Nucleophilic Displacement Polymerization

The thrust of our research has been to incorporate the BPC moiety into a polymer backbone that can impart flame retardancy without additives. The incorporation of this monomer into a thermoplastic has been approached in several ways including the following nucleophilic aromatic polymerizations, nucleophilic displacement under phase transfer conditions (PTC), " diene metathesis,and vinyl addition polymerization. ... 

TYPE OF POLYMERIZATION Nucleophilic displacement of activated aromatic halides in polar solvents by cilkali metal phenates or Friedel-Crafts processes examples include polycondensation of the potassium salt of hydroquinone and 4,4 -difluorobenzophenone in DMSO at temperatures up to 340°C and the polycondensation of 4,4 -difluorobenzophenone and silylated hydroquinone at 220-320°C. 

TYPE OF POLYMERIZATION Nucleophilic displacement of activated aromatic halides in polar solvents by alkali metal phenates or Friedel-Crafts processes. 

Recently, the above mentioned model reaction has been extended to polycondensation reactions for synthesis of polyethers and polysulfides [7,81]. In recent reports crown ether catalysts have mostly been used in the reaction of a bifunctional nucleophile with a bifunctional electrophile, as well as in the monomer species carrying both types of functional groups [7]. Table 5 describes the syntheses of aromatic polyethers by the nucleophilic displacement polymerization using PTC. 

Qiu et al. [11] reported that the aromatic tertiary amine with an electron-rich group on the N atom would favor nucleophilic displacement and thus increase the rate of decomposition of diacyl peroxide with the result of increasing the rate of polymerization (Table 1). They also pointed out that in the MMA polymerization using organic peroxide initiator alone the order of the rate of polymerization Rp is as follows ... 

This aminium radical salt in aqueous solution in the form of solvated radical salt is very stable and will not polymerize acrylonitrile even with CeHsCOONa to form the corresponding benzoate. Therefore, we believe that in the nucleophilic displacement, there must be some intermediate step, such as intimate ion pair and cyclic transition state, which will then proceed the deprotonation to form the active aminium radical ion [14], as shown in Scheme 1. The presence of the above aminomethyl radical has also been verified [15] through ultraviolet (UV) analysis of this polymer formed such as PAN or PMMA with the characteristic band as the end group. 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

Nucleophilic GTP proceeds by a dissociative mechanism in which the propagating centers are ionic—similar to other anionic polymerizations [Hertler, 1994, 1996 Quirk and Bidinger, 1989 Quirk et al., 1993 Webster, 1992, 2000]. The anionic propagating species XXVII is generated in low concentrations by nucleophilic displacement of the trimethylsilyl group by the nucleophilic catalyst (W+Nu ) ... 

Intramolecular nucleophilic displacements of halogen from u> -halohexylamines is an obvious route to saturated azepines and was the route by which hexamethyleneimine was first prepared. The method has since been superseded as a route to simple hexamethyl-eneimines but is still used for highly substituted derivatives (67MI51600). However, high dilution techniques are necessary in order to avoid polymerization intermolecular N-alkylations are minimized by using amine protecting groups, e.g. Af-tosyl. 

An AB monomer, 2-(4-hydroxyphenyl)-3-phenyl-6-fluoroquinoxaline [17], was prepared from the reaction of 4-hydroxybenzil and l,2-diamino-4-fluorobenzene and subsequently polymerized under aromatic nucleophilic displacement conditions in NMP. The resultant polymer exhibited an intrinsic viscosity of 1.23 dL/g, a Tg of 247 °C and thin film tensile strength and modulus at room temperature of 107 Mpa and 3.2 GPa, respectively [17]. The same AB monomer was also copolymerized with hydroquinone and 4,4 -difluorodiphenyl sulfone to yield a series of copolymers with interesting properties [17]. The same AB monomer was prepared and polymerized by other researchers to yield a polymer with an intrinsic viscosity of 0.65 dL/g and a Tg of 255 °C [18]. 

Poly(arylene ether triazole)s have also been prepared by heterocyclic-activated displacement polymerization [36], The 1,2,4-triazole unit sufficiently activated, albeit weakly, aryl fluorides for nucleophilic displacement. Several 3,5-bis(4-fluorophenyl)-4-aryl-l,2,4-triazoles were polymerized with various bis-phenols to yield polymers with Tgs from 185 to 230 °C [36]. The 1,2,4-triazole unit appears to be one of the more weakly activating heterocycles towards nucleophilic substitution polymerization. 

Diazotization in the presence of boron trifluoride enables diazonium tetrafluoroborates to be isolated from the reaction mixture and purified. Subsequent controlled decomposition produces the required fluoroaromatic. Although explosion hazards and the toxicity of the isolated salts are significant concerns with this process, known as the Balz-Schiemann process, 4,4 -di-fluorobenzophenone (BDF. 6) has been prepared by this route as a monomer for the production of the engineering plastic poly(ether ether ketone) , or PEEK , by condensation with 1,4-dihydroxybenzene in the presence of potassium carbonate. BDF 6 is superior to its chlorine analog because in aromatic systems the nucleophilic displacement of fluorine is more facile than that of chlorine, leading to a shorter polymerization time and a better quality product containing less degradation impurities. 

Thus, owing to their tendency toward polymerization and the difficulties often encountered in their preparation, systematic investigation of the chemistry of epiBulfvdea have been rate indeed. The nucleophilic displacement reactions reviewed in this section are often by necessity related to specific examples. Generalization of a given reaction should be viewed Cautiously until more detailed investigations are reported. 

Syntheses. The presence of the ether and imide functionalities provides two general approaches for synthesis. Polyetherimides can be prepared by a nucleophilic displacement polymerization similar to the halide displacement in polysulfone synthesis or by a condensation of dianhydrides and diamines that is similar to normal polyimide synthesis (see Polyimides). 

Nitro-DisplacementPolymerization. The facile nucleophilic displacement of a nitro group on a phthalimide by an oxyanion has been used to prepare polyetherimides by heating bisphenoxides with bisnitrophthalimides (91). For example with 4,4/-dinitro monomers, a polymer with the Ultem backbone is prepared as follows (92). Because of the high reactivity of the nitro phthalimides, the polymerization can be carried out at temperatures below 75°C. Relative reactivities are nitro compounds over halogens, A/-aryl imides over IV-alkyl imides, and 3-substituents over 4-substituents. Solvents are usually dipolar aprotic liquids such as dimethyl sulfoxide, and sometimes an aromatic liquid is used, in addition. 

As a variation on the base-catalyzed nucleophilic displacement chemistry described, polysulfones and other polyarylethers have been prepared by cuprous chloride-catalyzed polycondensation of aromatic dihydroxy compounds with aromatic dibromo compounds. The advantage of this route is that it does not require that the aromatic dibromo compound be activated by an electron-withdrawing group such as the sulfone group. Details of this polymerization method, known as the UUmann synthesis, have been described (8). 

Fig. 1. Nucleophilic substitutions as strategy for the use of unprotected, pharmacophore-rich reagents. FG, functional group Nu, nucleophile PG, protective group Pol, polymeric support X, leaving group for nucleophilic displacement.

Hydroxide ion opens the ethylene oxide by a nucleophilic displacement to initiate the polymerization. 

Bochkarev et al. [110] have recently described the anionic polymerization of HM(C6F5)3, M = Ge, Si, or Sn (Nc = 1, Nb = 3). Most noteworthy is the germanium system, where they have obtained self-limiting molecular-mass ranges of approximately 100000-170000, in spite of the mode of preparation. These workers hypothesize that deprotonation at germanium produces anions that nucleophilically displace fluorine atoms in para positions to produce a dendrimer structure of the following general formula ... 

Thus polystyryl carbanions and polyacrylonitrile carbanions prepared by anionic polymerization were reacted with cellulose acetate or tosylated cellulose acetate in tetrahydrofuran under homogenous reaction conditions. The carbanions displaced the acetate groups or the tosylate groups in a S v2-type nucleophilic displacement reaction to give CA-g-PS and CA-g-PAN. Mild hydrolysis to remove the acetate/tosylate groups furnishes the pure cellulose-g-polystyrene (Figure 3). 

The macromolecular silyl chloride reacts with sodium in a two-electron-transfer reaction to form macromolecular silyl anion. The two-electron-trans-fer process consists of two (or three) discrete steps formation of radical anion, precipitation of sodium chloride and generation of the macromolecular silyl radical (whose presence was proved by trapping experiments), and the very rapid second electron transfer, that is, reduction to the macromolecular silyl anion. Some preliminary kinetic results indicate that the monomer is consumed with an internal first-order-reaction rate. This result supports the theory that a monomer participates in the rate-limiting step. Thus, the slowest step should be a nucleophilic displacement at a monomer by macromolecular silyl anion. This anion will react faster with the more electrophilic dichlorosilane than with a macromolecular silyl chloride. Therefore, polymerization would resemble a chain growth process with a slow initiation step and a rapid multistep propagation (the first and rate-limiting step is the reaction of an anion with degree of polymerization n[DP ] to form macromolecular silyl chloride [DP +J, and the chloride is reduced subsequently to the anion). 

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