Polymerizations, cationic living type

Three other MEEP-type polyphosphazenes were synthesized by Allcock [622]. Polymers XIII and XIV were prepared via the cationic living polymerization of phosphoranimines, and polymers XV by ring opening polymerization. 

Most of the known cationic photoinitiators produce acid species in an irreversible reaction, and once formed these species continue to promote the polymerization reaction even after the end of irradiation. This behavior is of living type and is in contrast to the radical photoinitiated... 

The polymerization methods leading to linear diblock, triblock or segmented block copolymers are based on two general reaction schemes. In a first one, a or a, oj active sites are generated on a polymer chain poly A which then initiate the polymerization of a second monomer B. Such a polymerization can be of free radical, anionic or cationic type and preferably of living type which proceed without termination and transfer reactions. The concept of this synthesis is given in Figure 7.2. 

Linear polyethylenimine (LPEI), which contains only secondary amino functions, has been prepared in different ways. The one most used is the hydrolysis of poly(N-acyl ethyienimine), which is obtained by cationic polymerization of the corresponding N-alkyl or aryl oxazoline (see Chapter 4.24), (Scheme 5). This type of polymerization exhibits living characteristics so that polymers with controlled molecular weight and narrow distribution can be obtained. Consequently, the LPEI obtained from such a polymer will also have a... 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

Surface-initiated living cationic polymerization of 2-oxazolines on planar gold substrates has been reported by Jordan et al (Fig. 9). SAMs of initiators on a planar gold substrate have been used to initiate the living cationic ringopening polymerization of 2-ethyl-2-oxazoline. The polymer chain end was functionalized with an alkyl moiety by means of a termination reaction in order to form an amphiphilic brush-type layer. The resulting layers (thickness... 

Various types of well-defined block copolymers containing polypropylene segments have been synthesized by Doi et al. on the basis of three methods (i) sequential coordination polymerization of propylene and ethylene 83-m>, (ii) transformation of living polypropylene ends to radical or cationic ones which initiate the polymerization of polar monomers 104, u2i, and (iii) coupling reaction between iodine-terminated monodisperse polypropylene and living polystyrene anion 84). In particular, the well-defined block copolymers consisting of polypropylene blocks and polar monomer unit blocks are expected to exhibit new characteristic properties owing to the effect of microphase separation. 

However, polyphosphazene micelles are known. These are formed by sonication of aqueous suspensions of amphiphilic diblock copolymers—macromolecules in which one block is hydrophilic and the other is hydrophobic. Both blocks may be phosphazene-based, or one block can be a phosphazene and the other an organic polymer. Polymers of this type are assembled via the living cationic polymerization discussed earlier. The polymers shown as 3.95-3.99 illustrate the range of structures that have been studied and the size of the micelles formed.237-240... 

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