Cyclization Rates for Two Points in the Interior of a Polymer Chain Sam Lee and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1A1 Received September 10, 1996; Revised Manuscript Received January 22, 1997 X ABSTRACT: The fluorescence spectra and decay curves of a series of poly(ethylene glycols), EOx-Py- EO87-Py-EOx, were obtained in dilute solution. These pyrene-labeled polymers have a central chain of 87 (number average) monomer units separating the two pendant pyrene groups and two flanking end chains of equal length, x, where x ) 2, 12, 17, or 47 (number average) monomer units. In toluene, tetrahydrofuran, and benzene, small decreases in the cyclization rates of these polymers were observed with increasing length of the end chains. The rate of ring opening in the chain interior was found to be twice as fast as ring opening at the chain ends. These results are consistent with theoretical predictions of interior cyclization and the effects of excluded volume, respectively. Rates of end-to-end cyclization were found to be slower than expected by comparison to the rates of interior cyclization. Also, dilute solutions of interior-labeled polymer show small amounts of pyrene dimers which are preassociated intramolecularly. These results are believed to arise from differences in the local concentration of pyrene in interior- and end-labeled polymers. In water, there is evidence of hydrophobic association of the pyrene groups, and the extent of preassociation decreases with increasing chain length. Introduction One of the last unexplored issues about polymers in dilute solution concerns the relative rates of motion of the chain ends compared to two segments separated by a similar contour length, but in the chain interior. 1 It is well-known that monomer units at the chain end undergo local motions more rapidly than their counter- parts within the chain. 2 However, large-amplitude motion of sufficient scale to be described by Rouse and Zimm models is much more difficult to study experi- mentally. These are, for example, the motions leading to polymer cyclization. For many years, we have studied a variety of linear polymers labeled with pyrene at the chain ends. 3 In these systems, the kinetics of intramolecular pyrene excimer formation provides information on the dynamics and thermodynamics of end-to-end cyclization. How- ever, the case where the two excimer-forming chro- mophores occupy well-defined sites in the chain interior has never been examined. Excimer formation in these polymers involves the backbone-to-backbone cyclization of the chain. If a series of polymers labeled in the chain interior were available, their cyclization rates, k 1,interior , compared with k 1,end of a corresponding end-labeled polymer would provide information on the extent to which the hydrodynamic drag of the tails may retard interior cyclization (Scheme 1). Recently, we described the synthesis of a series of monodisperse poly(ethylene glycols) (PEGs), EO x -Py- EO 87 -Py-EO x , which are characterized by a central chain of fixed length spanning the two pyrene labels. 4 The central chain in each sample is flanked by end chains (or tails) of a different length. These polymers allow us to determine how the rate of internal cyclization is affected by the molecular weight of the end chain. There are theoretical studies of the conformation 5-7 and dynamics 1,8,9 of subchains located within the inte- rior of the chain contour. In θ solvents, the dimensions of the central chain that spans the two labeled segments is independent of the length of the end chains. Under excluded-volume conditions (i.e., good solvents), most theories 5,6d,f predict that the central chain swells more with increasing length of the end chains. Friedman and O’Shaughnessy have obtained scaling relationships for interior cyclization in the case where the relative position of the two cyclizing groups remains fixed; i.e., N end1 /N central and N end2 /N central remain constant while the total number of chain segments, N ) N central + N end1 + N end2 , can change. Interior cyclization of polymers having the structure of our labeled PEGsswhere N central remains fixed while N end1 ) N end2 is variedshas been considered by Perico and Beggiato. 1,8 They found a small, but asymptotic decrease in k 1 with increasing length of the end chains. This effect becomes more pronounced as the capture strength of the fluorescent probe increases and also as the flexibility of the chain decreases. This paper presents the results of our fluorescence studies of these labeled PEGs in dilute solutions with good solvents and compares their cyclization rates with the theoretical results of Perico and Beggiato (which refer to a θ solvent). The cyclization rate constants for these PEG bis(phosphates), EO x -Py-EO 87 -Py-EO x , are also compared to those of an end-labeled diester, Py- X Abstract published in Advance ACS Abstracts, March 15, 1997. Scheme 1 2633 Macromolecules 1997, 30, 2633-2641 S0024-9297(96)01351-4 CCC: $14.00 © 1997 American Chemical Society