Monofunctional anionic initiator

The principal anionic initiation processes are (a) nucleophilic attack on the monomer which produces one-ended (monofunctional) anions by addition of the initiator across the double bond of the monomer [see Eq. (8.3)] and (b) electron transfer by alkali metals that leads to two-ended (bifunctional) anions (see later). 

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]. 

Diacrylate Michael Adducts. The synthesis of difiinctional NVF Michael adducts from diacrylates works only under particular conditions. Most successful alkoxide catalysts for the monofunctional derivatives gave slow reaction rates and low conversions. Stronger bases such as butyl lithium did enhance the reaction rate, however, BuLi inevitably promoted anionic initiated polymerization as well and resulted in gel formation. It was found that sodium hydride gave the best overall result, giving good reaction rates and conversions to gel free diftmctional products (Scheme 5). 

The terms in Equation 1.3 (Malkin s autocatalytic model) are described in Nomenclature. In Malkin s autocatalytic model, the concentration of the activator, [A], is defined as the concentration of the initiator times the functionality of the initiator. For a difimctional initiator [e.g., isophthaloyl-bis-caprolactam, the concentration of the activator (acyllactam) is twice the concentration of the initiator]. The term [C] is defined as the concentration of the metal ion that catalyzes the anionic polymerization of caprolactam. In a magnesium-bromide catalyzed system, the concentration of the metal ion is the same as the concentration of the caprolactam-magnesium-bromide (catalyst) because the latter is monofunctional. 

Polyethylene oxide)macromonomers have thus been synthesized by two-step processes. Poly(oxyethylene)mo nomethyl ethers with widely varying molecular weights are commercially available. They are obtained by anionic polymerization of oxirane initiated by monofunctional alkoxides261 such as potassium 2-methoxyethanol. 

Monofunctional Initiators. AB, ABA, and multiblock copolymers can be synthesized by initiation of one monomer with a monofunctional initiator like n-butyl lithium. When the first monomer has been reacted, a second monomer can be added and polymerized. This monomer addition sequence can be reversed and repealed if the anion of each monomer sequence can initiate polymerization of the other monomer. The length of each block is determined by the amount of the corresponding monomer which was provided. Styrene-isoprene-styrene block copolymers can be made by this method by polymerizing in benzene solution and adding the styrene first. Addition of a small amount of ether accelerates the slow attack of dienyl lithium on styrene. 

Coupling Reactions. In this technique, a living AB block copolymer is made by monofunctional initiation and is then terminated with a bifunctional coupling agent likeadihaloalkane. ABA copolymers can be madeby joining AB polymeric anions ... 

The industrial manufacturing process for cyanoacrylate monomers is designed to generate pure organic compounds free of metals. The prepared monomer is functionally very reactive and is polymerised by several mechanisms of which the most common is by anionic methods. In most applications the initiation is usually carried out by the nucleophilic contaminant (water or moisture) found on most surfaces. These adhesives differ from other adhesives in that they are monofunctional and can homopolymerise rapidly at room temperature. A number of modifiers have been added to impart a range of desired properties and these include stabilisers, inhibitors, thickeners, plasticisers, tracers, colorants and preservatives. 

Most interesting from the standpoint of commercial development is the formation of block copolymers by the living polymer method. Sequential addition of monomers to a living anionic polymerization system is at present the most useful method of synthesizing well-defined block copolymers. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types. 

The order of monomer addition is important. For example, to prepare an AB type block copolymer of styrene and methyl methacrylate, st ene must be polymerized first using a monofunctional initiator and when styrene is completely reacted, the other monomer MMA must be added. The copolymer would not form if MMA were polymerized first, because living poly(methyl methacrylate) is not basic enough to add to styrene. The length of each block is determined by the amount of corresponding monomer which was provided. To produce ABA type copolymer by monofunctional initiation, B can be added when A is consumed, and A added again when B is consumed. This procedure is possible if the anion of each monomer sequence can initiate polymerization of the other monomer. Multiblock copolymers can also be made in this way. 

PEO prepared by anionic polymerization using the monofunctional initiator ((diphenylmethyl)potassium) has one -OH end group leading to the formation of linear-dendritic diblock copolymers (Scheme 102). 

Block Copolymer Synthesis by Three-Step Sequential Monomer Addition The preparation of block copolymers by sequential addition of monomers using living anionic polymerization and a monofunctional initiator is the most direct method for preparing well-defined block copolymers. Detailed laboratory procedures for anionic synthesis of block copolymers are available [37, 230], Several important aspects of these syntheses can be illustrated by considering the preparation of an important class of block copolymers (Scheme 7.22), the polystyrene-fe-polydiene-( -polystyrene triblock copolymers. 

Sequential monomer addition by using a monofunctional initiator (Figure 3(c)) could be followed only if the reactivity of both monomers is almost the same. The maaoinitiator formed from monomer A can polymerize monomer B and vice versa. Styrene (St) and dienes in anionic polymerization and various methacrylates in anionic or GTP are characteristic examples of this method. The sequential monomer addition route, due to partial termination and/or deactivation reactions, proposed from the addition of three different quantities of monomer allows the synthesis of asymmetric ABA ttiblock copolymers, where the two end blocks exhibit different molecular characteristics. 

With a similar approach it is also possible to obtain model networks with controlled amounts of very well defined pendant chains. Adding monofunctional B chains to the initial mixture of pre-polymer and crosslinker an + B + B, reaction is generated. This will result in the formation of linear pendant chains of length equal to that of the B, s. Furthermore, thanks to modem anionic polymerization procedures [12,14,15] it is possible to synthesize telequelic monodisperse chains of type B, and B that can be used to build model networks with elastic and pendant chains of any desirable molecular weight and narrow molecular weight distribution. 

In the first way, the polyvinyl block is obtained by anionic polymerization as described before except that the monofunctional initiator (cumylpotassium) is replaced by a bifunctional initiator (dimer dianion of a-methyl-styrene/K). Then the amination of the two living ends and the synthesis of the polypeptide blocks are performed as described before for AB copolymers. 

Varshney, S.K., Hautekeer, J.P., Fayt, R. et al. (1990) Anionic polymerization of (meth)acrylic monomers. 4. Effect of lithium salts as Ugands on the Uving polymerization of methyl methacrylate using monofunctional initiators. Macromolecules, 23,2618-2622. 

Kraton 1102 contains about 20 wt % uncoupled SB diblocks. This is because a coupling method was used to produce SBS triblock copolymer from SB diblock copolymer that was synthesized, via anionic polymerization, using a monofunctional initiator. 

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