Review pubs.acs.org/CR
Li+ Catalysis and Other New Methodologies for the Radical Polymerization of Less Activated Olefins Jan Merna,† Petr Vlček,‡ Victoria Volkis,§ and Josef Michl*,∥,⊥ †
Department of Polymers, University of Chemistry and Technology, Prague, Technická 5, 16628 Prague, Czech Republic Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Square 2, 16206 Prague, Czech Republic § Department of Natural Sciences, University of Maryland Eastern Shore, Princess Anne, Maryland 21853, United States ∥ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States ⊥ Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 11610 Prague, Czech Republic ‡
ABSTRACT: After a brief survey of conventional radical polymerization of alkenes, we review their Li+ catalyzed radical polymerization and their controlled radical polymerization. Emphasis is on homopolymerization, but related copolymerization of less activated monomers is mentioned as well.
CONTENTS 1. Introduction: Polymerization of Alkenes 2. Conventional Radical Polymerization 2.1. Early History 2.2. Ethylene Solvent Effect Polymerization in Water Emulsion 2.3. Higher Alkenes 3. Controlled Radical Polymerization 3.1. Fundamentals 3.2. Current State 4. Li+ Catalyzed Radical Polymerization 4.1. Basic Concepts 4.2. Discovery 4.3. Problems 4.4. Highly Branched Polyisobutylene 5. Conclusions and Outlook 5.1. Ethylene 5.2. Higher Alkenes Author Information Corresponding Author Notes Biographies Acknowledgments References
they are used in everyday life and in a huge number of technical applications. The chemistry and technology of polyalkenes is therefore under intensive study in numerous academic and industrial laboratories. Coordination polymerization initiated with complex catalyst systems of the Ziegler−Natta3,4 type is the most common process for the production of high-density polyethylene (HDPE), isotactic polypropylene, and linear lowdensity polyethylene (LLDPE), a random copolymer of ethylene with a small amount of higher olefins, such as butene, hexene, or octene. Another higher olefin, isobutylene, is a monomer well activated for industrially rare cationic polymerization leading to rubbery linear polyisobutylene (PIB). Initiation is done with a proton or a carbocation derived from water, an alcohol, or an alkyl halide in the presence of a co-initiator, mostly of the Lewis acid type. The polymerization is performed at very low temperatures to avoid transfer reactions and achieve the high molar mass that is needed for PIB to be rubbery. Butyl rubber (IIR), a copolymer of IB and isoprene, is also manufactured at low temperature.5,6 High-pressure radical polymerization7 is used widely for the manufacture of low-density polyethylene (LDPE). Radical polymerization of other less activated monomers to moderate or high molar mass polymers would offer many advantages but is generally considered very difficult. Our usage of the term ”less activated monomers” follows the literature8 (alkenes, vinyl
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Special Issue: Frontiers in Macromolecular and Supramolecular Science
1. INTRODUCTION: POLYMERIZATION OF ALKENES Polyalkenes such as polyethylene (PE), polypropylene, and a wide spectrum of their various copolymers represent more than 60% of commodity plastics manufactured in the world,1,2 and © 2016 American Chemical Society
Received: August 16, 2015 Published: January 13, 2016 771
DOI: 10.1021/acs.chemrev.5b00485 Chem. Rev. 2016, 116, 771−785
Chemical Reviews
Review
Scheme 1. Transfer to Monomer in Radical Polymerization of Allylic Monomers
of α particles.18 The small amounts of liquid products formed had the composition CnH2n.19 In one case, the molar mass was determined by cryoscopy as ∼230 g·mol−115 and the degree of polymerization was found to be roughly proportional to the reaction pressure.16 The formation of these liquids was attributed to radical-induced alkene oligomerization. In retrospect, this appears correct, with the possible exception of irradiation with α particles, in which ionic processes might have participated. Ethylene reaction with radicals formed from alkyl iodides by UV irradiation produced yields of oligomers that decreased with increasing pressure of the iodide and light intensity, and also in the order MeI, EtI, PrI, and i-PrI.20 A variation of temperature from 130 to 245 °C did not have much effect. Oligomerization of ethylene induced with tetramethyllead and tetraethyllead at ∼400 °C proceeded faster and to higher molar mass than that of propylene,21 but the main products still were the simple adducts of radicals to the double bond of the olefin. Alkene oligomers were formed upon heating acetaldehyde to 350 °C in ethylene22 and in propylene, 1-hexene, 1-butene, 2-butene, isobutylene, trimethylethylene, and tetramethylethylene.23 The same alkenes were oligomerized with radicals produced by irradiation of acetaldehyde.24 Thermal propylene oligomerization at 260−600 °C/
