The Cyclization of Polymer Chains in Solution

Department of Chemistry and Erindale College University of Toronto Ontario, Canada M5S lAl 

ABSTRACT. The cyclization properties of polymer chains serve as a useful vehicle for examining various theoretical predictions about the polymer conformation and dynamics. Cyclization phenomena are particularly sensitive to excluded volume phenomena. This chapter provides a review of pertinent theoretical concepts and a description of experiments, based upon luminescence spectroscopy, which allow one to examine the predictions of the theory. 

The cyclization properties of flexible chain molecules have been of interest to chemists since the early days of this century (1). 

Originally the concern focussed on idiat kinds of local factors (torsional strain, steric effects) affected the cyclization of flexible organic molecules. Then emphasis shifted to the entropic factors affecting polymer cyclization (2). More recently one has begun to appreciate that many topical issues in polymer theory, particularly excluded volume effects and models for chain dynamics, find rather vivid expression in cyclization-related phencxnena. Thus studies of cyclization equilibria and dynamics provides new insights into polymer behavior and new approaches to testing predictions of contemporary theory. 

Experimental methods which permit precise measurements to obtain cyclization equilibrium constants Key and cyclization rate constants key all depend upon spectroscopic studies of polymers covalently labelled with appropriate dye molecules (3). The various spectroscopic methods 

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We note at the outset that there are two types of excimer formation in the 1x10 M-PEG aqueous solution. The first type of excimer forming site (EFS) results intermolecularly by association between chromophores from two different polymer chains. The number of these sites is directly dependent on the local concentration of labeled chains. The second type of excimer site arises from association between aromatic rings on the same polymer chain. In the present case, this means that cyclization of the labeled PEG chain must occur. 

With the increased level of understanding of the cyclization dynamics as monitored by intramolecular excimer fluorescence, it is now possible to extend this probe to the study of systems more complex than dilute solutions. One such situation involves the structure and dynamics of macromolecular complexes formed between polymeric proton donors and acceptors in aqueous solution. For example, there has been widespread interest in the complexes formed between poly(ethylene glycol) and poly(acrylic acid) or poly(methacrylic acid) in aqueous solution (34, 35). A second, complicated morphological problem is to describe the configurational behavior of polymer chains adsorbed on colloidal particles. This research has relevance to the understanding of steric stabilization. One system of particular interest is the interaction of poly(ethylene glycol) and colloidal silica (36-40). 

To date, only three studies have appeared which examine the consequences of macromolecule concentration on the cyclization of labelled polymers. These have involved pyrene excimer formation end-to-end cyclization for polymers with Py groups on the chain ends, and internal cyclization for polymers containing Py- groups along the chain backbone. In none of these experiments are the chains long enough, or the solutions sufficiently concentrated, for entanglement effects to be important. 

In other words a labelled polymer in dilute solution in a good solvent would experience opposing effects on cyclization as the concentration of matrix chains is increased Excluded volume screening would lead to an... 

The power of these techniques derives from their sensitivity and versatility. One can study fast processes which are diffusion controlled and slower processes which depend upon a conformation equilibrium or pre-equilibrium. Molecules can be examined at very low concentrations. Perhaps the most important application, and the least explored, is the study of traces of labelled chains in semidilute and concentrated polymer solutions. These experiments give one the power to examine the effect of the environment on the cyclization of individual polymer chains. 

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