Other initiators

Isobutyl vinyl ether has recently been polymerised by iron sulphate following the calcination of the initiator at 700°C. The same treatment at 750°C gives an inactive salt. Obviously, the key point about the activity of this compound has to do with some particular crystalline structure which is modified between these two temperatures. 

Biswas and collaborators have investigated the polymerisation of N-vinylcarbazole by various oxides, and vanadates All these heterogeneous reactions were claimed to be cationic in nature, and tentative coordination-type mechanisms proposed. 

Bittles et al. ° carried out a detailed kinetic and mechanistic study of the polymerisation of styrene by commercial catalytic-cracking acid clays (Filtrol). Thisheter-c eneous process gave low DP s and displayed all the typical features of a cationic (or pseudocationic) polymerisation. 

Biswas and Maity recently reviewed the polymerisation of alkenyl monomers by molecular sieves. The cationic nature of these systems seems well established, but the mechanism of initiation and the possible role of residual water as cocatalyst need further investigation. 

Several more qualitative reports mentioning the use of curious initiators are to be found scattered in the literature, but often the supposedly cationic mechanism involved in these polymerisation is not clearly established. 

and Ledwith (17) have reported a study of the kinetics of the polymerization of THF catalyzed by Ph3C+SbCl3. The activation energy they obtained is 12 kcal/mole. Bawn, Bell, Fitzsimmons, and Ledwith (20) have compared the rates of polymerization initiated by Ph3C+SbCl T with those initiated by Ph3C+PF3. Their results are shown in Table 9. They conclude that there is not much difference in the rate of polymerization as a result of replacing SbCl3 by PFjj . 

From these studies it appears that the kinetics of polymerization of THF are closely approximated by equation 42. The equation does not always apply from the beginning of the polymerization and frequently cannot be applied before a steady-state of active centers is achieved. The initiator term, / , in this equation is often a function of several components. Only in the case of preformed trialkyloxo nium ions of the form R30+X is the initiation simple. These results suggest that in order to theoretically study the kinetics of polymerization of THF or to compare the kinetics of THF polymerization in the presence of different gegenions, it is desirable to use preformed trialkyl oxonium salts. Ideally 

An application of NMR for study of end-groups in polymers produced by anionic polymerization involved use of the spin-echo technique [23]. It was applied for detection of tert-hutyl end-groups in polyMMA made using unenriched di-t rt-butyl magnesium as initiator. 

The uncertainty about the mechanism has been resolved very elegantly by Zambelli et al [39] who prepared isotactic polypropylene using catalysts in which the initial active centre consisted of Ti- CH3 groups. The NMR 

Maleimide and derivatives such as N-phenylmaleimide can be polymerized with triphenylphosphine as initiator [41] to give products with the repeat unit (7). The NMR spectrum of the polymer dissolved in d -dimethylsulphoxide contained signals at —28.3 and —13.4ppm (referenced to H3PO4) attributed to phosphonium salts and ylids, respectively, thought to be present as end-groups (8) and (9). 

Maleic anhydride also can be polymerized with triphenylphosphine as initiator but the product is somewhat complex. The P NMR spectrum of the polymer of maleic anhydride and the effects on it of treatments with diazabicyclononene and trifluoroacetic acid suggest that all the phosphorus atoms in the polymer are present as phosphonium salts [41]. 

It is convenient to consider at this point the case of poly(aryl ether ether 

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Approximately 100 different organic peroxide initiators, in well over 300 formulations, are commercially produced throughout the wodd, primarily for the polymer and resin industries (22—27). A multiclient study covets the commercial producers and users of organic peroxides as well as other initiators, and their commercial markets and appHcations (28). 

Current Polymerisation Catalysts—Peroxide, and Other Initiators, muIti-cHent study. Catalyst Consultants, Inc., Spring House, Pa., Nov. 1991. 

Each company has a full line of products including PPG (diols) and glycerol adducts, as well as other initiator adducts. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Replace Lj by Lj — D, but retain Vf and all other initial values from the total-reflux calculation. 

Other initial conditions Initial liquid-phase compositions ... 

There are other initiator systems of lesser commercial importance. Cumene hydroperoxide is reported to cure acrylic adhesives in the presence of alkyl or pyridyl thioureas [105]. These initiators have been combined with a phosphated acrylate to promote adhesion to metal [106]. Thiourea-based initiators can be applied as a one-part on galvanized metal, where the metal surface provides the second part of the redox initiator [107]. 

Initiating events, in this study, initiate plant scram or setback. Other initiators, such as refueling discharge accidents, do not necessarily cause a reactor shutdown but may lead t< minor fuel damage and radioactive releases. The list of initiators for nuclear power plants has litf ance for HFBR because of size and design differences. A list of HFBR-specific initiators was developed from " st prepared with the HFBR staff, the FSAR, the plant design manual, the procedures manual, techn specifications, monthly operating reports, and the HFIR PRA (Johnson, 1988). 

Scheme (43)] have high activity and no other initiating species, which rise to homopolymers, are formed (i.e.. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

The early history of redox initiation has been described by Bacon.23 The subject has also been reviewed by Misra and Bajpai,207 Bamford298 and Sarac.2,0 The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when initiation is required at or below ambient temperature and drey are frequently used for initiation of emulsion polymerization. 

The use of ring substituted diacyl peroxides has also been reported.204 Both the aryl and aroyloxy ends possess the desired functionality. Other initiators used in this context include peroxides (eg. hydrogen peroxide). 

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et al.77,78 The use of related initiators based on silylatcd pinacols [e.g. 34, 35] has been reported by Crivcllo et a/.,7l>82 jjan( os et ai i anhexasubstituted ethanes and the derived macroinitiators are known to vary according to the degree of steric... 

Such products were observed more or less in the polymerization with other initiators (S11CI4, SbCls, CH3COBF4, Et30BF4, and EtOSC F). These phenomena are inconsistent with a usual equilibrium process if, as expected, the enthalpy of the polymerization is negative. Therefore, solubility and kinetic factors must play an important role. 

According to the data in Table 3, the t-BuCl/Me3 AI/MeCl system produces higher polymer yield and is active over a wider temperature range than any other initiator/solvent combinations. While yields obtained with f-BuCl and f-BuBr using MeCl are comparable up to about —40 °C, lower yields are obtained with f-BuBr than with t-BuCl below -40 °C. 

Other initiators of similar nature were described recently 139). Their low price may recommend them for industrial applications. 

This is just one example of several such DOD initiatives that might be built around scenarios that assume that basic chemical problems in materials could be solved by concentration of resources on fundamental research. One can just as easily imagine other initiatives, such as the following ... 

The BCs have been previously discussed by Gleaves et al. [1], Zou et al. [3], Creten et al. [9] and others. Initial condition (2) can he accepted because its statement of an initially clean surfece is an experimental statement. BCs (3, 4) are here further discussed with refenraice to the experimental apparatus. BC (3) states that the flux at the reactor Met is a delta fimction and is the approximation that pulse injection occurs over an inflnitely short time. This is discussed using experimental data on the speed of injection of the input pulse. BC (4) is the approximation that the gas concentration is zero outside the reactor tube. It implies tiM any gas eluting firom the reactor tube is immKiiately removed, that is, the approximation is that... 

Vinyl monomers may be polymerized at favorable rates in an aqueous medium containing an emulsifier and a water-soluble initiator. A typical simple Tecipe would consist of the following ingredients with their proportions indicated in parts by weight 100 of monomer, 180 of water, 2 to 5 of a fatty acid soap, and 0.1 to 0.5 of potassium persulfate. Cationic soaps (e.g., dodecylamine hydrochloride) may be used instead of the fatty acid soap, and various other initiators may replace the persulfate (e.g., hydrogen peroxide and ferrous ion, or a water-soluble organic hydroperoxide). 

Once this minimum pharmaceutical care is covered, pharmacists would be free to set up any other initiative. The erosion of earning power that this might spell for existing pharmacies could be offset by means of minimum market protection periods and by prohibiting chains of pharmacies without a single owner per pharmacy. 

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