Anionic ring-opening polymerisation

In anionic ROP, nucleophilic metal alkoxides initiate the polymerisation reaction. Some bulky alkoxides, however, will rather act as a base thereby introducing a new anionic centre on a monomer unit, which can subsequently act as the anionic initiator [30,31]. Two 

In the first scheme (Fig. 21.1, reaction a), the alkoxide attacks the ester group after which an acyl-oxygen bond is broken. In the other case [Fig. 21.1, reaction b), the alkoxide attacks the carbon atom adjacent to the alcohol residue of the ester, after which an alkyl-oxygen scission takes place. This last mechanism is of particular importance for the polymerisation of four-membered lactones (3-lactones] in which the first mechanism is disfavoured by stereo-electronic effects [32]. 

Control over the polymerisation reaction can be increased using less reactive Initiators, thereby disfavouring transesterification. One strategy applies sterically hindered initiators to oppose reaction occurring with growing polymer chains. Another strategy Involves the use of certain less electropositive metals. The latter will be further referred to as metal-insertion ROP. 

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Sequencing of siloxane units was also investigated for anionic ring-opening polymerisation of l,l-diphenyl-2,2,3,3-tetramethylcyclotetrasiloxane by 29Si-NMR [120]. 

Another anionic ring-opening polymerisation that can also be used for the synthesis of a poly(germasilane) as well as a polysilane has been described by Sakurai and co-workers and is shown in Scheme 11.4. They are formed in greater than 40% yield and as well as having narrow molecular weight distributions (Mn = 17,000, Mw/M = 1.3) the polymers are highly ordered, as evidenced by NMR spectroscopy. 

A methoxy-terminated PEG-/>-PLA copolymer (CHsO-PEG-fo-PLA) was synthesised using the anionic ring-opening polymerisation (ROP) of both ethylene oxide and D,L-lactide in anhydrous tetrahydrofuran (THE). A typical experimental procedure for the synthesis of CHsO-PEG-f -PLA is described below. 

Figure 21.1 Possible mechanisms for anionic ring-opening polymerisations of non-substituted lactones showing both acyl-oxygen scission (a) as well as alkyl-oxygen scission (b).

Haba O., Tomizuka H., Endo T, Anionic ring-opening polymerization of methyl 4,6-0-benzylidene-2,3-0-carbonyl-a-D-glucopyranoside A first example of anionic ring-opening polymerisation of five-membered cyclic carbonate without elimination of CO2, Macromolecules, 38, 2005, 3562-3563. 

Scheme 4,5 Initiation, propagation, and intramolecular transesteriiication reactions in anionic ring-opening polymerisation of trimethylene carbonate.

Many investigations concerning the ionic or the ring opening polymerisations of numerous monomers have been detailed in many books. However, to our knowledge very few studies about cationic or anionic polymerisations of fluoroalkenes have been investigated, except the well known anionic polymerisation of perfluorobutadiene as studied by Narita [53]. 

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... 

Ring opening polymerisation of (R,S) P-butyrolactone mediated by activated anionic initiators leads to a-PHB with well-defined chemical... 

Intra- and intermolecular transesterification reactions during ring-opening polymerisation initiated by anionic species. 

A development in the synthesis of macromolecular scintillators is the radical co-polymerization of alkenes with the fluorescent co-monomer 2-t-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole <89MI 406-04). Basic initiators cause ring opening of 2-phenyl-l,3,4-oxadiazolin-5-one with subsequent anionic polymerisation to form A-benzamido-1-nylon [-N(NHCOPh)CO-] <90PB583>. 

Poly(epoxides) are readily prepared by a ring-opening addition polymerisation catalysed by anionic, cationic or coordination catalysts. Consider the polymerisation of an epoxide bearing a single substituent ... 

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