Winnik studies

When considering the features of excimer formation in somewhat greater detail, some comment on this study seems appropriate. The key issue that one should keep in mind when considering these types of mimic studies, is how the lifetime of a pyrene moiety compares to the lifetime of a free radical. In the case of polystyrene, the polymer that the group of Winnik studied, this comparison is not very favorable. One propagation step of this monomer at a moderate temperature of 30 °C (average temperature used in excimer study) takes about 0.2 to 0.3 milliseconds. Not only can one conclude that a time-dependent Smoluchowski equation... 

Parallel to the search for synthetic methods to make rings of all sizes, kinetic studies have appeared with the aim of providing insight into physical aspects of ring closure. Earlier kinetic work has been reviewed by Salomon (1936a) and Bennett (1941), and more recent work by Illuminati and Mandolini (1981) and Winnik (1981a). 

The photochemistry of [26] was first studied by Breslow and Winnik (1969). The excited triplet state undergoes intramolecular hydrogen abstraction which quenches the excited state and shortens the triplet lifetime. Here cyclisation involves the chain interior, each CH2 group being inherently... 

Palmer Fry J. Am. Chem. Soc. 1970, 92, 2580. See also Mitsuhashi Miyadera Simamura Chem. Common. 1970, 1301. For secondary isotope-effect studies, see Winnik Stoute Fitzgerald/. Am. Chem. Soc. 1974, 96, 1977. 

Ringsdorf, H., J. Venzmer, and F. M. Winnik. 1991. Fluorescence studies of hydrophohically modi feH poly( isopropylacrylamides)Vlacromolecule 4 1678-1686. 

Zhao, C. L., M. A. Winnik, G. Riess, and M. D. Croucher. 1990. Fluorescence probe techniques used to study micelle formation in water-soluble block copolymekangmuir.6 514-516. 

Winnik FM, Regismond STA (1998) Fluorescence methods in the study of polymer-surfactant systems. In Kwak ICT (ed) Polymer-surfactant systems. Marcel Dekker, New York, p 267... 

Several different elegant studies have been concerned with the availability of remote hydrogens for intramolecular abstraction by a triplet benzoyl group. Breslow and Winnik first reported remote hydrogen abstraction from para-substituted benzophenones 79). Winnik has shown that the accessibility of... 

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

Since gas-phase reactions are free from complications arising from solvation effects, a convenient starting point for a meaningful analysis of structural effects on reactivity would be the study of cyclisation reactions in the gas phase. Unfortunately, quantitative evidence of this sort is scanty. A section in Winnik s review (Winnik, 1981a) is devoted to cyclisation and the gas-phase conformation of hydrocarbon chains. From the numerous references therein one obtains a substantial body of evidence pointing to a general resemblance of cyclisation reactions in the gas phase with cyclisation reactions in solution. However, as Winnik has pointed out, gas-phase reactions have not been studied so far with the same kind of detail that is possible for reactions in solution. As a result, any attempt at understanding the relations between structure and reactivity in the area of cyclisation reactions must still rely heavily upon solution chemistry data. 

Yang, J., Winnik, M. A., The Orientation Parameter for Energy Transfer in Restricted Geometries Including Block Copolymer Interfaces a Monte Carlo Study, J. Phys. Chem. B 2005, 109, 18408 18417. 

The photochemical aspects of carbonyl photochemistry remain important subjects of research. Wagner and Thomas have used CIDNP to elucidate radical formation from a,a,a-trifluoroacetophenone. Irradiation of benzophenone and its derivatives in the presence of molecules with abstractable hydrogen atoms can give rise to intensely fluorescent compounds. This effect may interfere with the observation of nanosecond-domain kinetics.Quantum yields and kinetic isotope effects in nanosecond flash studies of the reduction of benzophenone by aliphatic amines have been measured by Inbar et Rate constant data are given in Tables 13 and 14. Winnik and Maharaj have studied the reaction of benzophenone with n-alkanes through hexane to hexatriacontane is 3.9 0.2kcal for all chain lengths.The effects of substituents on the benzophenone on these reactions have also been examined. The reactions of phenylacetophenone when used as polymerization initiator have been reviewed by Merlin and Fouassier. ... 

Franzin CM, MacDonald PM, Polozova A, Winnik FM. Destabilization of cationic lipid vesicles by an anionic hydrophobi-cally modified poly(iV-isopropylacrylamide) copoljnner a solid-state P NMR and NMR study. Biochim Biophys Acta 1998 1415 219-234. 

Wilhelm M, Zhao C-L, Wang Y, Xu R, Winnik R-A. Poly(styr-ene-ethylene oxide) block copolymer micelle formation in water A fluorescence probe study. Macromolecules 1991 24 1033-1040. 

M. D. Croucher and M. A. Winnik in NATO ASI Series C Mathematical and Physical Sciences Scientific Methods for the Study cf Polymeric Colloids and their Applications, Vol. 303, F. Candau and R. H. Ottewill (eds), Kluwer, Dordrecht, 1990, pp 35-72... 

Winnik [49] used fluorescence measurements of transfer of the electronic excitation between donor-naphthalene and acceptor-pyrene chromophores attached to the same polymer chain for studies of thermoreversible phase separation of aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM). Dilute solutions of the doubly labelled polymer PNIPAM were heated from 277 K to 313 K, and the fluorescence emission intensity of pyrene (integrated spectrum) was measured when the system was excited with 290 nm, donor excitation, and when excited with 328 nm, acceptor excitation. Non radiative energy transfer between excited naphthalene and pyrene occurred in aqueous solution of the polymer. The increase in intensity of pyrene fluorescence when the solution was excited at 290 nm, shown in Figure 4.13, is due to a phase separation process at lower critical solution temperature (LCST). When the LCST was reached, the phase separation into polymer-rich and polymer-lean phases occurred. It was concluded that the collapse of the polymer chain leading to densification of polymer phase is followed by domination of intramolecular contributions to the energy transfer process. 

Reprinted from Polymer, Volume 31, F.M. Winnik, Phase transition of aqueous poly-(N-isopropylacrylamide) solutions a study by non radiative energy transfer, 2132, copyright 1990, with permission from Elsevier Science)... 

Picosecond fluorescence studies were applied by Winnik and co-workers [72] for studies of temperature-induced phase transition of pyrene-labelled hydroxypropylcellulose (HPC-Py) in water. Temperature dependence of the fluorescence emission ratio of excimer to monomer emission (Ie/Im) showed a significant increase of excimer emission in a temperature range 283-313 K, then a decrease to a constant value at 319 K. Two excimer bands were observed when time-resolved spectroscopy was used i) a broad, structureless band with a maximum at 420 nm and a corresponding lifetime of 250 ps and ii) the well-known band of pyrene excimer, with a maximum at 470 nm and a lifetime of 68 ns. In the initial time region, 0-150 ps, monomer emission was observed, with a simulation by a superposition of three components (377, 398 and 421 nm). They observed only one excimer emission above the LCST and that was with a maximum at 470 nm. They concluded that the LCST implies a complete disruption of the ordered microstructures, which were created in cold water. 

The other interesting method utilises fluorescence measurements. This approach has been mainly applied to latex film formation by Winnik and Wang [90]. In this technique, latex is prepared in two different batches. In one batch, the chains contain a donor group, while in the other, an acceptor group is attached. The interdiffusion of polymer chains between neighbouring latex particles is then studied by direct non-radiative energy transfer measurements. 

PRI Principi, T., Goh, C.C.E., Liu, R.C.W., and Winnik, F.M., Solution properties of hydrophobically modified copolymers of A-isopropylacrylamide and A-glycineaciyl-amide A study by microcalorimetiy and fluorescence spectroscopy. Macromolecules, 33, 2958, 2000. 

DIA Diab, C., Akiyama, Y., Kataoka, K., and Winnik, F.M., Microealorimetric study of the temperature-induced phase separation in aqueous solutions of poly(2-isopropyl-2-oxazolines), Macromolecules, 37, 2556, 2004. 

LAU Laukkanen, A., Valtola, L., Winnik, F.M., and Tenhu, H., Formation of colloidally stable phase separated poly(A -vinylcaprolactam) in water A study by dynamic light scattering, microcalorimetry, and pressitre pertirrbation calorimetry. 

An elegant innovation to aid the study of polymer/surfactant interaction was the introduction of a fluorescent label directly onto the polymer molecule by covalent bonding. [See reviews by Winnik (43,44).] This approach has been particularly useful in systems, such as combinations of nonionic polymers and nonionic surfactants, where interaction is weak. For example, pyrene-labeled hydroxypropylcellulose (HPC) gave evidence of association with weakly reactive OTG (n-octyl-P-D-thioglucopyranoside) but only at concentrations near its c.m.c. (119). Experiments with pyrene-labeled PNIPAM have been reported by Winnik et al. (120), who obtained evidence of noncooperative association of this polymer with anionic and cationic surfactants. A polymer that has been terminally labeled with pyrene groups is PEO (121) in mixtures with SDS at lower concentrations fluorescence data indicated the polymer chain cyclized. At higher concentration the pyrene groups were located in separate micelles. 

Kumacheva E, Rharbi Y, Winnik MA, Guo L, Tam KC, Jenkins RD. Fluorescence studies of an alkalme swellable associative polymer in aqueous solution. Langmuir 1997 13 182-186. 

Wang Y, Juhue D, Winnik MA, Leung OM, Goh MC. Atomic force microscopy study of latex film formation. Langmuir 1992 8 760-762. 

Ma, Y Farinha, J. P. S. Winnik, M. A. Yaneff, P. V. Ryntz, R. A. Compatihihty of chlorinated polyolefin with the components of thermoplastic polyolefin A study by laser scaiming confocal fluorescence microscopy. Macromolecules 2004, 37, 6544—6552. 

This technique of covalently fixed fluoroprobes, we developed in collaboration with Winnik s group [ 121 ], is similar to that described by Webber [ 117 ]. It is not only of interest for CMC determination but also for kinetic studies of micellar block copolymer systems [ 122]. 

Some specific features wiU be given in the following concerning the miceUization behaviors of PS-PEO and PEO-PS-PEO, studied in our group in coUaboration with Winnik and co-workers [159] and with BaUaulF and co-workers [160]. This type of copolymers were also examined by Yu and Eisenberg [ 161 ] especially for copolymer with high PS content, and by Khokhlov and co-workers [162]. 

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