Alkali-earth polymerization initiated

Most of the early mechanistic investigations of anionic polymerization were concerned with reactions taking place in liquid ammonia. The system liquid ammonia-alkali metals will be dicussed first, followed by a review of heterogeneous reactions taking place on alkali or alkali-earth surfaces. Thereafter homogeneous electron-transfer processes and the addition of negative ions to monomer will be discussed. Finally some esoteric reactions, such as initiation by Lewis bases, charge-transfer complex initiation, etc. will be briefly reviewed. 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

The polymerization of TFE with eoordination eatalysts of the organometalic type has been used on a laboratory seale oifly. Reiher et al. [666] deseribed the TFE polymerization in liquid NH3 initiated by alkali or alkali earth amides and aeetylides. In addition, the polymerization in the presence of EtsAl, PrsAl, EtAlCl2, and Et2Zn was studied. These eompounds are dissolved in hydroearbons, THF, dialiphatie ethers, dioxane, or mixtures of these. 

For vinyl monomers two methods can be used to initiate polymerization, both involve alkali metal derivatives, or more rarely alkaline-earth metal derivatives, and differ only by the mechanism of formation of the primary carbanionic... 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

A similar but direct electron transfer from the metal to the monomer is operative when alkali and alkaline-earth metals (e.g., sodium) are used as the initiator.169,173,177 In this case, however, initiation is slow relative to propagation because of the low metal surface area available, and this method is used only for special purposes. 1,1-Dipehylethylene, for example, forms a dianion that, for steric reasons, is not capable for further head-to-tail addition of the monomer, but it can be used to initiate the polymerization of other monomers.178... 

Synthetic routes include anionic, cationic, zwitterionic, and coordination polymerization. A wide range of organometallic compounds has been proven as effective initiators/catalysts for ROP of lactones Lewis acids (e.g., A1C13, BF3, and ZnCl2) [150], alkali metal compounds [160], organozinc compounds [161], tin compounds of which stannous octoate [also referred to as stannous-2-ethylhexanoate or tin(II) octoate] is the most well known [162-164], organo-acid rare earth compounds such as lanthanide complexes [165-168], and aluminum alkoxides [169]. Stannous-2-ethylhexanoate is one of the most extensively used initiators for the coordination polymerization of biomaterials, thanks to the ease of polymerization and because it has been approved by the FDA [170]. 

Alkali Metals The direct use alkali metals and alkaline-earth metals as initiators for anionic polymerization of diene monomers as first reported in 1910 is primarily of historical interest because these are uncontrolled, heterogeneous processes [4]. One of the most significant developments in anionic vinyl polymerization was the discovery reported in 1956 by Stavely and coworkers at Firestone Tire and Rubber Company that polymerization of neat isoprene with lithium dispersion produced high di-l,4-polyisoprene, similar in structure and properties to Hevea natural rubber [47]. This discovery led to development of commercial anionic solution polymerization processes using alkyllithium initiators. 

A second method describes the initial preparation of the polymer ligand, which is then converted into the polychelate by reaction with a transition metal ion [1,3-6,10-12]. An advantage of this procedure is that - in contrast to the polychelates - the polyligands are soluble in polar organic solvents. This method, which leads to quite pure polychelates because of the moderate reaction conditions, first uses the preparation of alkali metal salts of polymeric Schiff-base ligands followed by metallization with several transition metal ions or alkaline earth ions [11]. One example is the reaction of the polymeric ligand derived from 4,4 -dihydroxy-3,3 -diacetylbiphenyl and 4,4 -diaminodiphenyl-methane with Mn(II), Co(II), Cu(ll), Ni(II) and Zn(Il) to yield 2 [13]. 

Lactams may be converted to the corresponding nylons by water-initiated polymerization. The reaction may also be initiated by a base that requires anhydrous conditions and proceeds at high reaction rates [9,10]. Initiators used include carbonates, hydrides, alcoholates, and hydroxides of alkali and alkaline earth metals. The conversion of lactams to polyamides is also possible by a cationic process, entailing initiation in an anhydrous medium by either strong protonic acids or their dissociable salts [11]. 

Alkali Metals. The direct use of alkali metals and alkaline-earth metals as initiators for anionic polymerization of diene monomers as first reported in 1910... 

Anionic initiation has been accomplished in a variety of solvents, both polar and nonpolar. Typically, initiation can proceed by electron transfer reactions from alkali or alkaline earth metals, polycyclic aromatic radical anions, or alkali and magnesium ketyls. The other possibility includes the nucleophilic addition of organometallic compounds to the monomers. Related monofunctional initiators comprise alkyl derivatives of alkali metals or organomagnesium compounds such as Grignard reagents. Difunctional species are alkali derivatives of a-methylstyrene tetramer or the dimer of 1,1-diphenylethylene. An overview of the initiation process in carbanionic polymerization is given in Ref. [159]. 

CycHc carbonates such as 2,2-dimethyltrimethylene carbonate (DTC) are polymerized either anionicaUy or by insertion in a ring-opening fashion with a variety of initiating systems based on alkah metals (II, Na, K) [11], earth-alkali metals (Mg)... 

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