The Synthesis of Densely Grafted Copolymers by Atom Transfer Radical Polymerization

TL;DR: In this article, a macroinitiator with a grafting site at each repeat unit was used to obtain a broad molecular weight distribution of brush-like macromolecules using atom transfer radical polymerization (ATRP).

Abstract: Recent progress in the field of densely grafted, or “brush” (co)polymers has prompted a need to develop efficient methods to synthesize a wider variety of materials with the same basic architectural design. These brushlike macromolecules have been prepared previously using the macromonomer method.1-6 Macromonomers, usually prepared by anionic polymerization, were homopolymerized using conventional radical methods to maximize the number of branches possible from a linear backbone based on vinyl monomers. Upon fractionation of these materials using size exclusion chromatography, samples of narrow polydispersities were obtained which could then be cast on surfaces to form highly ordered thin films. To avoid the rigorous methods necessary for ionic polymerizations and sample fractionation, and to extend the variety of compositional content of these types of materials, atom transfer radical polymerization (ATRP) has been used to prepare similar macromolecular architectures. The approach described here involves grafting from a macroinitiator and can offer greater versatility in terms of both the length and the composition of the backbone and/or the side chains than previous methods which employed the synthesis of high molecular weight macromonomers and their subsequent polymerization by uncontrolled radical techniques; to obtain welldefined polymeric brushes required their fractionation, generally by SEC. To our knowledge, there are no known examples of using a macroinitaitor with a grafting site at each repeat unit to make well-defined polymeric brushes. Combinations of nitroxide-mediated, conventional free radical polymerization and ATRP to prepare graft copolymers from macroinitiators have been used previously.7,8 ATRP has also been combined with conventional radical polymerization to prepare amphiphilic graft copolymers9 and thermoplastic elastomers,10 as well. In each of these cases, however, the materials are loosely grafted, having been prepared from a macroinitiator which is a copolymer containing both initiation/ branch sites and spacing repeat units. Controlled radical polymerization and ATRP in particular afford access to materials of controlled molecular weight, predicted by the ratio of consumed monomer to initiation sites.11,12 This method also yields polymer segments of narrow molecular weight distributions13 in addition to being applicable to a host of vinyl monomers such as styrene, (meth)acrylates, acrylonitrile, etc.14 Thus, there are many possibilities which make its application to the area of brush (co)polymers appealing. Included here are preliminary synthetic data and AFM images which show that it is possible to prepare densely grafted copolymers using ATRP. Two approaches were used to prepare the macroinitiators, Scheme 1. The first involved conventional free radical homopolymerization of 2-(2-bromopropionyloxy)ethyl acrylate (BPEA)15 using AIBN in the presence of carbon tetrabromide to attenuate the molecular weight (Mn ) 27 300, Mw/Mn ) 2.3). By use of AIBN as an intiator to prepare the ATRP macroinitiator, a polymer with a broad molecular weight distribution was obtained. Such a macroinitiator would consequently result in the formation of brush polymers with broad molecular weight distributions, no matter how well controlled the polymerization of the side chains. Thus, the preparation of a well-defined macroinitiator was undertaken. In the second approach, trimethylsilylprotected 2-hydroxyethyl methacrylate (HEMA-TMS)16 was polymerized via ATRP and subsequently esterified with 2-bromoisobutyryl bromide (BriBuBr) in the presence of a catalytic amount of tetrabutylammonium fluoride (TBAF) to yield a different macroinitiator, poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (pBIEM)16 with controlled molecular weight and low polydispersity (Mn ) 55 500, Mw/Mn ) 1.3), Table 1. It should be noted that the macroinitiator prepared using ATRP was composed of a stiffer methacrylate structure and with a 2-bromoisobutyryl initiation site while the free radically prepared pBPEA contained an acrylate backbone and 2-bromopropionyl initiation sites. However, both types of initiating species have been shown to initiate styrene polymerization well.14 Both polymers were then used as macroinitiators for ATRP of styrene (S) and butyl acrylate (BA). Side chains with a degree of polymerization of about 40 from a macroinitiator of pBIEM with a Mn of approximately 50 000 (which contained about 200 initiation sites per Scheme 1 9413 Macromolecules 1998, 31, 9413-9415