Chain polymerization living cationic polymerizations

The possibility, through living cationic polymerization processes, to produce linear chain phosphazene copolymers [486]... 

The number-average degree of polymerization for a living cationic polymerization is defined as the concentration of monomer consumed divided by the total concentration of all propagating chains (dormant and active)... 

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... 

PS/PIB/PS block copolymers can be made by controlled-living cationic polymerization. In this polymerization process, the propagating chains are in equilibrium with the dormant species. A suit-... 

When the reactivity of the two monomers is similar and steric factors are absent Rcr=Rv. For instance IB and styrene (St) possess similar reactivity, therefore, diblock copolymers poly(IB-b-St) [7] as well as the reverse order poly(St-fo-IB) [8, 9] could be readily prepared via sequential monomer addition (Scheme 2). Moreover, identical reaction conditions (-80 °C and TiCl4 as Lewis acid) could be employed for the living cationic polymerization of both monomers. However, whereas the living PIB chain ends are sufficiently... 

PaMeSt chain end] >10) before the addition of IB. This is based on a recent finding that the living cationic polymerization of pClaMeSt can be accomplished under conditions identical to those used for the synthesis of poly (aMeSt-fc-IB) copolymer [22, 23]. Importantly, the living PpClaMeSt chain end is very stable and there is no loss of livingness even after 5 h under monomer starved conditions. This is attributed to the reduced tendency of intramolecular alkylation due to the particularly large deactivating effect of the p-chloro substituent on the 2,5-positions of the aromatic ring. 

Macromonomers have been synthesized by living cationic polymerization by three different techniques by the use of a functional initiator, employing functional capping agent or by chain end modification. 

The recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

An important advantage of the use of such added nucleophiles is that it allows controlled/living cationic polymerization of alkyl vinyl ethers to proceed at +50 to +70°C [101,103], relatively high temperatures at which conventional cationic polymerizations fail to produce polymers but result in ill-defined oligomers only, due to frequent chain transfer and other side reactions. Recently, initiators with functionalized pendant groups [137] and multifunctional initiators [ 138—140] have been developed for the living cationic polymerizations with added nucleophiles. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

Relative to living cationic polymerization, the structure of a-methylsty-rene is both advantageous and disadvantageous. Because of the additional methyl group on the a-carbon, the growing carbocation is tertiary and should be thermodynamically more stable, but it would also be prone to undergo j8-proton elimination (chain transfer) due to the increase in the number of abstractable protons. Another important aspect of this monomer is its low ceiling temperature that requires low temperatures for polymerization. 

Before the development of living cationic polymerization in the 1980s, Kennedy and his co-workers devised another way to synthesize end-functionalized polymers, which uses special reagents called inifer, or initiator-chain transfer agents [129]. The method is primarily for the synthesis of polyisobutene with a tertiary chlorine terminal, which is, however, a synthon for a variety of other functional groups. These developments have been reviewed extensively [1,3,130] and fall outside the scope of this chapter. 

A major advantage of the living cationic polymerization (2) process is that, because the chain ends are stiU active after... 

Kennedy et al. used living cationic polymerization from a tricumyl initiator to prepare an allyl-terminated 3-arm star of pIB, followed by hydroboration/oxida-tion to generate hydroxy chain ends which were esterified with 2-bromoisobutyryl bromide to generate the ATRP trifunctional macroinitiator (Scheme 58) [354]. They subsequently carried out ATRP of MMA in toluene using the Cu(I)/N-( -pentyl)2-pyridylmethanimine catalyst system with the addition of Cu° powder [242] to maintain a sufficient concentration of active Cu(I) [354]. Macroinitiators of Mn=9200 and 15,000 were prepared and both had narrow molecular weight distributions (Mw/Mn=1.15 and 1.09, respectively). The formation of block copoly-... 

Transformation of Anionic Polymerization into Cationic Polymerization. Richards et al. (26. 27, 73-75) proposed several methods for the transformation of a living anionic polymeric chain end into a cationic one. Such a process requires three distinct stages polymerization of a monomer I by an anionic mechanism, and capping of the propagating end with a suitable but potentially reactive functional group isolation of polymer I, dissolution in a solvent suitable for mechanism (2), and addition of monomer II and reaction, or change of conditions, to transform the functionalized end into propagating species II that will polymerize monomer II by a cationic mechanism (73). 

Figure 8.1 The first example of a living cationic polymerization of isobutene using cumyl acetate as initiator and BCI3 as activator in dichloromethane at —30°C. Number-average molar mass, polydispersity index (numbers in the plot), and number of polymer chains (inset) are reported as a function of the mass of PIB obtained. Source Reprinted with permission from Faust R, Kennedy JP. J Polym Sci A Polym Chem 1987 25 1847 [28]. Copyright 1987 John Wiley and Sons, Inc.

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