Ethers living polymerization

Because of these consecutive steps, initiation by iodine is slow. Purposeful addition of HI with I2 leads to significantly better initiation and, in the case of -butyl vinyl ether, living polymerization (49). 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

Various block copolymers have been synthesized by cationic living polymerization [Kennedy and Ivan, 1992 Kennedy, 1999 Kennedy and Marechal, 1982 Puskas et al., 2001 Sawamoto, 1991, 1996]. AB and ABA block copolymers, where A and B are different vinyl ethers, have been synthesized using HI with either I2 or Znl2. Sequencing is not a problem unless one of the vinyl ethers has a substituent that makes its reactivity quite different. Styrene-methyl vinyl ether block copolymer synthesis requires a specific sequencing and manipulation of the reaction conditions because styrene is less reactive than methyl vinyl ether (MVE) [Ohmura et al., 1994]. Both monomers are polymerized by HCl/SnCLj in the presence of (n-CrikjtiNCI in methylene chloride, but different temperatures are needed. The... 

Under certain conditions, irreversible chain-breaking reactions are absent and cationic ROPs of cyclic ethers proceed as living polymerizations. These conditions are found for polymerizations initiated with acylium and l,3-dioxolan-2-ylium salts containing very stable counterions such as AsFg, PFg, and SbClg or with very strong acids (fluorosulfonic and... 

In the early 1980s, Kennedy and his co-workers reported quasiliving polymerizations, which are phenomenologically akin to living polymerizations [57]. These processes involved slow and continuous monomer addition to a stirred initiator solution kept at a relatively low temperature. The monomers used therein included a-methylstyrene, isobutene, styrene, and alkyl vinyl ethers. In most cases, the number-average molecular weights steadily increased with the weight of the added monomer and the formed polymers had relatively narrow MWDs. 

With the HI/I2 initiating system, they also found that the polymerization of IBVE and related alkyl vinyl ethers (CH =CHOR R = methyl, ethyl, and higher alkyls) in nonpolar solvents led to results that conformed with living polymerizations, as summarized in Figure 16 [58,59] ... 

Initiating Systems with Nucleophilic Counteranions The first generation of initiating systems for vinyl ether controlled/living polymerizations consists of a protonic acid (HB, initiator) with a Lewis acid (MtX , activator), [63,69,70]. The reported examples ofHB and MtX ... 

The HB/MtX -initiated polymerizations of vinyl ethers are typically carried out in nonpolar media such as toluene and n-hexane (depending on the solubility of the products) at temperatures below 0° C. In some cases, however, polar solvents (e.g., methylene chloride) may be used at appropriate initiator/activator mole ratios [119] and, specifically with the HI/ZnI2 system, controlled/living polymerization is feasible even at room temperature ( + 25° C) [98,99]. 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

These salts suppress the free ionic (dissociated) growing species and accelerate the conversion of carbocations to covalent species (see Sections IV.B.3 and IV.B.3). The salt-mediated living polymerizations of vinyl ethers in polar media also provide useful systems in which polar monomers and their polymers that can be insoluble in nonpolar solvents can be polymerized successfully. 

J. Initiating Systems with Nucleophilic Counteranions For isobutene, this group refers almost exclusively to the BCh-based initiating systems without external additives. As listed in Table 3.A, combinations of BCl3 with tertiary esters, ethers (methoxides), or alcohols induce controlled/living polymerization of isobutene in CH3C1 or other solvents at temperatures below -30° C. Scheme 10 illustrates the proposed pathway for the polymerization initiated with the cumyl acetate (12)/BC13 system [35] ... 

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