Cationic polymerization isobutene

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Ionic Polymerization. Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media is not always clear since heterogeneous initiators are often involved. Further, it is much more difficult to obtain reproducible data because ionic polymerizations proceed at very fast rates and are highly sensitive to small concentrations of impurities and adventitious materials. Butyl rubber, a polymer of isobutene and isoprene, is produced commercially by cationic polymerization. Anionic polymerization is used for various polymerizations of 1,3-butadiene and isoprene. 

A highly obscure feature of cationic polymerization is the great phenomenological difference between aliphatic and aromatic monomers. The survey by Brown and Mathieson [84] of the behaviour of a very wide range of monomers towards trichloroacetic acid is particularly illuminating in this respect. Unfortunately, there are so few studies with aliphatic olefins that detailed comparisons must be confined to isobutene. It is well known that isobutene cannot be polymerised by conventional acids, such as sulphuric, perchloric, hydrochloric, or by salt-like catalysts such as benzoyl perchlorate, whereas all these catalysts readily give at least oligomers from aromatic olefins. Even when the same catalytic system, (e.g., titanium... 

The propensity of the C5 site towards electrophilic substitution has been exploited to prepare functionalized oligomers by cationic polymerization. Thus monomers like isobutene, s ene, the vinyl ethers, etc. polymerize in the presence of simple furan derivatives such as 2-methyl furan to give essentially short chains (DP between 2 and 100 depending on the specific experimental conditions) with a terminal furan ring as a result of predominant transfer onto the C5 position of the added furan compound (20). 

Fig. 3.54. Carbenium ion additions to isobutene as key steps in the cationic polymerization of isobutene. The dashed arrow corresponds to the overall reaction.

Intermolecular additions of carbenium ions to olefins give polymers. Such a reaction is used in industry, for example, in the cationic polymerization of isobutene (Figure 3.43). One of the rare cases of an intermolecular carbenium ion addition to an olefin without polymer formation occurs in the industrial synthesis of isooctane (Figure 3.44). 

The cationic polymerization of vinyl monomers such as isobutene, styrene, ot-methylstyrene, indene, and vinyl ethers is generally impeded by several transfer processes65 . The molecular weight of the polymer is not determined by the molar ratio of monomer to initiator. Therefore, the methods that are based on the long lifetime of the active sites at the chain ends cannot be applied here. 

Nevertheless, a few years ago, Kennedy 66 69) developed a method yielding co-functional polymers by cationic polymerization of vinyl monomers. The principle of the socalled inifer method is to kinetically favor transfer to the initiating species with respect to all other kinds of transfer reactions (especially the transfer to monomer). A typical initiating system is composed of an allyl or benzyl halide and boron trichloride BCl3. This mixture behaves like an alkenium tetrachloro-borate and readily initiates the polymerization of monomers such as isobutene or a-methylstyrene. The efficiency of the halide as a transfer agent depends on the lability of the C—Cl bond and on the molar ratio [RC1]/[BC13],... 

Finally the fact that polymerization occurred in this system is not peculiar at all. The catalyst of the Russian authors was composed of 1 2 AlEt3 TiCl4 whereas the American workers also used Al/Ti ratios of 2. These compositions yield acid catalysts which explains the reported activity toward isobutene. In other words, these catalysts must have contained free TiCl4 or EtAlClg. When the Al/Ti ratio was raised to 24, no polymerization occurred because no acidic species were present any more 153). The observation 151) that increasing molecular weights were obtained with decreasing temperature is also typical for cationic polymerizations. 

An artificial rubber may be made by cationic polymerization of isobutene using acid initiation with BF3 and water. What is the mechanism of the polymerization, and what is the structure of the polymer ... 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Carbenium ions apparently are formed =100 times less efficiently than radicals, with both radical and carbocationic polymerization operating simultaneously when kinetically possible. Although isobutene does not polymerize radically, styrene readily polymerizes by a radical mechanism. Thus, radical polymerization of styrene dominates in wet systems where the cations are trapped by water to form inactive oxonium ions. Polymerization is =100 times faster in super-dry systems, demonstrating that cationic polymerization must dominate, and that the rate constant of cationic polymerization is approximately 104 times higher (=100 x =100) than that of the radical polymerization ( radical) = 80 mol- -L-sec-1 at 25° C). 

Since its discovery for vinyl ethers and isobutene in the 1980s, the scope of controlled/living cationic polymerization has been expanded rapidly in terms of monomers and initiating systems. Figure 18 shows a partial list of representative monomers for which controlled/living cationic polymerizations are available. They cover virtually all classes of cationically polymerizable vinyl compounds, such as vinyl ethers, isobutene, styrene and its derivatives, and A/-vinylcarbazole. A rough estimate indicates that the total number of monomers for controlled/living cationic polymerization... 

In 1986 Faust and Kennedy reported the first example of controlled/living cationic polymerization of isobutene, which was initiated by a cumyl ace-... 

More recently, Kennedy reported another initiating system that controls styrene polymerization with an added nucleophile 2,2,4-trimeth-ylpentyl chloride (TMP-Cl)/TiCl4 with N,N-dimethylacetamide (DMA) in CH3Cl/methylcyclohexane (4 6 v/v) mixture at -80° C [165]. The use of another additive, 2,6-di-ferf-butylpyr idine (proton trap), is described as beneficial. The molecular weight and MWD are controlled in this system, but the role of the added DMA is still ambiguous [166]. This system with the aliphatic (erf-chloride was designed to extend to the synthesis of isobutene-styrene block copolymers via sequential cationic polymerization (Chapter 5). 

Figure 24 Typical multifunctional initiators for controlled/living cationic polymerizations. See also Table 3 for references with isobutene. ...

As the range of styrene derivatives for living cationic polymerization expands (Chapter 4, Section V.C), a variety of block copolymers with sty-renic segments have been synthesized. Most of the reported examples involve combinations of styrene derivatives with vinyl ethers or isobutene. Some examples of styrene derivative-vinyl ether block copolymers are listed in Fig. 6 [16,87-89]. Monomers that can form similar block copolymers with isobutylene are listed in Fig. 7 (Section III.B.3). 

As pointed out already, rather few end-functionalized poly isobutenes have been obtained from isobutene via living cationic polymerization, whereas abundantly via the inifer method followed by various chemical reactions to convert the resulting tertiary chloride terminal (Section IV.A.3) [3],... 

Recently, a functional initiator method has been reported for isobutene, where the initiator is CHsOQO)—Ar—C(CH3)2—Cl (Ar = t-BuC6H3) [156]. In the presence of TiCl4 (activator) and N,N-dimethylace-tamide (as an added nucleophile), the cumyl-type moiety of the initiator initiates living cationic polymerization the acetate moiety serves as the protected carboxylic acid. 

A class of end-functionalized polymers with polymerizable terminal groups are generally called macromonomers. By both functional initiator and terminator methods, a variety of macromonomers have been synthesized in living cationic polymerization of vinyl ethers, styrenes, and isobutene, as summarized in Table 3 [16,31,147,149-151,155,158-171]. Some of these macromonomers are used in the synthesis of graft polymers (Section VI.C). 

As summarized in Chapter 4, Section V.E.l, a variety of multifunctional initiators are currently available for the living cationic polymerizations of vinyl ethers [83,188,189], alkoxystyrenes [149,190], and isobutene [191-201], and up to tetraarmed polymers with controlled arm lengths are prepared by the use of these initiating systems. Scheme 9 exemplifies such a synthesis for vinyl ethers [188]. The details for the design of these initiating systems are found in Chapter 4. 

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 major industrial production of polymers obtained by cationic polymerization of alkenes is related to the 2-methylpropene (isobutene or isobutylene) homo- and copolymers which can be classified into three families ... 

Cationic polymerization is applied almost exclusively to monomers with olefinic double bonds. Susceptible are double bonds whose carbon atoms carry electron-donating substituents such as alkyl groups. Thus, isobutene with two methyl groups adjacent to the double bond polymerizes readily, propene with only one is sluggish, and ethene with none is inert a-methyl styrene is more reactive than styrene vinyl ethers are reactive, but vinyl chloride is not. The most important commercial product is butyl rubber, produced by copolymerization of isobutene with small amounts of isoprene, initiated by A1C13, BF3, or TiCl4 [82]. 

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