Isobutene Polymerization Using a Chelating Diborane Co-Initiator

Abstract

Lewis acidic diborane 1 (J. Am. Chem. Soc. 1999, 121, 3244-3245) is highly effective for both proton- and cationogen-initiated isobutene polymerization in hydrocarbon media at low temperature. Reactions of diborane 1 with cumyl chloride and cumyl methyl ether were studied by variable-temperature 1H and 19F NMR spectroscopy. At low temperatures stable ion pairs 2a and 2b are formed; at higher temperatures these ion-pairs form phenyl-1,3,3-trimethylindan (3) with concomitant release of HCl to form 1 in the case of 2a or degradation of the anion (2b). Reaction between Ph3C-Cl and diborane 1 resulted in the generation of an ion-pair 4 consisting of the Ph3C cation very weakly associated with the chelated, mu-Cl counteranion as revealed by X-ray crystallography.

Alkyl Zinc Chlorides as New Initiators for the Polymerization and Copolymerization of Isobutene

“…Classical systems that provide high molecular weight PIBs include BCl 3 ,11 chelating boranes12 and a variety of transition‐metal halides, e.g., TiCl 4 , VCl 4 and FeBr 3 13. Organometallic Lewis acids paired with weakly coordinating anions, notably AlMeCp 2 /B(C 6 F 5 ) 3 ,14 [SiMe 3 ][B(C 6 F 5 ) 4 ],15 (C 5 Me 5 )TiMe 3 /B(C 6 F 5 ) 3 ,1c, 16 and ${\rm Cp'}_{\rm 2}$ ZrX 2 /[CPh 3 ][B(C 6 F 5 ) 4 ] (X = Me, H; Cp′ = C 5 H 4 SiMe 3 )17, 18 have also proved to be highly effective initiators, as have proton initiated systems coupled with perfluoroaryl borate anions 19, 20.…”

Arylzinc Complexes as New Initiator Systems for the Production of Isobutene Copolymers with High Isoprene Content

“…Mixtures of alkyl aluminum halides with alkyl halides are very efficient, although gel formation is still evident at higher concentrations of IP, and there is significant chain branching, even at À70 8C. Other examples include BCl 3 , [1,5] chelating boranes, [6] and a variety of transition-metal halides, for example, TiCl 4 , VCl 4 , and FeBr 3 . [7][8][9] Organometallic Lewis acids paired with extremely weakly coordinating anions, such as [AlCp 2 Me]/B(C 6 F 5 ) 3 , [10] [SiMe 3 ][B(C 6 F 5 ) 4 ], [11] [Cp*TiMe 3 ]/B(C 6 F 5 ) 3 (Cp* = C 5 Me 5 ), [12] and [(Cp R ) 2 [13][14][15] have also been found effective.…”

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“…Initiation probably involves protonation of monomer by a strong BA ( 5 ) that is generated in situ from 3 and adventitious moisture within the monomer droplet (Scheme ); however, direct evidence for initiation by 5 has not yet been obtained. Initiation by such species under nonaqueous reaction conditions has been implied from stopping experiments involving addition of sterically hindered pyridines (e.g., 2,6‐di‐ t ‐butylpyridine)19, 21, 32, 33 to systems using other PFLA coinitiators where adventitious moisture was believed to be the initiator. Such studies are not conclusive evidence of this type of initiation process in light of the fact that sterically hindered pyridines were found to decompose carbocations paired with weakly coordinating anions derived from PFLAs 19, 34, 35.…”