Greener, cleaner polymerization of isobutene

Abstract

Methylaluminoxane (MAO)/silyl halide-based initiator systems greatly improve the sustainability of isobutene (IB)polymerizations by eliminating halogenated solvents, reducing energy consumption and removing the need for postpolymerization purifi cation. These solvent-free systems give rise to moderate yields of high-molecular-weight (MW) polymers at convenient temperatures without conventional toxic solvents. In addition, facile separation of MAO and polyisobutene (PIB) was accomplished with supported MAOs. The resulting system gave enhanced activity and high yields of medium-to-high-MW polymer at temperatures up to ambient. The MW-temperature profi le for MAO-based systems approximates that for γ-radiation and perfl uoroarylated Lewis acid-based systems; however, the former does not require high-purity monomer and is more economical and easily scalable. Modeling experiments involving 2,4,4-trimethyl-1-pentene indicate that silyl halides undergo ligand-exchange type reactions with MAO where halogen attached to silicon is replaced with a methyl substituent from MAO with concomitant production of chloraluminoxane and initiation of cationic polymerization in an ill-defi ned manner. IntroductionCationic polymerization 1-3 is a useful technique for the synthesis of a number of important materials including terpenic resins 4 and isobutene (IB)-based polymers. 5 From the standpoint of green chemistry, 6 this methodology has a number of inherent limitations that have proven diffi cult to surmount. Historically, the inverse relationship between polymer molecular weight (MW) and temperature (T) has necessitated energy intensive cooling for the synthesis of high-MW products. 7,8 Another challenge is that the Lewis acid (LA) coinitiators commonly used as a component of the initiator system remain trapped inside the product mixture. This combined with the hydrolytic instability of most LAs 9-11 precludes recycling of the coinitiator, adds post-polymerization purifi cation steps (requiring a larger plant footprint) and generates waste. Finally, given the ionic nature of the species involved in cationic polymerization, most systems only operate effi ciently in polar solvents, which are often halogenated (e.g. methyl chloride) and ultimately toxic and detrimental to the environment. 12Although numerous advances have been made over the years in improving the MW-T profi le for IB polymerizations, the majority of these systems require expensive initiator systems, solvents/monomers of ultrahigh purity and temperatures below −20°C. 13-46 Thus, a longstanding goal has been the development of initiator systems capable of producing high-MW polymers at temperatures closer to ambient in neat monomer or aliphatic solvents of moderate purity, while using a low cost and/or recyclable LA coinitiator that does not contaminate the product. Recently, a number of reports have appeared on the use of alkylaluminoxane coinitiators for the synthesis of high-MW grades of IB polymers at elevated reaction temperatures. 47-51 Of these, the most promising...

Pilot Plant Study on the Polymerization of Isobutene Using Silica-Supported Methylaluminoxane

“…Benchtop scale reactions showed that the polymerization of neat IB with supported SSMAO in conjunction with a carbocation synthon could proceed with explosive violence. 22 The rate at which polymerization occurred at ambient T to give high conversion was directly related to the propensity for the carbocation synthon to undergo ionization and found to decline as follows 1,4-dicumyl chloride (∼1−2 s) > 2-bromobutane (∼5−10 s) > ethyl bromide (∼5−10 min) > dichloromethane (∼2−3 h) > chloroform (∼10−12 h). 13 Identical polymerization runs in n-hexane were found to be more controllable, and of the aforementioned initiators, 2-BrBu had the most favorable degree of activity, effecting full conversion within 1 h once the proper ratio of the coinitiator to initiator was determined (Table 1).…”

“…The synthesis of silica-supported MAO was carried out in a manner similar to that described by Jacobsen et al 25 and is detailed in a previous publication by this research group. 22 This involves the reaction of calcined Davison Grade 11 silica gel (28−200 mesh) with a toluene solution of MAO, followed by the removal of volatiles under reduced pressure with heating. Although no issues were encountered in the preparation of this material, it should only be attempted by trained professionals.…”

“…Polymer molecular weight data was determined by size exclusion chromatography and light scattering using a setup identical to that described in earlier publications. 22,26,27 Polymers were eluted with tetrahydrofuran, and a dn•dc −1 value of 0.11 was used for polyisobutene. 1 H NMR of polymers was conducted in a manner identical to that previously reported where polymers were dissolved in CDCl 3 and run on a 300 MHz or higher instrument.…”

“…1 H NMR of polymers was conducted in a manner identical to that previously reported where polymers were dissolved in CDCl 3 and run on a 300 MHz or higher instrument. 22,26,27 The spectra obtained contained signals at ∼1.11 ppm (gem-dimethyl), ∼1.42 ppm methylene, and small peaks within the vicinity of ∼5.2−4.6 ppm corresponding to the chain ends. There are essentially equivalent percentages of endo-to exo-olefinic end-groups.…”

Indefinitely active aqueous cationic polymerizations, part I: Low cost, recyclable initiator systems

“…HPAs have been used as initiators of cationic polymerization; however, their MW‐T profiles are substandard, and they are not cost competitive to traditional LAs (e.g., BF 3 ) . As part of earlier investigations on supported methylaluminoxane and heterogeneous PFLA based initiator systems, we also studied activated forms of HPAs as solid acid initiators with limited success…”