Polymerization scheme

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

Oxiranones (a-lactones) 81JA686, 80AG(E)276), e.g. (6), are highly reactive, readily polymerizing (Scheme 18), possibly via a zwitterion (18). Such a species (19) would also account for the rearranged products (20) and (21) from (22 Scheme 19). 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

The reaction between benzoyl peroxide and A,A-dimethylaniline has been the subject of many examinations over the years. The following mechanism of initiation is fairly well accepted in the polymerization of styrene. It seems likely that a similar mechanism is followed for other free-radical polymerizations (Scheme 5). 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

Bamford, Eastmond and coworkers have employed metal complexpolymeric halide redox systems to initiate block and graft copolymerization. The polymeric halides can be synthesized by a variety of techniques, including radical polymerization,281 anionic polymerization (Scheme 7.28),"so... 

Polymerizations of methacrylic monomers in the presence of methacrylic macromonomers under monomer-starved conditions display many of the characteristics of living polymerization (Scheme 9.36). These systems involve RAFT (Section 9.5.2). However, RAFT with appropriate thiocarbonylthio compounds is the most well known process of this class (Section 9.5.3). It is also the most versatile having been shown to be compatible with most monomer types and a very wide range of reaction conditions.382... 

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

It follows from considerations of this nature that the form of the molecular size distribution obtained in bifunctional condensation is unaffected by such variations as portion-wise addition of one reactant to the other, polymerization followed by partial degradation, and so forth. Virtually all conceivable polymerization schemes involving reactions... 

A dozen years later, another breakthrough was reported the use of Zr(IV)-boratabenzene complexes as catalysts for olefin polymerization (Scheme 24).40... 

There are four basic reactions that terminate the polymerization (Scheme 6) are ... 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

The first example of a hybrid polymer that contains both [NS(0)C1] and [NPC12] units, 141a, was obtained as a pale yellow elastomer by ring-opening polymerization (Scheme 10).305 In the presence of GaCl3, this polymerization process proceeds quantitatively at room temperature in CH2Cl2-306... 

The equilibrium constant for ATRP, XATRp= k/kd, provides critical information about the position of dynamic equilibrium between dormant and active species during polymerization (Scheme 4). The relative magnitude of KATRp can be easily accessed from the polymerization kinetics using ln([M]0/[M]t) vs.t plots, which provide values... 

Scheme 1.3 Polymerization scheme showing the migratory insertion mechanism as well as the possible occurrence of the chain back-skip. The possible formation of agostic Mt-H bonds is...

The iso- and syndiotactic isomerism in the insertion polymerization of dienes (for 1,2 polymerization of generic dienes and for cis-1,4 polymerization of 4-monosubstituted or of 1,4-disubstituted monomers) would be determined, according to the polymerization scheme proposed by Porri and co-workers,181 182 by the relative orientations of the two ligands (diene monomer and allyl terminal of the growing chain) in the preinsertion catalytic intermediates. 

A larger blue shift in fluorescence was observed for alkoxycarbonyl-substituted PTs 400 and 401. The polymers were prepared from 2,5-dibromo-substituted monomers by two methods (i) Ullmann reaction with Cu powder and (ii) Ni(0)-mediated polymerization (Scheme 2.63) [485]. Both polymers have similar molecular weights (Mn 3000), although the Cu-prepared polymers showed higher quality and lower polydispersity. PL emission maxima for the Cu-prepared polymers 400 and 401 were red-shifted, compared to the Ni-prepared polymers (by 13-15 nm ( 0.05 0.06 eV) in solution and 25-30 nm ( 0.08 O.lOcV) in films, Table 2.4). This demonstrates that the properties of the polymer depend on the preparation method and, consequently conclusions from small shifts of 0.05-0.1 eV in PL EL energies of the materials, prepared by different methods, should be made with care. 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

FIGURE 35. Polymerization schemes of octatetrayne derivatives and cycloaromatization of 1,6-didehydro[10]annulene (98). Reprinted with permission from Reference 53. Copyright (1994) American Chemical Society... 

Following the general approach of Oosawa and Kasai (1962), one may assume that the following set of equilibria define the polymerization scheme of tubulin protomers ... 

Similar block copolymers, i.e., poly(S- -CL), poly(BD- -CL) as well as po-ly(S-b-BD- -CL) ABC triblock copolymers have recently been prepared by Sta-dler et al. by sequential anionic polymerization (Scheme 20) [76]. Addition of... 

Moreover, alcohol functionalities have been introduced into the polynor-bornene (PNB) backbone by copolymerization of norbornene with a few percent of 5-acetate norbornene and subsequent acetate reduction. After transformation of the pendant hydroxyl functions into diethyl aluminum alkoxides, sCL has been ring opening polymerized (Scheme 31). Owing to the controlled/ liv-ing character of both polymerization processes the isolated poly(NB- -CL) graft copolymers were characterized by well-defined composition, controlled molecular weight and branching density, and narrow MWD (PDI=1.2-1.4) [92]. 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

Poly(methyl methacrylate) with a variable degree of polymerization anchored to silica surfaces was synthesized following the room temperature ATRP polymerization scheme described earlier [45,46]. In the main part of Fig. 25 we plot the variation of the PMMA brush thickness after drying (measured by SE) as a function of the position on the substrate. Thickness increases continuously from one end of the substrate to the other. Since the density of polymerization initiators is (estimated to be 0.5 chains/nm ) uniform on the substrate, we ascribe the observed change in thickness to different lengths of polymer chains grown at various positions. 

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