Cationic ring-opening polymerisation

In contrast to anionic ROP, cationic ring-opening polymerisations are far less utilised due to the poor control over the reaction parameters. Cationic ROP was first reported by Penczek et al. for the polymerisation of -caprolactone and /3-propionolactone [52]. Afterwards, various initiating systems have already been reported on which can be divided in four classes including alkylating 

The mechanism becomes more complex in the case of acylating reagents as both the endo- as well as the exocylic oxygen are able to react [53]. When the endoclyclic oxygen is involved, a six-membered intermediate can be formed. The reaction of this intermediate with additional monomer units can result in both ester as well as anhydride functionalities present at the chain-end as shown in Fig. 21.4. 

In case of Lewis acid initiation, a cationic ROP will only occur if the counterion is not too nucleophilic [54]. When nucleophilic counterions are present, the metal-insertion mechanism will be followed instead, as already reported for several catalytic systems including ZnCl2 [55], TiCl4 and AICI3 [56]. 

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Beside the natural fermentation process, dextran can be synthesised chemically via a cationic ring-opening polymerisation (ROP) of levoglucosan (1,6-anhydro-/3-D-glucose), a pyrolysis product of polyglucans (Fig. 8, [83]). 

Fig. 8 Synthesis of dextran via a cationic ring-opening polymerisation of 1,6-anhydro-2,3,4-tri-0-allyl-/J-D-glucopyranose (path A) and l)6-anhydro-2,3,4-tri-0-benzyl-/J-D-glucopyranose path B) [84,85]...

Poly(methinimin) (-(HC=N)-) formation through cationic ring opening polymerisation of l,3,5-triazme >. 

Polytetrahydrofuran (PTHF) is a polyether obtained by cationic ring opening polymerisation of tetrahydrofuran (THF), a five membered cyclic ether ... 

Tetrahydrofuran (THF), the cyclic monomer used for the synthesis of polytetramethyleneglycols (see Chapters 7.1-7.3) by cationic ring opening polymerisation, was produced in the earlier stages of this technology from furfurol. Furfurol results from the acid hydrolysis of pentosanes existing in many agricultural wastes (corn on the cob, straw and so on). 

Figure 21.3 Proposed mechanism for cationic ring-opening polymerisations initiated through alkylating agents. Adapted from [30].

The most widely applied catalyst systems in organocatalytic ROP include N-heterocyclic carbenes [62, 64, 67], guanidine and amidine bases [51,65] and phosphorane-based compounds [68,69]. Strong organic acids are excluded from this section as their mechanism follows the one described for cationic ring-opening polymerisations [70, 71]. 

Pencsek, S., Kubisa, P., and Matyjaszewski (1985) Cationic Ring Opening Polymerisation, Vol. 2. 

An investigation was carried out on the initiating behaviour of functional tosylates and triflates in cationic ring-opening polymerisation of 2-methyl-1,3-oxazoline. The tosylates and triflates were prepared and tested with a number of initiators to achieve the best results with respect to molecular weight and polydispersibility. 24 refs. AUSTRIA EUROPEAN UNION WESTERN EUROPE Accession no.829955... 

Scheme 4.3 Proposed mechanism of cationic ring-opening polymerisation of six-membered cyclic carbonates.

Zhao Y, Tmhlar DG (2007) How well can new-generation density functionals describe protonated epoxides where older functionals feil J Org Chem 72(1 ) 295-298 earlier PR, Deora N, Crawford TD (2006) Protonated 2-methyl-1,2-epoxypropane a challenging problem for density functional theory. J Org Chem 71(4) 1592-1597 Frenking G, Kato H, Fukui K (1975) An MO-theoretical treatment of the cationic ring-opening polymerisation. I. Ethylene oxide. Bull Chem Soc 45(1) 6-12... 

RUD 12] Rudati P.S., Mueller D.C., Meerholz K., Preparation of insoluble hole-injection layers by cationic ring-opening polymerisation of oxetane-derivatized tiiphenylamine dimer for otganic electronics devices , Procedia Chemistry, vol. 4, pp. 216-223, 2012. 

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