Butene, cationic polymerization

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

Monomers for manufacture of butyl mbber are 2-methylpropene [115-11-7] (isobutylene) and 2-methyl-l.3-butadiene [78-79-5] (isoprene) (see Olefins). Polybutenes are copolymers of isobutylene and / -butenes from mixed-C olefin-containing streams. For the production of high mol wt butyl mbber, isobutylene must be of >99.5 wt % purity, and isoprene of >98 wt % purity is used. Water and oxygenated organic compounds iaterfere with the cationic polymerization mechanism, and are minimized by feed purification systems. 

The ionic liquid process has a number of advantages over traditional cationic polymerization processes such as the Cosden process, which employs a liquid-phase aluminium(III) chloride catalyst to polymerize butene feedstocks [30]. The separation and removal of the product from the ionic liquid phase as the reaction proceeds allows the polymer to be obtained simply and in a highly pure state. Indeed, the polymer contains so little of the ionic liquid that an aqueous wash step can be dispensed with. This separation also means that further reaction (e.g., isomerization) of the polymer s unsaturated ot-terminus is minimized. In addition to the ease of isolation of the desired product, the ionic liquid is not destroyed by any aqueous washing procedure and so can be reused in subsequent polymerization reactions, resulting in a reduction of operating costs. The ionic liquid technology does not require massive capital investment and is reported to be easily retrofitted to existing Cosden process plants. 

Such isomerizations are sometimes desired and sometimes are the cause of or explanation for unwanted structures. In the cationic polymerization forming poly(l-butene), nine different structural units have been found. Classical 1,2-hydride and 1,2-methide shifts, hydride transfer, and proton elimination account for these structures. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Kennedy and Thomas (1) first reported the synthesis of a crystalline poly(3-methyl- 1-butene) by cationic polymerization at —130°C. Preliminary HNMR studies indicated that the polymer was not simply a tactic modification of the conventional 1,2-polymer but, in fact, possessed a repeat structure which resulted... 

The polymerization kinetics of propylene, 3-methyl-l-butene, and 4-methyl-l-pentene can be described by Eq. (12) and it is felt that this scheme may be generally valid for cationic polymerization of olefins as there is no reason to suspect that a fundamental difference in polymerization mechanism exists in the case of the three monomers cited above as compared with other cationically polymerizable olefins. 

The detailed composition of poly(3-methyl-l-butene) and poly(4-methyl-l-pentene) produced by cationic polymerization has been investigated using high resolution 300 MHz H NMR and 20 MHz 13C NMR spectroscopy. It has been confirmed that both monomers polymerize by a cationic isomerization polymerization involving intramolecular hydride shifts. The composition of poly(3-methyl- 1-butene) obtained by cationic polymerization at — 130° C has been shown to be predominantly -(-CH2—CH2—C(CH3)2. ... 

The proposed model is similar in general form to those proposed for the cationic polymerization of propylene (16) and 3-methyl-1-butene (4), and in fact, could possibly be extended to all cationic polymerizations of olefins. 

Another type of ionic species was proposed by Uelzmann (92). Uelzmann suggested that from trialkylaluminum and titanium tetrachloride, TiClg)+ and (R3A1C1) caused cationic initiation on the titanium followed by an anionic propagation on the aluminum ion. Bestian and co-workers (70) proposed similar cationic intermediates which propagate by anionic shifts. These steps are the opposite of the anionic initiation and cationic propagation proposed in this review for the butene-1 polymerization. 

Figure 2 Free radical polymerization (top) and anionic polymerization (middle) of styrene and cationic polymerization of iso-butene (bottom). For the nature of initiators X see text.

In the large mqority of the cationic polymerizations of heterocycles the propaption step proceeds with complete inversion of conf uration. Ihis vras diown for the first time by Vandenberg in the polymerization of ds- and trans-2,3-butene oxides as... 

In the cationic polymerization of heterocycles, a similar phenomenon was observed by Goethals in the polymerization of propylene sulfide and trans 2,3-dimethyl-thiirane. The latter monomer polymerizes rapidly and quantitatively to a linear polymer which is then relatively slowly converted into 3,4,6,7-tetramethyl-l, 2,5-tri-thiepane (J67a). In this particular process, the macroring formation is a practically irreversible reaction and differs in this sense from the equilibrium processes discussed so far. The irreversibility is due to the formation of one molecule of cis-butene per one molecule of a cyclic trithiepane ... 

Account for the fact that 1-butene can be used to control polymer molecular weight in cationic polymerization of isobutylene. 

The cationic polymerization of 3-methyl-l-butene produced a polymer whose NMR spectra consisted of only two singlets. Propose a structure for the polymer consistent with the NMR and suggest a mechanism for this polymerization. 

Ketley (10) has shown that 1,1-dimethylcyclopropane gives the same polymer by cationic polymerization as that obtained from 3-methyl-l-butene. He assumed a tt complex mechanism for initiation. 

M. Sangermano, S.N. Falling, and J.V. Crivello, Photoinitiated cationic polymerization of epoxy monomers in the presence of poly(3,4-epoxy-1-butene). J. Macromol. Sci. Pure Appl. Chem. 2002, A39(ll), 1279-1294. 

The cationic polymerization of propylene, 1-butene, and higher 1-alkenes yields only very low molecular weight polymers DP < 10 - 20) with highly complicated strucmres that arise due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer, and elimination, besides chain transfer during polymerization. In the polymerization of ethylene, initiation involving protonation and ethylation is quickly followed by energetically favorable isomerization ... 

The kinetic influence of water on cationic polymerization is complicated. In some systems, e.g., i-butene/TiCU in dichloromethane (Biddulph et al., 1965),... 

Explain why a random copolymer is obtained when 3,3-dimethyl-1-butene undergoes cationic polymerization. 

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