Radical Copolymerization of Isobutylene and Ethyl Acrylate with LiCB₁₁Me₁₂ Catalyst

Abstract

The radical homopolymerization of isobutylene (IB) is unusual but can be accomplished under catalysis with LiCB 11 (CH 3 ) 12 (1) in poorly coordinating solvents at ambient pressure and temperature. 1-3 It yields a novel highly branched form of polyisobutylene (b-PIB) with molecular weights in the thousands (up to ∼2.5 Â 10 4 ) according to GPC relative to polystyrene standards. Presently we examine the use of the LiCB 11 (CH 3 ) 12 catalyst with azo-tertbutane (ATB) radical initiator for the copolymerization of IB with ethyl acrylate (EA), as previously briefly mentioned in a short communication. 2 We find that it leads to an unusual somewhat branched nonalternating high molecular weight copolymer with up to ∼56 mol % of IB.EA and IB can be copolymerized, but it is difficult due to different reactivity. 4 IB is normally polymerized cationically to a linear polymer with AlCl 3 in halogenated hydrocarbons in the presence of traces of water. 5 In contrast, ethyl acrylate is ordinarily polymerized under radical initiation, sometimes under catalysis with rare earth metal complexes 6 or other metal-containing species. [7][8][9][10] The poor copolymerization potential of IB with acrylic esters in a radical process is apparent from the monomer reactivity ratio. The Alfrey-Price Q-e value 11 reactivity ratio of IB is more than 10 times lower than that of EA. 12 Still, conventional radical copolymerization of the two monomers results in a poly(ethyl acrylate) (PEA) containing a fair number of IB units. 13,14 For example, thermal or UV peroxide initiated radical copolymerization of IB and EA provides a low molecular weight product containing 20-30 mol % of IB. 14 The situation is improved by addition of strong Lewis acids, such as AlR n Cl 3-n , 15-17 BF 3 , 18 SnCl 4 , 18 ZnCl 2 , 18 etc. A high molecular weight alternating copolymer containing 50 mol % of IB was obtained using alkylaluminum and alkylboron halides as catalysts. 19 Metal-catalyzed coordination polymerizations are highly effective for olefins, but less so for polar acrylic monomers. Although recently, functionality tolerant late transition metal catalysts were used to catalyze the copolymerization, 20,21 EA/IB copolymers are still produced commercially by free-radical polymerization because other methods are too costly. 22' EXPERIMENTAL SECTION Materials. IB (Airgas) was pure grade (>99%) and was dried over activated 3 Å molecular sieves. EA (Aldrich) was distilled, dried over 4 Å molecular sieves, and stored in a Schlenk tube. 1,2-Dichloroethane (DCE, Aldrich) was dried with P 2 O 5 and distilled twice before use. ATB (Aldrich) was ∼97% pure. EA-d 5 and IB-d 8 were ordered from CDN Isotopes Inc. LiCB 11 Me 12 23 was dried under reduced pressure (∼0.04 mbar) at 100°C overnight before use. It contained residual sulfolane in a molar ratio sulfolane:LiCB 11 Me 12 = ∼1:10.Polymerization. Polymerization was conducted in a 25 mL stainless steel pressure reactor with a Teflon insert, a stir bar, and an accurate pressure gauge. In a typical reaction, the dried cata...

Radical polymerization of 1‐alkenes catalyzed by lithium salts of carboranes

“…Although interactions of ester group oxygen with the lithium cation are known and were described in the literature,12, 13 no precipitate was formed upon mixing the ABIPE solution with Li[CB 11 Me 12 ]. Thus, if any DCE soluble ABIPE–Li[CB 11 Me 12 ] complex was formed, it might still be catalytically active in the radical polymerization process, given that the presence of other esters, ethyl acetate, ethyl acrylate, poly (ethyl acrylate) does not prevent the lithium catalyzed radical polymerization of isobutylene 14. Using this azo initiator, monomer conversion reached about 6% after 24 h at 50 °C and only low‐molecular‐weight oligomers with M n < 1000 were formed.…”

Li+Catalysis and Other New Methodologies for the Radical Polymerization of Less Activated Olefins

“…Li + catalysis was first demonstrated a long time ago, when acrylonitrile containing a dissolved Li + salt was shown to undergo catalyzed radical polymerization (Scheme ). Later, it was shown for homopolymerization − of acrylates and similar more activated monomers, and their copolymerization − with less activated monomers, and it has been considered common knowledge for some time . Since alkenes activated for the complexation of Li + by the presence of suitable substituents with lone pairs, such as acrylonitrile or methacrylate, usually already undergo radical polymerization well in any event, metal ion catalysis is not critically needed and is of interest primarily as a possible source of stereocontrol.…”

“…In the absence of the initiator or of the lithium salt, or in the presence of a radical inhibitor or a lithium complexing solvent, polymerization did not take place. 162,163 A closer look at the polymers formed from propylene (very highly branched, molar mass ∼30000 g•mol −1 ), 164 higher alkenes (molar mass below 1000 g•mol −1 ), 165 and most of all, isobutylene (very highly branched, see below), 161,165,166 as well as the copolymerization of isobutylene with ethyl acrylate (with up to ∼50 mol % IB content, highly branched, molar mass ∼30−50000 g•mol −1 ), 149,165 gradually revealed that it is difficult to reproduce some of the initially reported molar mass values. It ultimately became clear that the yields, molar masses, and at times even the structures of the polymers were a very sensitive function of the composition of the catalytic solution and that painstaking control was required to obtain reproducible results.…”

UV-Assisted Li+-Catalyzed Radical Grafting Polymerization of Vinyl Ethers: A New Strategy for Creating Hydrolysis-Resistant and Long-Lived Polymer Brushes as a “Smart” Surface Coating

“…25 There have been some experiments demonstrating that the radical polymerization, especially for less activated monomers, could be promoted in the presence of lithium salts. 26,27 Very recently, direct controlled radical homopolymerizations of VEs were performed in water with LiOH as a mediator. 28 These results suggest that a specific interaction exists between Li + and CC, and perhaps to some extent, it would dominate over chain transfer steps and lead to highermolecular-weight PVEs.…”