Dynamics cyclization

J. L. Garcia Fernandez, A. Rey, J. J. Freire and I.F. de Perola, Cyclization dynamics of flexible polymers - numerical results from Brownian dynamics. Macromolecules, 23 (1990) 2057-2061. 

For ideal chains W(0) should decrease as chain length N. For very long chains experiencing full excluded volume, the exponent is predicted to take the value -1.92. Measurements of cyclization equilibria should allow these predictions to be tested. The theory of cyclization dynamics is less advanced. The prediction of the classic treatment of Wilemski and Fixman (5) suggests that the... 

The specific fluorescent probe technique used in the present research is excimer formation between pyrene groups terminally attached to both ends of poly(ethylene glycol) (PEG) chains. Excimer formation in such a labeled polymer may result from both intramolecular and intermolecular interactions. A large body of literature over the past decade deals with intramolecular cyclization as detected by fluorescence techniques. This work has come largely from three groups. Cuniberti and Perico (24, 25) were the first to explore the possibility that excimer fluorescence could be used as a probe of end-to-end cyclization dynamics. Later, Horie and co-workers (26, 27) used triplet annihilation as a probe of cyclization in long chains. By... 

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). 

Li XB, Winnik MA, Guillet JE (1983) A fluorescence method to determine the solubility parameters delta-h of soluble polymers at infinite dilution—cyclization dynamics of poly-mras. Macromolecules 16(6) 992-995. doi 10.1021/ma00240a032... 

Information on cyclization dynamics is obtained when the reaction between A and Q is so fast that it occurs on every encounter the reaction becomes diffusion controlled. The most convenient means of carrying out such experiments is via fluorescence or phosphorescence quenching (4) where A is excited electronically to A with light of an appropriate wavelength and Q is chosen to be a diffusion-controlled quencher of A. One has to establish that Q and A indeed react at the diffusion controlled rate. 

This chapter will review experiments and theory pertinent to the use of excited state quenching processes to study cyclization phenomena in polymers. Most of the emphasis will be on cyclization dynamics, but this will be examined in comparison with results and expectations for corresponding cyclization equilibria. 

The theory of cyclization dynamics was first presented by Wileaski and Fixman [WF] (5). A number of curious features of the theory prompted detailed attention by Doi (11), by Perico and Cuniberti (12), and by others (13). The theory is developed in terms of the bead-and-spring Rouse-Zimm [RZ] model (14). Unrealistic in detail, this model is quite useful for describing low frequency, large flmq[>litude chain motions. The RZ model, figure 2, treats the chain as a series of n beads connected by (n-1)harmonic springs of root-mean-squared length b. 

The data available for testing the theory of cyclization dynamics derive almost exclusively from experiments on polystyrene. 

This is of interest because synthetic procedures have been developed for placing identical chromophores on the chain ends. For example, Winnlk has prepared and examined several such types in his excimer studies on cyclization dynamics, described earlier in these proceedings. 

Figure 4 Dynamic dilution of sample for HPLC analysis of amidation/cyclization reaction. Transfer solvent methoxyethanol flow rate 1 ml/min. Detection refractive index.

Under the same reaction conditions macrogelation occurs later in the polymerization of 1,3-DVB. Moreover, the [r ] of the microgels from 1,3-DVB is much smaller than that from 1,4-DVB. The exponent a of Mark-Houwink equation for the 1,3-DVB polymers in toluene was found to be only 0.25 [250] and 0.29 [251 ] compared with 0.48 for 1,4-DVB polymers obtained under similar reaction conditions [230]. The delay of the gel point and the small hydro-dynamic volumes of 1,3-DVB microgels, compared with 1,4-DVB microgels also illustrate that the extent of cyclization is much higher in 1,3-DVB polymerization. 

Once the spectroscopy and dynamics of parent singlet phenylnitrene were understood, we began a systematic study of the effect of substitution on the kinetics of singlet phenylnitrenes. For most of the aryl azides of interest " the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same (Fig. 9). With these nitrenes, cyclization to substituted benzazirines is the rate-limiting step of the process of nitrene isomerization to ketenimine in a manner similar to the parent phenylnitrene. The only exception, o-fluorophenylnitrene, will be examined in detail in the last section of this review. 

Singleton and coworkers took up the ene cyclization reaction of ene-allene (Scheme 4) and carried out combined experimental-computational investigation.43 The ene reaction had been known to show mechanistic uncertainty, in particular whether it proceeds via a concerted or stepwise route, and therefore provided a challenge for dynamics study. KIE measurement for the reaction of 22 (Ri = R.2 = TMS) in toluene at 50°C gave kcus/kcm of 1.43, which was smaller than what was normally observed in concerted ene reactions. However, the isotope effect was too large to support a stepwise ene reaction. Thus, this was in line with the idea that the mechanism is near the concerted-stepwise borderline. 

Inter-ring metal migrations, dynamic NMR studies, 1, 412 Intracyclic germanium-carbon bond formation large rings, 3, 706 small rings, 3, 703 Intramolecular Alder-ene reactions with metals, 10, 576 with palladium, 10, 568 with rhodium, 10, 575 with ruthenium, 10, 572 with transition metal catalysts, 10, 568 Intramolecular allylations, in cyclizations, with indium compounds, 9, 679... 

In the first case, achiral IV-methacryloylthiobenzanilide 39 formed (E,Z)-conformation of the imide moiety and crystallized in a chiral fashion. The solid-state photoreaction gave optically active 3-lactam 40. The dynamic molecular rearrangement for cyclization was elucidated on the basis of direct comparison of the absolute configuration of both the starting material and the photoproduct (Scheme 11).[25][38]... 

A study of kinetic isotope effects, theoretical calculations, and dynamic trajectories of the C(2)-C(6) (Schmittel)/ene cyclization of enyne-allenes has been reported.178 For (74), the isotope effect l=h/ l=d = 1-43 is consistent with a highly asynchronous... 

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