Two General Methods for the Synthesis of Thiol-Functional Polycaprolactones G. Carrot and J. G. Hilborn* Polymer Laboratory, Swiss Federal Institute of Technology-Lausanne, MX-D Ecublens, CH-1015 Lausanne, Switzerland M. Trollsås and J. L. Hedrick* Center of Polymeric Interfaces and Macromolecular Assemblies, IBM Research Division, Almaden Research Center, 650, Harry Road, San Jose, California 95120-6099 Received February 11, 1999; Revised Manuscript Received April 29, 1999 ABSTRACT: We report here two different methods for preparing thiol-functional polymers. The first method consists of esterifying the hydroxyl-terminated polyesters with a thiol-protected mercaptoacetic acid. The Sangers reagent (2,4-dinitrofluorobenzene) was used to protect the mercaptoacetic acid and was removed with mercaptoethanol under mild conditions. The second technique involves the preparation of a protected thiol-functional initiator, the R-(2,4-dinitrophenylthio)ethanol, that could then be used in the polymerization of -caprolactone. This “functionalization from initiation” technique could also lead to the preparation of well-defined multibranched polymers with a thiol group at the focal point after the polymerization with a mercapto-initiator carrying dihydroxy or tetrahydroxy functionalities as initiating sites. The ultimate motivation in the preparation of such functional polymers is to elaborate nanocom- posites by using the polymers as macroligands in the synthesis of nanoparticles. Introduction The manipulation and control of surface properties of polymeric thin films has been an important theme in many areas of research including biotechnology, microelectronics, microfabrication, and others. 1 Our interest here is in using thin polymer films to tailor curved surfaces. In particular, we are interested in supramolecular arrangements based on nanoscopic entities, such as semiconductor nanosized clusters capable of self-assembly. Such heterosupramolecular entities should consist of macromolecular ligands at- tached to the cluster surface. One way to tailor surface properties is the deposition of thin polymeric films, and most examples are based on physisorption of either homopolymers or block copolymers. 2 Typically, for such films the interaction between the surface and the polymer is not strong, and desorption can occur in a good solvent or as a result of other compounds competing for the adsorption sites. Alternatively, grafting of polymers to the surface by forming a covalent bond provides significantly greater adhesion of the polymer chains to the substrate. 3 This “grafting to” technique has been accomplished by end- functionalized polymers that react with a substrate, for example, silane-functional polymers grafting to silica surface, 3d,e peptides grafting through the amide linkage to appropriately substituted substrates, 3f and thiol- functional polymers grafting to gold-coated substrates. 3g Indeed, the high reactivity of sulfur with many inorganic species such as gold or copper has brought interest to the use of molecules or macromolecules containing a free thiol. 4 Although many synthetic routes for the preparation of organic thiols are well-known, only a few examples of well-defined polymers with thiol functionality have been reported. 4g,i,5 The limited synthetic procedures available may be a consequence of the stringent condi- tions thiols require for their preparation due to their high reactivity and their strong tendency to dimerize. Ring-opening polymerization (ROP) methods allow the preparation of well-defined polyesters with func- tional end groups. 6 The “coordination-insertion” mech- anism of the polymerization initiated by aluminum triisopropoxide (Al(O i Pr) 3 ) implies that a hydroxy end group is systematically present in the ω-position, as a result of the hydrolysis of the chain-growing site. Polycaprolactone (PCL) macromonomers have been obtained from the esterification of this ω-hydroxy end group with methacrylic acid. 7a Likewise, we have re- ported preliminary studies on the conversion of the ω-hydroxy end group to a thiol group through a con- densation reaction with a protected mercapto acetic acid, followed by a deprotection step. 7b Aternatively, an R-functionality can be introduced in a controlled way by the use of functional aluminum alkoxide initiators prepared by reaction of triethylalu- minum (AlEt 3 ) with an alcohol. 7a,8 In addition, func- tional initiators have been synthesized through the exchange of the alkoxy groups of Al(O i Pr) 3 with the desired functional alcohol. 9 Here, we detail a similar approach to thiol-functional PCL using derivatives of mercaptoethanol as an initiator. Both methods, used to functionalize linear polycapro- lactone (PCL), are described in this paper. To this end, our objective is to prepare a series of polymers that serve as stabilizers for the controlled growth of the semicon- ductor nanoparticles. This should lead to the formation of heterosupramolecular structures where the particle spacing is controlled by the molecular weight and the particle size by the grafting density of the polymer chains. * To whom correspondence should be addressed. 5264 Macromolecules 1999, 32, 5264-5269 10.1021/ma990198b CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999