Initiators anionic polymerization

The preparation of ABA triblock polymers requires use of a telechelie bisthiol prepared by termination of anionic polymerization initiated by a difunctional initiator. The relative yields of homopolymer, di- and triblock obtained in these experiments depend critically on conversion.273... 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

Block copolymers of (R,S)-(3-butyrolactone and eCL have been synthesized by combining the anionic ROP of the first monomer with the coordinative ROP of the second one (Scheme 15) [71]. The first step consisted of the synthesis of hydroxy-terminated atactic P(3BL by anionic polymerization initiated by the alkali-metal salt of a hydroxycarboxylic acid complexed with a crown ether. The hydroxyl end group of P(3BL could then be reacted with AlEt3 to form a macroinitiator for the eCL ROP. 

In the relatively few anionic polymerizations initiated by neutral nucleophiles such as tertiary amines or phosphines the proposed propagating species is a zwitterion [Cronin... 

Anionic polymerization Initiated by electron transfer (e.g., sodium-naphthalene and styrene In THF) usually produces two-ended living polymers. Such species belong to a class of compounds called bolaform electrolytes (27) In which two Ions or Ion pairs are linked together by a chain of atoms. Depending on chain length, counterion end solvent, Intramolecular Ionic Interactions can occur which in turn may affect the dissociation of the ion pairs Into free ions or the llgand-lon pair complex formation constants. 

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

However, the efficiency of this compound as an anionic polymerization initiator is rather poor, as compared with that of standard lithiumorganic initiators. 

The temperature has a decisive effect on the anionic polymerization initiated by DPHLi (10) or benzyllithium in THF. As soon as the temperature is increased from —78 °C up to —40°C, the livingness is lost. Compared to MMA and other methacrylates, tBuMA is an exception for which the anionic polymerization remains under control at temperatures as high as 25 °C. This remarkable behavior is accounted for by the steric hindrance of the ester group, which prevents side reactions from occurring at an appreciable rate. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Oxetanes are rather basic materials and are inert to attack by bases. However, as stated above, anionic polymerization of thietanes has been reported [67], Significantly, in anionic polymerizations initiated by organo-lithium compounds the propagating end is a carbanion rather than a thiolate species viz. 

Effect of Solvents and Reaction Conditions Synthesis Capabilities Block Copolymers Functional End-Group Polymers Initiation Processes in Anionic Polymerization Initiation by Electron Transfer Initiation by Nucleophilic Attack Mechanism and Kinetics of Homogeneous Anionic Polymerization Polar Media Nonpolar Media... 

Anionic polymerization initiated by uncharged nucleophiles has been observed in several systems. The first example of such a reaction probably was furnished by the work of Horner et al.236), who initiated polymerization of nitroethylene and of acrylonitrile by trialkyl phosphines. The following mechanism was postulated ... 

The polymerization kinetics of vinyl sulfonamide were studied by Schultz and Schle-ssmann71 using various initiators. Phenyl lithium, a typical anionic polymerization initiator, was the preferred one, a high rate being achieved at — 50 °C. 

Bywater, S., Worsfold, D.J., 1967a. Alkyllithium anionic polymerization initiators in hydrocarbon solvents. J. Organomet. Chem. 10 (1), 1-6. 

Figure 10.3a shows the MALDI-TOF mass spectrum of a PMMA sample obfained by anionic polymerization (initiator cumyl lithium, termination by methanol). ° It can be seen that the peak due to singly charged ions is at 48 kDa and that the peak due to dimeric ions is at 94 kDa. The figure also gives fhe values for M and obfained by viscomefry and SEC, which agree with the values obtained by MALDI. 

One important class of anionic-polymerization initiators are the organometallic compounds, especially those of lithium and sodium (see Chapter 5 to review the general properties of the metalloalkyls). Grignard reagents can induce polymerization of acrylonitrile and acrylate esters, but not of hydrocarbons. Initiation by organosodium compounds often involves in situ generation of the active species from sodium metal. Sodium metal catalyzes the polymerization of dienes, and the mechanism has been shown to involve a dianion formed by dimerization of the diene radical anion. An alternative initiation system for anionic polymerization involves... 

Anionic polymerization initiators can function in two different ways. In the simplest case, the added initiator contains the actual polymerization-initiating species. For example, in the polymerization of styrene induced by potassium amyl, styrene adds directly onto the amyl anion the amyl group becomes one of the end groups of the polymer ... 

Chloral, CChCHO, can be anionically or cationically polymerized. The polymerization is initiated above the ceiling temperature of 58° C and then allowed to proceed well below the ceiling temperature. Phosphines and lithium /-butoxide are especially suitable as anionic polymerization initiators, whereas tertiary amines only produce poly (chlorals) of low thermal stability. Anionic copolymerization of chloral with excess isocyanates produces alternating polymers, as is also the case for the cationic copolymerization of chloral with trioxan. 

Difunctional initiators such as sodium naphthalene can be employed to prepare triblock ABA block copolymers. Difunctional initiators produce "living pol3mieric dianions which are capable of adding a second monomer at each end. On the addition of a second monomer the ABA structure results. For example, consider the preparation of poly(butadiene-b-styrene-b-butadiene) by anionic polymerization initiated by sodium naphthalene. 

A brief description of the chemistry of these materials is described on the article entitled Alkyl-2-cyanoacrylates. Commercial adhesive products are usually based on the ethyl ester but methyl, n-butyl, allyl -methoxyethyl and -ethoxyethyl are also important. Formulated adhesives consist of essentially pure monomer with relatively small amounts of property-modifying additives. The curing reaction is anionic polymerization, initiated by traces of alkaline material present on most substrate surfaces, particularly in conjunction with low levels of surface moisture (see Chain polymerization). 

Anionic polymerization initiator sodium naphthalene, tetrahydrofuran solvent <1.1 (17)... 

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