatomic thermoplastic polymer with molecular formula (–OCH2CH2CH2–)n. It's polymerized from oxetane monomer, which is four-membered cyclic ether.

History
Needed chemistry was observed and developed through 1930’s and 1940’s. The very first polymerized oxetane was 3,3-bis(chloromethyl)oxetane followed by other 3,3-disubstitued derivatives during 1950‘s. Unsubstitued oxetane itsef was polymerized in 1956.

Monomers
Tens of oxetane derivatives have been synthetized and many of them are polymerizable. Reasons for inabbility to polymerize are different basicity and ring strain caused by different electron and bulkiness of substituents also as their position. Major 3-substitued and 3,3-disubstitued monomers are summarized in the oxetane article.

Polymerization
Ring strain of unsubstitued oxetane is 107 kJ/mol. That’s twenty times more, than non-polymerizable six-membered tetrahydropyran. Oxetane polymerizes vis cationci, ring-openning mechanism. Special oxetanes are polymerizable by other mechanisms.

Propagation centre is tertiary oxonium ion, mainly initialized by Lewis acids, trialkyl oxonium salts, carbocationic salts and others. Strong acids tends to generate secondary oxonium ions, which are unreactive, thus they are not initiators of first choice. On the other hand super acids (eg. HSO3F)a re fantastic initiators of cationic polymerization of cyclic ethers, such as oxetane.

For sufficient stability of propagation centre, an counterion X– of low nucleopholocoty is required, such asSbCl6–, PF6–, AsF6– or SbF6–. First polymerizations were conducted with compounds consisting of BF4– or BF3OH– counterions.

Polymerization is very fast and thus preparation of lower molecular weight products (also with desired functional end groups) wasn’t performed unitl today.

Side reactions
Unsymmetrically substitued oxetanes polymerizes according to ability of attacking one or both a-carbons of the propagation centre. Unsubstitued and 3-substitued derivatives polymerize in symetricall manner. 2-substutituented derivatives form all main structural units meaning head-to-tail, head-to-head and tail-to-tail. However with right conditions and initiation systém used a stereospecific propagation can be achieved.

Oxygens of the main chain posses enought reactivity to attack oxonium propagation centre to either form cyclic oligomers (usually tetramers) or to depolymerize. These reactions within one molecule are refered as backbitting. During polymerization of unsubstitued oxetane, attack of two growing chains may occur, in very small number, to form a acyclic oxonium ions. This process is so called temporary termination. (obrazek vpravo) Mentioned side reactions compete in speed with propagation. The faster the propagation, the less side reactions take place. Speed of propagation depends on polymerized monomer, iniciation system used and polymerization conditions set.

Substitued polyoxetanes
A series of substitued oxetanes have been synthesized and polymerized. The very first polymerized oxetane was 3,3-bis(chloromethyl)oxetane.