Metathesis Polymer Cyclization with Telechelic Poly(THF) Having Allyl Groups

. Topologically unique macromolecules comprising a single cyclic and multicyclic polymer units have gained growing interest due to their distinctive behaviors from linear and branched counterparts.1-5 Polymer cyclization was performed first through the coupling reaction of a bifunctional linear polymer precursor, such as a bifunctionally living polymer like polystyrene, with a complementarily reactive bifunctional reagent, typically dihaloalkanes or dihalosilanes (Scheme 1).6-9 This bimolecular reaction should be conducted under high dilution to suppress the intermolecular chain extension reaction and at the same time under rigorous stoichiometric balance between large and small molecules. Hence, the process is often kinetically circumvented to limit this straightforward polymer cyclization process from wide applications. A psuedo-unimolecular reaction by an “electrostatic self-assembly and covalent fixation” technique was recently developed as an alternative polymer cyclization process (Scheme 1).5,10-14 Thus, a linear polymer precursor having moderately strained onium salt groups, typically five-membered cyclic ammonium or six-membered bicyclic ammonium salt groups, carrying appropriately reactive counteranion like dicarboxylate, was employed. The cations and anions always balance the charges, and the selective nucleophilic ring-opening reaction occurred at an appropriately elevated temperature under dilution, to convert the ionic interaction into the permanent covalent linkage. A strictly unimolecular polymer cyclization process using an R,ω-heterobifunctional linear polymer precursor was developed by Deffieux et al. and applied further by several groups (Scheme 1).15-19 In this process, the two end groups of the polymer precursor become complementarily reactive with each other after the deprotection or the activation, and the reaction under dilution gives a cyclic polymer product. The cyclization efficiency was notably improved since the inherent stoichiometric balance is maintained between complementarily reactive groups located within the same polymer molecule, and the reaction follows the unimolecular kinetics depending solely on the concentration of the polymer precursor. Nevertheless, the synthesis of the heterobifunctional polymer precursor often requires multistep processes involving protection/deprotection of reactive end groups, limiting its synthetic usefulness. We show here a novel strictly unimolecular polymer cyclization process involving bifunctional polymer precursor having identical reactive end groups (Scheme 1). Thus, a linear polymer precursor having allyl groups was prepared and subjected to a metathesis condensation, also known as a ring-closing metathesis (RCM), under dilution in the presence of a Grubbs catalyst, ruthenium(II) dichloride phenylmethylene bis(tricyclohexylphosphine) [RuCl2(PCy3)2(dCHPh)]. The metathesis condensation process has so far been successfully applied for the cyclization of smallto medium-sized substrates having terminal allyl groups20,21 and for the synthesis of a variety of topologically unique molecules like catenanes and knots22-24 as well as for the polycondensation of acyclic dienes (ADMET) to produce a variety of functional polymer materials.25-29 Furthermore, the equilibrium between cyclic and linear chains during a ring-opening metathesis polymerization (ROMP) has been a subject of continued studies.30 In the present communication, we demonstrate a versatile means to produce large-sized cyclic polymers using a readily accessible precursor, i.e., telechelics having allyl groups, in the presence of a commercially available metathesis catalyst. Results and Discussion. Telechelic poly(THF) having allyl end groups, 1, was prepared by the end-capping reaction of a bifunctionally living poly(THF)12 with sodium allyloxide (for details, see Supporting Information). 1H NMR analysis of 1, having the MW of 5060 and the PDI of 1.27, showed characteristic signals due to allyl protons at 5.17 and 5.90 ppm (Figure 1, top). The metathesis condensation was subsequently performed under reflux in methylene chloride at polymer concentration of 0.2-2.0 g/L in the presence of a Grubbs catalyst. The catalyst was charged in the comparable molar quantity to allyl groups at the polymer chain ends * Corresponding author: Tel +81 3/5734-2498, Fax +81 3/57342876, e-mail ytezuka@o.cc.titech.ac.jp. Scheme 1 8667 Macromolecules 2002, 35, 8667-8669