IB is the most studied monomer that can only polymerize by a cationic mechanism. The living carbocationic polymerization of IB was first discovered by Faust and Kennedy using organic acetate/BCl3 initiating system in CH3Cl or CH2Cl2 solvents at –50 to –10 °C.23,24 Living carbocationic polymerizations of IB to date are based on BCl3, TiCl4, and organoaluminum halide coinitiators. The activity of the BCl3-based system is greatly solvent dependent, that is, sufficient activity only occurs in polar solvent. In less-polar solvents, the solvation of the counteranion does not promote ion generation and the binary ionogenic equilibrium is strongly shifted to the left. Therefore, the concentration of growing cations is extremely small, resulting in negligible polymerization rates. However, since PIB is poorly soluble in polar solvents at low temperatures, the molecular weights are limited with the BCl3-based initiating systems.
A wide variety of initiators, organic esters, halides, ethers, and alcohols have been used to initiate living polymerization of IB at temperatures up to –10 °C. The true initiating entity with ethers and alcohols is the chloro-derivative arising by fast chlorination. The polymerization involving the BCl4− counteranion is very slow, measured in hours, compared to the fast polymerization by protic impurities, and in the absence of proton scavenger, the monomer is consumed mainly by this process. In the presence of proton trap or EDs, similar rates, controlled molecular weights, and narrow MWDs (PDI ∼ 1.2) have been reported.37 According to kinetic studies, the polymerization is first order both in respect to monomer and BCl3.37 The absence of common ion salt effect in polymerizations involving the BCl4− counteranion suggests that propagation is mainly via the ion pairs, and the contribution of free ions, if any, is negligible.67
Organic esters, halides, and ethers have been used to initiate living polymerization of IB at temperatures from –90 up to –40 °C. In conjunction with TiCl4, ethers are converted to the corresponding chlorides almost instantaneously, while the conversion of esters is somewhat slow.34 According to Chen et al.,68 alcohols are inactive with TiCl4 alone but have been used in conjunction with BCl3 and TiCl4. BCl3 converts the alcohols to the active chloride, which is activated by TiCl4. In contrast to Chen et al., Puskas and Grassmuller69 reported chlorination of alcohols and initiation by TiCl4 alone.
Under well-dried conditions, PIBs with controlled Mns up to ∼ 60 000 and narrow MWDs could be prepared in the absence of any additives in nonpolar solvent mixtures and low temperatures.33 PIBs with Mns up to 150 000 and Mw/Mns as low as 1.02 have been obtained in the presence of proton trap or Lewis bases. The polymerization is first order in monomer but second order in TiCl4, due to dimeric counteranions,33 although first-order dependency was reported at [TiCl4] < [initiator, I0].70 The consequence of the second-order rate dependence is that although excess of TiCl4 over the initiator halide is not required to induce polymerization, at low initiator concentrations to obtain high Mn, acceptable rates are only obtained when high TiCl4 concentrations (16–36 times [I0]) are used. Living polymerization of IB was also reported with the TiCl4/TiBr4 mixed71 coinitiator that yields mixed Ti2Cln + 1Br8 – n− counteranions. By the stepwise replacement of Cl to Br, however, the Lewis acidity decreases, which results in a decreased ionization rate constant and therefore decreasing overall rates of polymerization with decreasing TiCl4/TiBr4 ratio.
Organoaluminum compounds have also been employed for the living cationic polymerization of IB using 1,4-bis(1-azido-1-methylethyl)benzene/Et2AlCl/CH2Cl2 at –50 °C to produce a living polymerization of IB for Mn < 50 000 where the presence of an ED like DMSO is not necessary.72 Another polymerization system based on Et2AlCl and tertiary alkyl halide initiators has been reported but requires the use of an 80/20 (v/v) nonpolar/polar solvent mixture.73 The first example of Me2AlCl-catalyzed living polymerizations of IB was presented using conventional tertiary alkyl chloride initiators and 60/40 (v/v) nonpolar/polar solvent mixtures. PIBs were prepared with Mn = 150 000 and Mw/Mns = 1.274 even in the absence of additives such as proton traps or EDs. The ‘living’ nature of these polymerizations has been demonstrated at –75 to –80 °C in both 60/40 (v/v) hexane/CH2Cl2 and hexane/methyl chloride (MeCl) solvent systems. Recently, the living polymerization of IB was also reported using TMPCl/DTBP/hexanes:MeCl solvent mixtures/–80 °C using Me2AlCl, Me1.5AlCl1.5, or MeAlCl2.75 With the latter two coinitiators, the polymerization was extremely fast and completed in seconds that necessitated special considerations for reaction control.