Polyphosphazenes polymerization mechanism

This effect has also been observed in polyphosphazenes containing alkyl- or phenyl-carborane as pendent groups.12 A typical synthetic route to poly(phenyl-carboranyl-di-trifluoroethoxy-phosphazene) having pendent phenyl-carborane groups is shown in scheme 4. A substantial improvement in the thermal stability of the polymer was observed. This is attributed to a retardation of the ring-chain de-polymerization mechanism due to steric hindrance effects of the carborane units, inhibiting helical coil formation. 

It will be clear from the discussion so far that ring-opening polymerization is a crucial step in the synthesis of most polyphosphazenes. Hence, a knowledge of the mechanism of this process is important for widening the scope of this field, optimization of... 

A very promising variant on this type of condensation polymerization involves the use of monomers that possess groups X and Y, which can be eliminated from the same molecule. This circumvents the need for careful control of reaction stoichiometry. Moreover, in certain cases, polymerization of monomers of this type can follow a chain-growth type of mechanism that leads to high molecular weights much more easily. Such processes (Scheme 1.1, Route C) have not yet been explored for the formation of metal-containing polymers, but are well-established for the synthesis of certain classes of polymers based on main-group elements such as polyphosphazenes (1.2) and polyoxothiazenes (1.4) [12, 16, 26]. 

Aspects of phosphazene research " and of the history of phosphazenes have been examined. The prototrope equilibrium (Scheme 13) between the polyaminophosphines (NH form) and the polyiminophosphines (polyphosphazene or PH form) has been studied by Quantum Mechanical ab initio calculations. When R = H, the energy difference is in favour of the NH tautomer, but if R = NH2 the more stable is the polyphosphazene form. In fact, the preference for the later increases with the electronegativity of the R substituent, and in agreement with the experimental facts, the calculations showed that the polymerization of the monophosphazanes (1) (see Section 1) should be favourable when the electronegativity of R is about 3. The polyhydrido phosphazene has an helical structure with small bond alternation. The calculations also support the mechanisms for the formation of the polyhydridophosphazene (3) from tris(amino)phosphine P(NH2)3 (la) via its phosphazene (NH2)2HP=NH (2a) tautomer. ... 

The synthesis of hydrido-amino phosphazenes can be carried out directly from aminophosphines. Thus, the dialkylamino(amino)phosphine (Me2N)2PNH2 (generated in situ by amminolysis of the corresponding chlorophosphine) underwent a fast kinetically controlled polycondensation process without cross-linking in solution at low temperature to form the low P-hydrido(dimethylamino)polyphosphazene [N P(H)(NMc2)] (186) with absolute = 41.000 (PDI = 1.5) in a planar cis-trans or twisted helical conformation. The proposed mechanism for the polymerization was supported by ab initio calculations on the model (H2N)2PNH2 reported in ref. 1 (see above). 

The derivatization reactions of the poly(alkyl/aryIphosphazenes) afford many new polyphosphazenes with a variety of functional groups and significantly extend the versatility of this class of polymers. Furthermore, the chemistry described above demonstrates that it will be possible to incorporate a number of side-groups into the polymer to enhance the thermal, mechanical, and electrical properties. A large number of graft copolymers of polyphosphazenes should also be accessible via initiation of anionic polymerizations by the polymer anions. 

Modifications in the preparation of polyphosphazenes by the condensation polymerization of N-silylphosphoranimines are also reported. The preparation of a series of new poly(alkyl/arylphosphazenes), relevant chemistry of the Si-N-P precursors, and catalysis by phenoxide anions are presented (NeUson, Chapter 18). Matyjaszewski (Chapter 24) discusses the use of fluoride catalysts to prepare polyphosphazene random and block copolymers with alkoxy and fluoroethoxy substituents and proposes mechanisms for their formation. 

The synthesis of N-silyl-phosphoranimines bearing mixed alkoxyalkoxy and trifluoroethoxy substituents is reported. The polymerization kinetics and the proposed anionic mechanism are also discussed. These monomers are utilized for the preparation of polyphosphazene random copolymers by the simultaneous polymerization of two phosphoranimines. Block copolymers have also been synthesized by addition of a second phosphoranimine after conversion of the first. Evidence for copolymer formation includes and 3lp NMR, SEC, solubility and DSC data. The differences between analogous random and block copolymers are discussed. 

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