Acrylates, alkyl anionic polymerization

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymerization of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkali metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Useful initiating systems for acrylate polymerization have been prepared from complexes of ester enolates with alkali metal alkoxides (104,105). 

Classic anionic polymerization must be conducted at too low temperature for commercial feasibility. Anionic polymerization of hindered acrylates and methacrylates with stable large counterions works at temperatures close to those required for condenser cooling. The anionic polymerization of lower alkyl acrylates and methacrylates is still a problem. 

If the monomers contain reactive groups that could be attacked by carbanions they will not be suitable for anionic polymerizations. Halogen-containing vinyl monomers are difficult to polymerize in these systems because of the elimination of alkyl halides. Interactions between many initiators and the carbonyl groups of methacrylates or acrylates necessitate the use of special reaction conditions, like very low temperatures, for the anionic polymerization of these monomers. 

MMA toward an alkyl radical [302]. Because of its low electron density (e=2.7) [302], this fluoromonomer readily undergoes anionic polymerization with amines and organic and inorganic salts in the presence of 18-crown-6 [301, 303]. However, anionic initiators commonly used for polymerization of MMA such as butyllithium and Grignard reagents fail to polymerize this monomer due to Sn2 addition-elimination [303,304]. All-acrylate polymers for 157 nm imaging have been prepared by radical terpolymerization of 2-trifluoromethyl-acrylate and methacrylate or acrylate [288] (Fig. 90). 

The exceptionally fast rate of anionic polymerization of cyanoacrylates in the presence of a base, including water, made this class of monomers unique among all acrylic and vinyl monomers. Of the alkyl cyanoacrylate family of monomers, fhe mefhyl- and ethyl-esters are used extensively in industrial and consumer-type adhesives. Meanwhile, the isobutyl, fi-butyl, and n-octyl cyanoacrylate esters are used clinically as blocking agents, sealants, and/or tissue adhesives in different parts of the world due to their much lower toxicity as compared to their more reactive methyl coxmterpart. 

Schlaad, H., Muller, A. H. E., Kolshom, H., and Kruger, R.-P., Mechanism of Anionic Polymerization of (Meth)acrylates in the Presence of Aluminium Alkyls. 3. MALDI-TOF-MS Study on the Vinyl Ketone Formation in the Initiation Step of Methyl Methacrylate with ferf-Butyl Lithium, Polymer Bull., 35, 177, 1995. 

Baskaran D (2003) Strategic developments in living anionic polymerization of alkyl (meth) acrylates. Prog Polym Sci 28 521-581... 

Coordinated anionic polymerizations of methyl methacrylate with diethyliron-bipyridyl complex in nonpolar solvents like benzene or toluene yield stereoblock polymers. In polar solvents, however, like dimethylformamide or acetonitrile, the products are rich in isotactic placement. There are many reports in the literature on polymerizations of acrylic and methacrylic esters with Ziegler-Natta catalysts. " The molecular weights of the products, the microstructures, and the rates of the polymerizations depend upon the metal alkyl and the transition metal salt used. The ratios of the catalyst components to each other are also important. ... 

Although organometallic catalysts have attracted the most attention lately with regard to the anionic polymerization of alkyl methacrylates, it is important to recognize that alkali metal alkoxides in a variety of media can initiate the polymerization of both alkyl methacrylates and alkyl acrylates. Especially in view of the recent discovery and reporting of controlled, living acrylate and methacrylate polymerizations via group transfer... 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate (86). Thus, using oligomeric (o -methylstyryl)lithium as initiator in THF at —78°C, the molecular weight distribution (M /Mn) of the polymer was 3.61 in the absence of lithium chloride but 1.2 in the presence of lithium chloride ([LiCl]/[RLi] = 5). In the presence of 10 equiv of LiCl, f-butyl acrylate was polymerized with 100% conversion and 95% initiator efficiency to provide a polymer with a quite narrow molecular weight distribution (My,/Mn = 1.05). More controlled anionic polymerizations of alkyl methacrylates are also obtained in the presence of lithium chloride. Other additives, which promote controlled pol5unerization of acylates and methacrylates, include lithium f-butoxide, lithium (2-methoxy)ethoxide, and crown ethers (47,48). The addition of lithium chloride also promotes the controlled anionic polymerization of 2-vinylpyridine. 

Fayt, R. Forte, R. Jacobs, C. Jerome, R. Ouhadi, T. Teyssie, Ph. Varshney, S. K. New initiator system for the living anionic polymerization of tert-alkyl acrylates. Macromolecules 1987, 20, 1442-1444. 

Also, it has been shown that the polymerization of alkyl cyanoacrylates can be driven forward by tertiary amines such as pyridine whereby the active mechanism is thought to move through a zwitterionic pathway [JOH 81]. Through work emanating from the authors laboratory, it has also been shown that weak photobase generator systems can be used to initiate a zwitterionic pathway toward a photoinitiated anionic polymerization of cyano acrylates. A-Alkoxypyridinium and N-phenacylpyridinium type salts were described as the active initiators for these systems (Diagram 3.15) [ARS 96, ONE 99, KAS 03a]. 

Monomers Several general categories of vinyl monomers are suitable for anionic polymerizations. These include aromatic monomers (2-vinylpyridine, styrene), conjugated dienes (butadiene, isoprene), and alkyl methacrylates. In the case of vinyl monomers, adjacent substituents that stabilize an anion are most suitable for anionic polymerizations. Examples include substituents found in styrene, butadiene, isoprene, alkyl methacrylates, or cyano acrylate which stabilize propagating anions by electronic effects. 

It should be noted that tBA is currently the only acrylate monomer that undergoes living anionic polymerization to afford polymers with up to a few 10 gmor or higher values rmder suitable conditions. This means that it is necessary to synthesize functional terf-alkyl esters of acrylic acid, as shown in Figure 31, in order to achieve the living anionic polymerization of protected acrylate... 

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