Monomer absorption

In addition to the dependence of the intennolecular potential energy surface on monomer vibrational level, the red-shifting of the monomer absorption as a fiinction of the number of rare gas atoms in the cluster has been studied. The band origin for the Vppp = 1 -t— 0 vibration in a series of clusters Ar -HF, with 0 < n < 5, was measured and compared to the HF vibrational frequency in an Ar matrix (n = oo). The monomer vibrational frequency Vp p red shifts monotonically, but highly nonlinearly, towards the matrix value as sequential Ar atoms are added. Indeed, roughly 50% of the shift is already accounted for by n = 3. 

Polymer formation from monomeric butadiene lipid vesicles is demonstrated by the decrease of the monomer absorption at 260 nm as well as by GPC of the residue of a freeze dried polymer vesicle dispersion. The latter method was also used for proving the formation of polymeric vesicles from the methacryloyl lipids. 

PVA). In methanol, an intense monomer absorption maximum is located at 870 nm. In the polymer film, at a dye polymer loading ratio of 1 to 5, the 870 nm solution maximum is broadened and split into two bands due to aggregation, centered at 700 and 950 nm respectively. However, no evidence of microcrystal formation in the film was detected upon examination with an optical microscope and thus, if present, microcrystals are less than 1000 A in diameter. 

In the case of butadienic lipids, the polyreaction was followed by a decrease of the strong monomer absorption (260 nni)25). Disappearance of vinyl protons in the 2H NMR spectrum proved polymer formation for vesicles made from vinylic, acryloylic, and methacryloylic surfactants l3,16). 

The spectra presented in Fig. 12.7 and 12.8 demonstrate that a polarization pattern is preserved during a fast addition of radicals to a monomer. Absorptive (A) polarization observed under direct photolysis of Pis (Fig. 12.2) results in the A pattern of a spectrum of the spin adduct (Fig. 12.7a), and the E/A pattern observed under sensitized photolysis (Fig. 12.3) results in an E/A pattern of a spectrum of the... 

A direct measure of the optical absorption coefficient a is the optical density OD defined by OD = log Iq/I = 0,434od (Iq and I are the incident and the transmitted light intensities and d is the thickness of the crystal). Weak polymer absorption in the range from 600 to 400 nm is present in the original monomer crystals due to weak thermal polymerization reactions. The absorption of the linear polymer molecules (which are homogeneously distributed within the partially polymerized monomer diacetylene crystals) increase during UV-irradiation at room temperature due to photopolymerization reactions. In contrast to the monomer absorption, the polymer... 

The photoinitiation reactions is possible only upon excitation of the monomer molecules M of the monomer crystal. No further dimer formation is possible when the crystal is irradiated below the monomer absorption band located at 310 nm. 

Direct excitation of the molecules M is performed with UV-light energies below the monomer absorption edge hv < hv (with wavelength 320 nm < K < 370 nm).. The photoaddition polymerization reaction obtained in this way at 10 K is shown for the optical absorptions of the AC-centres (b, c, d,...) in Fig. 15. After preparation of the b photoproduct (see the procedure of Fig. 6) the sequence of the AC centres b->c->d->e->f gis obtained upon 364 nm-irradiation. Simultaneously chain termination reactions leading to the stable oligomers 5 and e are observed. The same reactions are also observed with the diradicals in the ESR and optical spectra. After preparation of the dimer diradical and subsequent 364 nm-irradiation, very effective addition polymerization reactions are observed parallel with chain termination reactions leading to AC-molecules. The same effects are valid for the dicarbene DC species described below (Section 3.2). 

A recent study of mass transfer controlled kinetics of propene polymerization by TiCl3/AlEt3 catalyst has shown the monomer absorption rate to be proportional to the speed of stirring and to fall with increase in conversion [289]. 

Most of the lanthanide complexes that readily polymerize ethylene are inactive in propylene polymerization [48]. Thus, the complexes [ (Me3Si)2NC (NPr )2 2Ln(/i-H)]2 (Scheme 25) were also tested in catalysis of propylene and styrene polymerization. The yttrium derivative had low activity in propylene polymerization. Over a period of 2 h the monomer absorption reached 58 mol per mole of catalyst, whereupon the catalytic activity was lost. Complexes with Ln = La, Sm, Dd, and Yb were even less active and became inactive after 15-20 min. In styrene polymerization, only derivatives of smallest lanthanide metals showed catalytic activity. The Lu complex initiated polymerization of styrene (20°C, neat styrene, 5% of Lu complex), and 90% conversion was reached in 6 days. The polystyrene obtained had a high molecular weight Mn = 811,000gmor = 1250,000gmoF ), a narrow molecular-weight... 

Aqueous acidic solutions of either stilbazolium cation exhibit only "monomer" absorption and fluorescence at concentrations of 0.005M or lower. Increasing the stilbazolium concentration in aqueous HCI (0.1-1.0M) to 0.01 M or higher results in a red-shifted, broadened and structureless "excimer" fluorescence and a slightly broadened absorption spectrum (Table 1). Irradiation of these concentrated stilbazolium solutions result in a decrease in the trans- cis isomerization and the formation of the a dimer as the principle photoproduct for both cations (Tables 2 and 3). Although both the cis isomer and the a dimer are formed from each cation in the concentrated homogeneous solution, only the 5 dimer forms from cation 1 while the p dimer and small amounts of the e dimer form from cation 2. 

DMF precludes using ESR to follow the polymerization because of high dielectric loss due to the solvent. However, the polymerization can be followed by optical density since the system is homogeneous and the monomer absorption does not interfere in the visible region. 

The permanent absorption exhibited by the irradiated polymer is quite different from that shown by the polymer itself, which absorbs only weakly at 300-350 m/. Absorptions below 300 m/ are difficult to interpret because of the monomer absorption. It is suggested that for the most part both transient absorption and permanent absorption are caused by the radical ... 

As deseribed above, elassieal infrared speetroseopy using grating speetrometers and gas eells provided some valuable information in the early days of eluster speetroseopy, but is of limited scope. However, the advent of tunable infrared lasers in the 1980s opened up the field and made rotationally resolved infrared speetra aeeessible for a wide range of species. As for mierowave spectroscopy, tunable infrared laser speetroseopy has been applied both in gas eells and in moleeular beams. In a gas cell, the inereased sensitivity of laser speetroseopy makes it possible to work at much lower pressures, so that strong monomer absorptions are less troublesome. 

Infrared spectroscopy can also be earned out in molecular beams. The primary advantages of beam speetroseopy are that it dispenses almost entirely with monomer absorptions that overlap regions of interest, and that the complexes are... 

Molecular assemblies of dyes often show hypsochromic or bathochromic shifts of the main absorption bands if the n systems are touching each other ( 4 A) or if the distance is less than 6-7 A. This shift can be explained by the exciton model, which assumes resonance interaction between the dipoles of excited states. The most simple case is the interaction between two such dipoles of excited state of linear dyes, e.g., carotenoids. The interaction of two stacked chro-mophores then leads to a short wavelength shift (stack or H aggregate) of the monomer absorption bands the interaction of two chromophores in lateral position (Scheibe or Jelly or J aggregate) leads to a long-wavelength shift (Fig. 1.5.15). 

In the absorption spectrum of the dimer two bands appear at lower and higher energy with respect to the monomer absorption. The sphtting depends on the electronic coupling V,j and the relative intensity of the bands on the relative position and orientations between the two electric transition... 

Fig re 7. In situ FTIR spectra illustrating the disappearance of the monomer absorption at 657 cm and simultaneous polymer absorption increase at 703 cm ... 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

If the assumptions of the kinetic derivation are valid, the polymerization rate Vp should, according to Equation (19-20), increase with the concentration of true active centers [C ] as well as with the fraction of catalyst surface/mon occupied by the monomer. The term ATa[A] in Equation (19-23) becomes negligibly small for constant transition metal halide surface and very small metal alkyl concentration. Then, to a first approximation, /mon is constant. The concentration of active centers, and consequently v also, should increase with increasing metal alkyl concentration, and finally—after all catalyst surface active centers have become occupied—should become constant. But with increasing metal alkyl concentration there is stronger competition between metal alkyl absorption and monomer absorption. Consequently,/mon and Vp must decrease. Thus, the polymerization rate Vp should pass through a maximum with increasing metal alkyl concentration, and there is experimental evidence for this. The polymerization rate should be proportional to the metal alkyl concentration for constant metal alkyl/transition metal halide ratios. 

Multilayers of diyne fatty acids and their cadmium salts readily polymerize if they are exposed to U V-light of wavelengths 300 nm, The action spectrum of the photopolymerizationindicates two maxima at X, = 256 and 242 nm, which are in good agreement with the first two maxima of the monomer absorption. Hence, jAoto-reactivity is primarily due to photon absorption by the monomer diyne unit. 

Upon y-irradiation the complex crystals polymerized under retention of the angle crystalline character. Maximum conversion to polymer of 65 % could be obtained The photoresponse curve of the UV/VIS initiated polymerization, shown in Fig. 20, indicates high reactivities in the region of the monomer absorption at wavelengths 300 nm, and in the region between 380 and 450 nm, due to a sensitizing effect of the phenazine molecules... 

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