Chapter 34
Boroxyl-Based Radical Initiators and Polymerization
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T. C. Chung and H. Hong Department of Materials Science and Engineering, The Pennsylvania State University, University Park, P A 16802
This paper discusses a new family of living free radical initiators, alkylperoxy-dialkylborane (C-O-O-BR ), which show living polymerization of acrylate and methacrylate monomers at ambient temperature. The C-O-O-BR species can be prepared by in situ selective mono-oxidation of an asymmetrical trialkylborane (-C-BR ) with a control amount of oxygen. Apparently, in the presence of polar monomers the C-O-O-BR engages spontaneously hemolytic cleavage to form active alkoxyl radical (C-O*) and "stable" boroxyl radical (*O-BR ), due to the delocalization of the free radical with the empty p-orbital of boron. The alkoxyl radical is active in initiating the polymerization of vinyl monomers. On the other hand, the stable borinate radical may form a reversible bond with the propagating radical site to prevent undesirable termination reactions. The living polymerization was characterized by predictable polymer molecular weight, narrow molecular weight distribution, and the formation of telechelic polymers and block copolymers by sequential monomer addition. 2
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© 2003 American Chemical Society
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
481
Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch034
482 The control of polymer structure has been an important issue in polymer synthesis, both for scientific interests and industrial Appl.ications. Living polymerization provides an optimal means of control that can result in polymers with well-defined molecular structures; i.e. desirable molecular weight and polymer chain ends, narrow molecular weight distribution, and the formation of block and graft polymers. Free radical polymerization is particularly interesting due to its compatibility with a wide range of functional groups. Early attempts to realize a living free radical polymerization involved the concept of reversible termination of the growing polymer chains by iniferters (i,2), such as N , N diethyldithiocarbamate derivatives. However, this approach suffered from poor control of the polymerization reaction and the formation of polymer with high polydispersity. The first living radical polymerization was observed in reactions involving a stable nitroxyl radical, such as 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) (5-5), which does not react with monomers but forms a reversible end-capped propagating chain end. Usually, the reactions have to be carried out at an elevated temperature (>100 °C) to obtain a sufficient concentration of propagating radicals for monomer insertion. Subsequently, several research groups have replaced the stable nitroxyl radical with transition metal species or reversible chain transfer agents as the capping agents to mediate living free radical systems. These polymerization reactions follow the mechanisms of atom transfer radical polymerization (ATRP) (6-8) or reversible additionfragmentation chain transfer (RAFT) (9), respectively. Overall, these systems have a central theme-reversible termination via equilibrium between the active and dormant chain ends at an elevated temperature.
electron withdrawing
electron donating
In the past decade, we have been studying a new free radical initiation system based on the oxidation adducts of organoborane and oxygen, which may contain boroxyl radical - a mirror-image of the stable nitroxyl radicals (shown above). Our early interest in the borane/oxygen radical initiator stemmed from the desire to develop a new effective route in the functionalization of polyolefins (i.e. ΡΕ, PP, E P , etc.) (70), which was a long-standing scientific challenge with great potential for industrial Appl.ications. The unexpected good control in the incorporation of borane groups to polyolefin and the subsequent radical chain extension by the incorporated borane groups promoted us to examine this free radical polymerization mechanism in greater details.
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
483
Borane Containing Polyolefins and Polyolefin Graft Copolymers We firstly studied the copolymerization of α-olefin with borane containing α-olefin monomers (77, 12) by Ziegler-Natta and metallocene catalysts. The incorporated borane groups pending along the polymer chain were interconverted to polar functional groups, such as O H , N H , etc., as illustrated in Equation 1. Due to the unique combination of good stability of trialkylborane to the transition metal catalysts, good solubility of borane in hydrocarbon medium, and the improved copolymerization ability of single-site catalyst systems, a broad range of functional polyolefin copolymers have been prepared with an relatively well-defined molecular structure, i.e. narrow molecular weight and composition distributions and a controlled amount of functional groups.
Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch034
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The major drawback in this functionalization process is the cost of borane compound and the undesirable reduction of crystallinity and melting temperature of P E and PP due to the side chains. To lessen these concerns, our research turned to converting the borane groups to the radical initiators that can carry out graft-from radical polymerization, as illustrated in Equation 1, which incorporate hundreds of functional groups (via monomers) to polyolefin by each borane group. Such a process not only dramatically increased the efficiency of borane groups, but also reduced the number of side chains. The resulting graft copolymers (13, 14) were found to be very effective compatibilizers in polyolefin blends and composites. With the proper choice of borane moieties, such as alkyl-9-BBN having a stable bicyclononane double chair-form structure and a linear alkyl group to the polymer chain, the mono-oxidation by oxygen is selective at the linear alkyl group to form C - 0 - 0 - B R species (75). Next, these moieties initiated free radical graft-from polymerization of functional monomers (such as acrylic and methacrylic monomers) at ambient temperature to form polyolefin graft copolymers containing polyolefin and functional polymer segments. The unexpected good control in the chain extension process (almost no homopolymer or crosslinked material) prompted us to further examine this free radical polymerization mechanism. 2
Borane-terminated Polyolefins and Diblock Copolymers One of the ideal reactions to examine borane/oxygen initiators was to prepare borane-terminated polyolefin (16-18) containing only one borane group. The efficiency of the borane group in the radical polymerization can be easily determined by the formation of diblock copolymer, and the polymerization profile (molecular weight and molecular weight distribution) of the forming diblock copolymers provides a wealth of detail about the radical polymerization process. Several methods were developed for the preparation of borane-terminated polyolefins. The most effective and convenient route involves H - B R chain 2
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Selective Monoxidation
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Functional Monomers
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NaOH/H 02
Living Free Radical Polymerization
Equation 1
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