Highly Branched Polypropylene via Li⁺-Catalyzed Radical Polymerization

Abstract

b S Supporting Information W hile radical polymerization techniques are well suited for polar monomers, they are not used to polymerize simple R-olefins as the allylic hydrogens on the latter are subject to chain transfer in the presence of a reactive propagating radical. 1 Thus, with the exception of ethylene produced commercially at very high pressures and temperatures (and a few reports of propylene under similar intense conditions 2,3 ), only short oligomers of simple alkenes such as propylene or 1-hexene can be obtained using conventional radical polymerization techniques. Against this backdrop, our group recently reported that a variety of simple alkenes could, in fact, be polymerized under radical conditions at ambient temperatures and pressures in the presence of a lithium cation in the form of LiCB 11 (CH 3 ) 12 . 4 By all indications, including the use of radical and ionic traps, the polymerization proceeds as a radical process. We believe that Li þ coordination to the alkene in weakly ligating organic media alters the relative rates of propagation and degradative chain transfer in favor of propagation (Scheme 1).Following these initial reports, we began investigating this system more fully with isobutylene, whose six allylic hydrogens render it a most unlikely candidate for radical polymerization. 5 This system proved quite unique, as it appears that both a radical process and a cationic one can be induced simultaneously. Under nonoxidative conditions, the radical process produced a polyisobutylene (b-PIB) with a highly branched architecture, unlike anything previously reported or prepared commercially. Under oxidative conditions, the radical process still occurred and produced b-PIB, while a concurrent cationic one generated perfectly linear l-PIB that differed from ordinary PIB only in that a carborate anion was covalently attached at the chain end. 6 Under these conditions, LiCB 11 (CH 3 ) 12 acted not only as a catalyst but also as a reagent and could not be recovered from the reaction mixture quantitatively or at all. A concurrent study of LiCB 11 (CH 3 ) 12 -catalyzed radical polymerization of 1-hexene and 1-octene has so far only revealed the formation of low molecular weight oligomers. 7 The contrast to the polymerization of propene described below is striking, and other reaction conditions are still being examined.Here, we report on our investigation of the nature and architecture of polypropylene (PP) that is generated under nonoxidizing conditions in the weakly ligating solvent 1,2-dichloroethane (DCE), using azo-tert-butane (ATB) initiator. Figure 1 illustrates the 13 C NMR spectrum of a sample of b-PP generated at moderate pressures (15 atm) and temperatures (80°C) in DCE solvent with ATB initiation under LiCB 11 (CH 3 ) 12 catalysis (for precise experimental conditions see Supporting Information). Polymer yield in this reaction was around 40%. The b-PP formed is very highly branched, as can be seen by the plethora of resonances observed in the spectrum between 10 and 45 ppm, which is unli...

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

“…Nevertheless, only low‐molecular‐weight oligomers with M n < 1000 were detected as a result of radical polymerization of all tested 1‐alkenes at elevated temperatures, regardless of the initiator used, and for these longer alkenes we were not able to reproduce the initial results 1. This contrasts with the reproducibility found for the short alkenes, isobutylene4 and propylene 5…”

“…With nonoxidizing initiators, this alkene yields a previously unknown hyperbranched polymer with molecular weights in excess of 2 × 10 4 g/mol, and with oxidizing initiators, it yields a mixture of the same hyperbranched polymer with ordinary linear polyisobutylene with molecular weights up to 5 × 10 4 g/mol, even at temperatures as high as 80 °C. In another recent investigation,5 it was found that propylene also readily forms a very highly branched polymer, with molecular weights up to 3 × 10 4 g/mol. Moreover, copolymers of isobutylene and ethyl acrylate were synthesized by radical copolymerization using the same Li[CB 11 Me 12 ] activator 2, 4…”

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

“…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.…”

Oxidatively Induced Concurrent Cationic and Radical Polymerization of Isobutylene in the Presence of LiCB11Me12

“…Catalysis with 1 can also be used for radical copolymerization of IB with ethyl acrylate. , Most recently we have examined in more detail the radical polymerization of propene, which produces highly branched polypropylene . In contrast, 1-hexene and 1-octene yield only relatively low molecular weight oligomers under the conditions used recently .…”

“…3,8 Most recently we have examined in more detail the radical polymerization of propene, which produces highly branched polypropylene. 9 In contrast, 1-hexene and 1-octene yield only relatively low molecular weight oligomers under the conditions used recently. 10 Admittedly, until the detailed composition of the presumably colloidal catalytic solutions containing 1 in solvents of low polarity is better understood, it will be difficult to control these polymerization reactions.…”