Article pubs.acs.org/Macromolecules
Synthesis of High Molecular Weight Polymethylene via C1 Polymerization. The Role of Oxygenated Impurities and Their Influence on Polydispersity Jun Luo, Ruobing Zhao, and Kenneth J. Shea* Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States S Supporting Information *
ABSTRACT: The living C1 polymerization of sulfoxide ylides initiated by organoborane is one of the few methods for controlling the molecular weight, polydispersity, and topology of simple hydrocarbon polymers. However, the synthesis of linear hydrocarbon polymers (polymethylene) with molecular weights >50 kDa via this method often results in some erosion of polymer polydispersity (PDI 1.3−2.0). In the absence of known chain transfer or termination steps, the origin of the PDI erosion remained a mystery. Here, we report that the PDI erosion can be attributed to small quantities of a borinic ester (R2BOR) impurity that arises from the oxidation of the trialkylborane initiator/catalyst (R3B) by trace oxygen. The propagation rate of R2BOR is substantially lower than that of R3B. Since the oxidized initiator/catalyst would produce very little polymer during the course of polymerization, the low reactivity of R2BOR alone could not account for the increased PDI. However, we have found that during the course of the polymerization (10 min) R2BOR will complex with ylide and undergo 1,2-oxygen migration. The resulting species R2BCH2OR is a more reactive initiator/catalyst than its borinic ester precursor R2BOR. The catalyst transformation introduces reactive growing polymer chains into the system after initiation and throughout the remaining polymerization, contributing to the formation of lower MW polymer. These results are supported by a computational study of the activation energies of the rate-limiting steps. The introduction of a less oxygen-sensitive amine−borane complex initiator/catalyst minimizes this complication and provides a method for synthesizing high-MW, low-PDI polymethylene.
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olyethylene (PE) is the world’s most common plastic.1 The majority of commercial PE falls in a molecular weight range of 10 000−300 000 g/mol. Because of its commercial importance, polymer scientists have a long-standing interest in the influence of polymer microstructure, including molecular weight, branch content/distribution, and architecture, on the physical properties of PE.2−5 Despite extraordinarily efficient catalysts for the synthesis of simple hydrocarbon polymers such as PE and polypropylene (PP), the controlled synthesis of these materials still presents a challenge. The polyhomologation reaction, a living C1 polymerization of ylides, is one of the few methods to control the molecular weight and topology of simple hydrocarbon polymers.6−8 The C1 monomers are mostly carbene precursors such as diazo compounds and sulfoxide ylides.9,10 The applications of C1 polymerization for the synthesis of telechelics,11,12 substituted carbon backbone polymers,13−17 and block copolymers18−21 have also been reported. Using C1 polymerization, the simplest hydrocarbon polymer, polymethylene, can be prepared by methods that include BF3-mediated polymerization of diazomethane22 and organoborane-catalyzed polymerization of sulfoxide ylides.23 In this article, we will focus on the polymerization of sulfoxide ylides in the presence of organoborane initiators. In an attempt to produce high-MW polymethylene via borane-mediated polymerization of dime© 2014 American Chemical Society
thylsulfonium methylide, some erosion of polydispersity has been noted (PDI = 1.3−2.0). Although this PDI is still relatively low, in the absence of known chain transfer or termination steps, the origin of the PDI erosion remained a mystery. This article explores the origin of the PDI erosion. We have considered several contributing factors including small quantities of a borinic ester impurity, arising from oxidation of the trialkylborane R3B initiator/catalyst by trace oxygen. For the synthesis of low-MW polymethylene (50 kDa). The use of decalin as a cosolvent maintains homogeneity during formation of this highly crystalline polymer. In addition, trace amounts of oxygen results in the production of borinic ester 5. Based on both experimental and computational studies, the borinic ester 5 does not produce polymethylene directly, since it is much less reactive compared to trialkylborane 2. However, under the polymerization conditions, borinic ester 5 can undergo 1,2-O migration forming a new trialkylborane initiator/catalyst 7O during the polymerization. Since 7O would “kick in” during the polymerization after initiation by trialkylborane 2, these species are responsible for production of the lower MW fraction of the product. The reaction is no longer a “single site” catalyst 5490
dx.doi.org/10.1021/ma501206a | Macromolecules 2014, 47, 5484−5491
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dx.doi.org/10.1021/ma501206a | Macromolecules 2014, 47, 5484−5491
