Initiators in anionic polymerizations

Organolithum compounds (lithium alkyls) are the most valuable initiators in anionic polymerization.120168 169172-175 Since living anionic polymerization requires the fastest possible initiation, sec- or ferf-butyllithium is usually used. Lithium alkyls add readily to the double bond of styrene [Eq. (13.32)] or conjugated dienes and form free ions or an ion pair depending on the solvent ... 

ABSTRACT. A new class of protected hydroxyl containing functionalized initiators were recently disclosed by the Defense Evaluation and Research Agency (DERA). These novel initiators have the general structure TBS-0-(CH2)n-Li. Excellent solubility in hydrocarbon solvents was exhibited by these materials which allowed the preparation of telechelic, high 1,4-microstructure polybutadienes. The two-step synthesis of these functionalized initiators from commercially available raw materials will be presented in detail. The first step involved reaction of an omega-haloalcohol with /-butyldimethylsilyl chloride, in the presence of an acid acceptor, to form the precursor. This precursor was then reacted with lithium metal in a hydrocarbon solvent to afford a solution of the functionalized initiator. The thermal stability of these initiators in hydrocarbon solution will also be presented. The application of the precursors and functionalized initiators in anionic polymerization of dienes will be briefly discussed. 

Uses Catalyst in mfg. of rubbers and plastics based on styrol-butadiene, polyisoprene, and polybutadiene initiator in anionic polymerization of styrene and conjugated dienes, thermoplastic elastomers organic synthesis... 

In a few cases, the actual polymerization starter is not identical with the added initiator. In anionic polymerizations with strong bases such as t-C H OK in dimethyl sulfoxide, the base first reacts with the sulfoxide and forms the DMSO anion as the actual polymerization-starting species ... 

Michael additions are thought to be responsible for initiation in anionic polymerization ... 

Diphenylmethane is the conjugate acid of the diphenylmethyl carbanion, and the equilibrium acidity constants (Ka) have been measured both directly and indirectly in the gas phase and in solution [3]. The most extensive investigations of the effect of structure on acidity for carbon acids have been carried out in DMSO using a carbon indicator method to determine relative acidities and this scale was anchored with potentiometric measurements to provide an absolute scale of acidities [3, 43]. A summary of relevant pKg values for various carbon acids is shown in Table 2. The data in Table 2 are especially relevant for considering the reactivity of 1,1-diphenylmethyl carbanionic species as initiators in anionic polymerization. In general, an appropriate initiator for a given monomer is an anionic species that has a reactivity (stability) similar... 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

Some new initiators soluble in hydrocarbons were described during the last few years. Organo-lithium compounds form 1 1 complexes with alkyls of Mg 134,135), Zn 136) or Cd l36), and their usefulness as initiators of anionic polymerization of styrene and the dienes was established 137). 

The term catalyst is often misused in anionic polymerization. These mechanisms require the use of initiators that differ from catalysts in that they are not regenerated at the end of the reaction. The similarity between initiators and catalysts is that they both create a situation that permits polymerization via a reduction in the activation energy of the process. 

Polystyrene/polyethylene oxide dendrimers were prepared by ATRP using tri- and tetra (bromomethyl) benzene as the initiators [207]. Each bromine end-group of the resulting stars was transformed first to two - OH groups and subsequently to potassium alcholate, as shown in Scheme 114. These - OK sites served to initiate the anionic polymerization of EO. The synthesized dendritic copolymers were found to display monomodal and narrow molecular weight distribution. 

In anionic polymerization, the initiators are either Bronsted or Lewis acids. In Bronsted acid, H+ is the effective catalyst while in Lewis, co-catalyst is involved to give catalytically active species, e.g. 

Atmospheric oxygen may initiate in situ polymerization [55] as well as hght as exemplified by the photo-induced isomerization/polymerization of (Z,Z)-muconate anions in the gallery space of LizAl LDH [61] and the photoisomerization of indolinespirobenzopyran [62]. 

Sequential addition of monomer works well in anionic polymerization for producing well-defined block copolymers [Morton, 1983 Morton and Fetters, 1977 Quirk, 1998 Rempp et al., 1988]. An AB diblock copolymer is produced by polymerization of monomer A to completion using an initiator such as butyllithium. Monomer B is then added to the living polyA carbanions. When B has reacted completely a terminating agent such as water or... 

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

TABLE 8-10 Effect of Solvent and Initiator Concentration on Stereochemistry in Anionic Polymerization of Isoprene at 20° Ca... 

Well developed is the anionic polymerization for the preparation of olefin/di-olefin - block copolymers using the techniques of living polymerization (see Sect. 3.2.1.2). One route makes use of the different reactivities of the two monomers in anionic polymerization with butyllithium as initiator. Thus, when butyl-lithium is added to a mixture of butadiene and styrene, the butadiene is first polymerized almost completely. After its consumption stryrene adds on to the living chain ends, which can be recognized by a color change from almost colorless to yellow to brown (depending on the initiator concentration). Thus, after the styrene has been used up and the chains are finally terminated, one obtains a two-block copolymer of butadiene and styrene ... 

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