Isobutene Polymerization Using Chelating Diboranes: Polymerization in Aqueous Suspension and Hydrocarbon Solution

Abstract

The use of the chelating diboranes o-C6F4[B(C6F5)2]2 (1) and o-C6F4(9-BC12F8)2 (2: 9-BC12F8 = 1,2,3,4,5,6,7,8-octafluoro-9-borafluorene) for the polymerization of isobutene (IB) in aqueous suspension or in hydrocarbon solution was studied. Polymerizations in aqueous suspension provided polymer of moderate MW and at variable conversion and were dependent on temperature, mode of diborane addition, the presence of surfactant, and the acidity of and nature of the anion present in the aqueous phase. The T dependence of MW over the T range −80 to −20 °C was studied in aqueous suspension, and higher MW polymer was formed at lower T. The hydrolysis and methanolysis of diboranes 1 and 2 was studied by NMR spectroscopy. Reactions of diborane 1 with excess MeOH or water afford solutions containing oxonium acids [o-C6F4{B(C6F5)2}2(μ-OR)][(ROH) n H] (7: R = H, n > 2; 3: R = Me, n = 3). When diborane 1 is present in excess over water or MeOH, degradation of the diborane is observed. In this case the products are o-C6F4{B(C6F5)2}H (5) and (C6F5)2BOH 7 or (C6F5)2BOMe 4, respectively. In the case of diborole 2, o-C6F4(9-BC12F8)B(2-C12F8-2′′-H)(μ-OH)·7H2O (17) and o-C6F4(9-BC12F8)B(2-C12F8-2′′-H)(μ-OMe) (11) were isolated from reactions of 2 with water and MeOH, respectively, and were characterized by X-ray crystallography. None of these degradation products effect IB polymerization in aqueous suspension. As a model for initiation of polymerization, the reaction of diborole 2 with 1,1-diphenylethylene (DPE) was studied. Addition of MeOH at low T results in efficient formation of the ion-pair [Ph2CMe][o-C6F4(9-BC12F8)2(μ-OMe)] via protonation of DPE. Polymerizations in hydrocarbon media were exothermic and rapid and gave quantitative yields of polymer even at very low concentrations of diborane 1. The T dependence of MW was studied in hydrocarbon solution and showed non-Arrhenius behavior. This was explained by competitive chain transfer to monomer at elevated T and chain transfer to molecular water at lower T.

A Catalyst Platform for Unique Cationic (Co)Polymerization in Aqueous Emulsion

“…Unfortunately,t hese catalysts required the use of highly concentrated electrolyte aqueous solutions as reaction media, allowing polymerizations exclusively at very low temperatures (below À40 8 8C). [13][14][15] These conditions are clearly not those of the conventional emulsion technique,aspresented here.…”

“…[12] Despite obvious progresses in terms of reaction rates,t he molar masses of synthesized polymers were still not higher than 3000 gmol À1 , alimit value above which interfacial polymerization ceases. [1] M(C 6 F 5 ) 3 (M = Al, Ga) [13] or more complex aromatic boranes (C 6 F 4 -1,2[B(C 6 F 5 ) 2 ], [14] C 6 F 4 -1,2[B(C 12 F 8 ) 2 ]) [15] successfully polymerized isobutylene [13][14][15] and styrene [15] into high-molarmass polymers (M n = 10-100 kg mol À1 ). Unfortunately,t hese catalysts required the use of highly concentrated electrolyte aqueous solutions as reaction media, allowing polymerizations exclusively at very low temperatures (below À40 8 8C).…”

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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.…”

“…Subsequent to the work of Sawamoto and coworkers it was discovered that the chelating diborane (1,2‐C 6 F 4 [B(C 6 F 5 ) 2 ] 2 , 1 ) was stable in excess methanol19–21 after initial attempts to generate the mono‐methanol adduct19, 22 failed. This discovery led to the development of conditions for aqueous polymerization of IB and its copolymerization with isoprene (IP)9, 19–21, 23 and until recently was the only aqueous cationic olefin polymerization system capable of producing high MW grades of polymers. Following work on chelating LAs, a number of systems based on the monodentate analog of 1 , [i.e., B(C 6 F 5 ) 3 , tris(pentafluorophenyl)boron, 2 ] in the presence or absence of a carbocation synthon have been shown to be capable of producing low MW polymers.…”

Poly(isobutylene) Nanoparticles via Cationic Polymerization in Nonaqueous Emulsions

“…In addition, there are many well‐known advantages to using emulsion polymerizations, including greater control over the polymerization rate, prevention of localized superheating and reactor fouling, defined chemical structures, and controlled molecular weights . Since the polymerization of IB is commonly performed at extremely low temperatures (≈−40 to −100 °C), most emulsion attempts have utilized electrolyte solutions to prevent the freezing of the aqueous dispersed phase . This has led to several limitations, such as low yield due to termination reactions in the polymerization processes and the use of synthetically challenging borate catalysts …”