Pendant-functionalized vinyl ether

Another important role of the pendant-functionalized vinyl ethers is that they can be precursors of initiators for living cationic polymerization of other vinyl ether and styrene derivatives, from which polymers with terminal functional groups can be prepared (see Section IV). 

This method involves the use of functional initiators with a protected or unprotected functional group. When the functional group is unreactive under the polymerization conditions, protection is not necessary. Functional vinyl ethers have extensively been used in the Bving cationic polymerizations of vinyl ethers and these functional poly(vinyl ethers) can be derivatized to the desired functionahty by simple organic reactions. Vinyl ethers carrying a variety of functional pendant groups, in a general form (Scheme 26.1) ... 

These applications are all based on an advantage of living cationic polymerization that one can readily prepare well-defined living polymers even from monomers carrying polar functional pendant substituents. Typical examples of functionalized vinyl ethers include ... 

By the same strategy, we have prepared a variety of star-shaped multi-armed poly(vinyl ether)s by our living cationic polymerization. As illustrated below, the nature of the arms can be varied by changing the structures of the pendant substituents (derived from functionalized vinyl ethers j8), including hydrophobic ( )/ hydrophilic (23), amphiphilic (24), and terminally functionalized (25). 

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines, N-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

Aldol group transfer polymerization of ferf-butyldimethylsilyl vinyl ether [62] was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the aldehyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the ferf-butyldimethylsilyl groups, poly(vinyl alcohol) branches. 

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. 

Almost all of the initiating systems discussed in this section can be applied to the living cationic polymerizations of vinyl ethers that carry a variety of functional pendant groups, in a general form [42,43,65,66,180] ... 

Scheme 1 Typical examples of the synthesis of pendant-functionalized poly(vinyl ethers) [23,29],...

Since the development of living cationic polymerizations of alkyl vinyl ethers (Chapter 4, Sections IV and V.A), considerable efforts have been made to synthesize and polymerize vinyl ether derivatives carrying polar functional substituents, and thereby it is now possible to obtain a variety of pendant-functionalized poly(viny) ethers) of controlled molecular weights and narrow MWDs [1,2,13], Figure 3 lists typical examples of vinyl ethers carrying various pendant functionalities for which living cationic polymerizations are available [14-35]. These monomers are synthesized most conveniently from 2-chloroethyl vinyl ether, now commercially available,... 

Figure 3 Vinyl ethers with pendant functional groups for which living cationic polymerization is feasible. The numerals in brackets attached to each entry indicate reference numbers.

The pendant functions listed in Figure 3 are often useful and of synthetic interest per se. For example, methacrylate and acrylate esters are polymerizable (cross-linking sites) [19-21] the cinnamate is photorespon-sive (for the photo-induced dimerization of its unsaturated groups) [20] oligo(oxyethylene) [25-27] and carbohydrate groups [35] give hydrophilic and water-soluble polymers, whereas perfluoroalkyl moieties [32-34] enhance hydrophobicity. Thus, poly(vinyl ethers) with cinnamate functions... 

Among these, in particular, the acetate [17] and the silyloxyl [31] derivatives are often used as the protecting groups for the hydroxyl (alcohol) function. For example, polymers of 2-acetoxyethyl vinyl ether are readily transformed into a polyalcohol, poly(2-hydroxyethyl vinyl ether), by alkaline hydrolysis [17]. Due to the polar pendant functions, the polymers are of course hydrophilic and often water-soluble, and serve as hydrophilic segments in so-called amphiphilic polymers, as will be discussed later (Sections III.D and VI.B.5). Other important protecting groups include the malonate [23] and the imides [29,30], which lead to polymeric carboxylic acids and amines, respectively (Scheme 1). 

Despite these examples, one can readily notice that the living cationic polymerization of pendant-functionalized styrene derivatives has been studied much less extensively than those of vinyl ethers, and further progress is anticipated accordingly. 

As seen in Scheme 2 (A), the most of the syntheses have been carried out with the HI/I2 and HX/ZnX2 (X = halogen) initiating systems, because these systems can effectively polymerize a large variety of vinyl ethers, including those with pendant functions, into well-defined living polymers [1]. In this way, the sequential living cationic polymerizations of two vinyl ethers are mostly "reversible i.e., both A - B and B - A polymerization sequences are operable. This is in sharp contrast to the block copolymerization of a vinyl ether with a styrene derivative or isobutylene (see below), where such reversibility often fails to work. 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Some recent syntheses employ the first method (A), where, for example, living cationic polymerizations of isobutene [222], (f-butyl)dimethylsilyl vinyl ether [223,224], and 2-methyloxazoline [225] are initiated from appropriate pendant functional groups. 

The intramolecular coupling between the hydroxyl functions of the HOVE units of block A and the pendant vinyl ether functions of block C was then achieved in presence of pyridinium para-toluene sulfonic acid salt (PTSA) as catalyst. The reaction between hydroxyl functions and vinyl ether groups yield the rapid formation of acetal links between the A and C sequences, as described in Scheme 21.7. 

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