Transfer to solvent

Here the polymerisation solvent acts as a transfer medium. 

Consider the following example of polystyrene homopolymerisation in carbon tetrachloride solvent  

The solvent participates in the reaction resulting in the termination of the propagating chain, and the formation of a new free radical species. 

The CCI3 radical so formed, is active and may initiate polymerisation. 

The frequency with which transfer occurs will depend on the chemical structure of the solvent, the monomer and the solvent radical. 

With polymerizations in solvents S the rate of the transfer Iso has to be taken into account. From equation (20-115) with = %,m + 

Utr s under otherwise equivalent conditions as in Section 20.3.2, we obtain 

The solvent transfer constants vary by several orders of magnitude (Table 20-7). The more readily transferable atoms that there are per molecule (H in the benzene, toluene, ethyl benzene series), the weaker the bond (carbon tetrachloride, carbon tetrabromide), and the more resonance-stabilized the resulting radical (triphenylmethane, fluorene pentaphenyl-ethane), the higher is the transfer constant. 

Transfer, which is sometimes very marked, can be used industrially 

Ethylene Propylene Isobutylene Allyl acetate Styrene 

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Figure 6.8 Effect of chain transfer to solvent according to Eq. (6.89) for polystyrene at 100°C. Solvents used were ethyl benzene ( ), isopropylbenzene (o), toluene (- ), and benzene (°). [Data from R. A. Gregg and F. R. Mayo, Discuss. Faraday Soc. 2 328 (1947).]...

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

HCl in Methanol or Ethanol Containing a Trace of Water. When a little HCl is dissolved in methanol, nearly all the protons are transferred to solvent molecules to form (CI130H2)+ ions. If now a certain amount of water is mixed with this solution, each water molecule provides for a proton a vacant level that lies considerably deeper than that occupied in the (CHaOH2)+ ion. Consequently, as we saw in Sec. 36, many of the protons are transferred, to form (H30)+ ions. If a hydrogen electrode is dipping into this solution, the falling of the protons will be... 

Conventional, AIBN - wnth transfer to solvent MALDI-TOF PNVp. M... 

Various side reactions may complicate RAFT polymerization. Transfer to solvents, monomer and initiator occur as in conventional radical polymerization. Other potential side reactions involve the intermediate radicals 165 and 167. These radicals may couple with another radical (Q ) to form 271 or disproportionate with Q to form 270. They may also react with oxygen. The intermediate radicals 165 and 167 are not known to add monomer. 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

Decay of donor-acceptor complex through electron transfer to solvent molecules yielding primary radical ion pairs ... 

The determination of large values of the rate constant ratio ks/kpfrom the low yields of alkene product that forms by partitioning of carbocations in nucleophilic solvents. These rate constant ratios may then be combined with absolute rate constants for the overall decay of the carbocation to give absolute values of kp (s ).14 16 For example, the reaction of the l-(4-methylphenyl)ethyl carbocation in 50/50 (v/v) trifluoroethanol/water gives mainly the solvent adducts and a 0.07% yield of 4-methylstyrene from proton transfer to solvent, which corresponds to kjkp = 1400. This can be combined with ks = 6 x 109 s V4 to give kp = 4.2 x 106 s l (Table 1). 

The addition of two ortho-methyl groups to Me-[8+] to give Me-[10+] results in a 60-fold increase in the rate constant kp for proton transfer to solvent.27... 

Substituent effect on the stability of the transition state for deprotonation (D, Figure 6). The addition of two ortho-methyl groups to Me-[8+] to give Me-[10+] results in an increase in kp for proton transfer to solvent from 1.4 x 106 s 1 to 8.3 x 107 s 1 (Table 1). There should be relatively little steric hindrance to the reaction of solvent with the /1-hydrogens of Me-[10+] because these are relatively distant from the ortho-methyl groups. However, the twisting about the CAr-Ca bond that minimizes steric interactions between the methyl... 

Chain-transfer constants, 25 571t Chain-transfer rate constants, 19 832 Chain-transfer rates, 19 839 Chain transfer to solvent (CTS), 23 385 Chalcanthite, 7 772 Chalcogenide glasses, 12 575, 584 semiconductivity in, 12 587 Chalcogenides acidic, 12 190-191 gallium, 12 359 in photocatalysis, 19 75 plutonium, 19 691 zirconium, 26 641... 

With some metal complexes, e.g. Fe(CN)6", where a clear CTTS (charge transfer to solvent) band is evident, photoexcitation can cause direct photoionisation and the creation of the solvated electron. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

A pulse radiolysis of Ag solution was studied, and the behavior of formed silver atom (Ag ) and dimer cation (AgJ) was measured [80]. The absorption band for dimer shows a significant red shift with increasing temperatures, which implies to the CTTS (charge transfer to solvent) character of the band. 

Solution Polymerization. By adding a solvent to the monomer-polymer mixtnre, heat removal can be improved dramatically over bnlk reactions. The solvent mnst be removed after the polymerization is completed, however, which leads to a primary disadvantage of solution polymerization. Another problem associated with radical chain polymerizations carried out in solution is associated with chain transfer to the solvent. As we saw in Section 3.3.1.2, chain transfer can significantly affect the molecular weight of the final polymer. This is particnlarly trne in solntion polymerization, where there are many solvent molecules present. In fact, chain transfer to solvent often dominates over chain transfer to other types of molecnles, so that Eq. (3.79) reduces to... 

The only alkyl group reported as reductively removed is the di-phenylmethyl substituent from N3 in compound 165.153 Deoxygenation of N-oxides can be done thermally (heating in toluene with oxygen transfer to solvent) for compounds of type 166,44 or more commonly by use of phosphorus derivatives, as described for compounds 32, 33,45 166,44 and 167.61 Halogens can be reductively removed from [l,2,3]triazolo-[4,5-6]pyridines 160,146 168,220 and 169,142 and from pentachloro derivative 164, giving dichloro compound 170.208 Removal of thiol groups by reduction... 

The preferentially solvating component is indicated by an asterisk ( ). K is for the formation of a solvated species containing the second mentioned solvent component. CTTS = charge transfer to solvent. 

The absorption spectra of anions are very sensitive to the composition of solvents in which they are embedded. In general, they are solvated, i.e. they are surrounded by a solvent shell. The molecules composing the solvent shell constantly exchange position with those in the bulk of the solvent. In these transitions, an electron is ejected not into the orbitals of a single molecule, but to a potential well defined by the group of molecules in the solvation shell. Such transitions are known as charge- transfer-to-solvent (CTTS) transitions. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

From the steady state fluorescence spectrum of indole in water a fluorescence quantum yield of about 0.09 is determined. Since the cation appears in less than 80 fs a branching of the excited state population has to occur immediately after photo excitation. We propose the model shown in Fig. 3a). A fraction of 45 % experiences photoionization, whereas the rest of the population relaxes to a fluorescing state, which can not ionize any more. A charge transfer to solvent state (CITS), that was also introduced by other authors [4,7], is created within 80 fs. The presolvated electrons, also known as wet or hot electrons, form solvated electrons with a time constant of 350 fs. Afterwards the solvated electrons show no recombination within the next 160 ps contrary to solvated electrons in pure water as is shown in Fig. 3b). 

This simple oxidoreduction reaction involves complex OH - water molecules interactions whose the spectral signatures are assigned to Charge-Transfer-To-Solvent states (CTTS states). Indeed, the anionic precursor of the hydrated OH radical represents an interesting system for the direct investigation of elementary redox events in a protic molecular solution. 

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