Use of telechelic fluorinated diiodides to obtain well-defined fluoropolymers
Introduction
Fluorinated polymers exhibit a unique combination of high thermal stability, chemical inertness (to acids, bases, solvents and petroleum), low dielectric constants and dissipation factors, low water absorptivities, excellent weatherability and a good resistance to oxidation and ageing, low flammabilities and very interesting surface properties [1], [2], [3].
Therefore, such products are involved in many applications (aerospace, aeronautics, engineering, optics, textile finishing, microelectronics) in spite of their high price but are undergoing an increasing market.
Well-architectured polymers can be prepared either from controlled radical polymerisation [4], [5] or from polycondensation or polyaddition. In these two last processes, telechelic or α,ω-difunctional (i.e., the functional groups are located at both extremities of the chain) precursors are required to obtain high molecular weight materials with satisfactory properties [6]. In the case of fluorinated telechelics, the literature describing their synthesis is abundant and it was reviewed several years ago [7], [8].
Two kinds of fluorinated telechelics exhibiting different properties are suggested and hence the polymers prepared from them can be involved in different applications. On the one hand, those possessing the fluorinated group in a lateral position about the polymeric backbone are searched for their enhanced surface properties but can not be used as thermostable materials resistant to oxidation and to chemicals. On the other hand, those containing the fluorinated chain in the backbone exhibit much better thermal properties and are also resistant to aggressive media, to UV radiations and to ageing.
Various strategies have been proposed to prepare this second family of fluorinated telechelics. In 1969, Rice and Sandberg [9] used the dead end copolymerization of vinylidene fluoride and hexafluoropropene in the presence of telechelic peresters. More recently, such a process was successfully achieved from hydrogen peroxide [10].
Nowadays, among fluorinated telechelics prepared on an industrial scale only three commercially available polyethers have already been obtained as follows:
- 1.
the anionic polymerisation of perfluoroepoxides (especially hexafluoropropylene oxide) led to Krytox® produced from Du Pont de Nemours Company [11], [12] as follows;
- 2.
the photooxidation of perfluoroolefins (e.g., tetrafluoroethylene [13], hexafluoropropene (HFP) [14] and perfluorobutadiene [15]) was performed by the Ausimont Company yielding functional or nonfunctional Fomblin® oligomers ranging from 1000 to 2000 in molecular weights [16] (where represents a function;
- 3.
the Daikin Company commercialises Demnum® oligomers, which can be inert or functional, from the ring opening polymerisation of fluorinated oxetanes followed by a fluorination to obtain thermostable oils [17].
A part from fluorinated polyethers, original fluorotelechelic oligodiols as random copolymers containing vinylidene fluoride (VDF) and HFP base units, e.g. HOCH2CF2(CH2CF2)p[CF(CF3)CF2]qCF2CH2OH have been synthesised by Chan et al. [18].
In addition, the synthesis of telechelic fluorinated diols from direct fluorination [19] of hydrogenated diols, produced in pilot plants, has successfully been achieved [20].
In spite of the lack of industrial fluorotelechelics (different from polyethers) about fluorinated nonfunctional homo and copolymers, the nowadays tendency shows that a growing interest is still required, as evidenced by numerous investigations [7], [8]. It seems that many reactions involve α,ω-diiodoperfluoroalkanes as reactants, to prepare telechelic intermediates preserving the fluorinated chain in the backbone.
Hence, it was worth summarising the usefulness of these diiodides as precursors of well-defined fluoropolymers. The objective of that review is to reshuffle surveys in this field of research. It is composed of three parts, each of them being dependent of both others in the chain of the preparation of these special fluoropolymers. The first one describes the various routes leading to α,ω-diiodofluoroalkanes while the second one overviews the syntheses of fluorotelechelics from them. Then, how can one use these original telechelics to prepare well-defined polymers? This last part also details the properties and applications of these latters.
Section snippets
Synthesis of α,ω-diiodo(per)fluoroalkanes
Although perfluoroalkyl iodides are widely used as reactants for the synthesis of fluorinated derivatives [21], [22], this is not the case for α,ω-diiodoperfluoroalkanes, because of the difficulty of synthesis, and their corresponding price.
However, a large variety of preparations of α,ω-diiodoperfluoroalkanes has been investigated and can be gathered in two different ways: from organic synthesis (mainly by chemical change of fluorotelechelics) and from telomerisation reactions of fluoroolefins
Functionalisation of α,ω-diiodo(per)fluoroalkanes for the obtaining of fluorinated telechelics
Several ways are possible to enable the diiodides to be functionalized. Basically, these routes can be gathered into four families:
- 1.
Direct transformation of both iodine atoms into functions, as follows (Met designates a metal):
- 2.
Functionalisation from the bis(monoaddition) of the diiodides to ω-functional-α-ethylenic derivatives ( and R designate a functional and a spacer groups, respectively):
The first
Synthesis of well-architectured fluoropolymers from fluorinated telechelics
Telechelic diols, diacids, diisocyanates, diamines, dienes are among the most promising starting materials for the synthesis of well-architectured fluoropolymers via condensation polymerisations or polyaddition polymerisations.
Several examples of preparation of such various fluoropolymers are presented in the following sections.
Conclusion and perspectives
Although perfluoroalkyl iodides are quite versatile compounds involved in many investigations to introduce a perfluorinated group on a molecule thanks to its low CF2–I bond dissociation energy, α,ω-diiodofluoroalkanes have not got same fate since less researches have been performed from them. This is mainly due to their difficult synthesis and their price. However, it can be assumed that these drawbacks may be overcome by the improvement of the synthesis which should make them readily
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