Anionic polymerization with sodium

Eor comparison, polystyrene and poly(4-vinylpyridine) are prepared by anionic polymerization with sodium naphthalene as initiator. Poly(4-vinylpyridine) precipitates from THE the mixture is poured into 200 ml of diethyl ether and the polymer filtered off.The polymer is then reprecipitated from pyridine solution into a ten-fold amount of diethyl ether and dried in vacuum. 

It has been demonstrated that 2,5-bis-(chloromethyl)-l,3,4-oxadiazole can undergo anionic polymerization. With sodium alcoholate, poly(l,3,4-oxa-diazole-2,5-diyl-l,2-vinylene) is formed, however the reaction cannot be controlled even at temperatures as low as —40°C. Instead, the exothermic reaction can be controlled by performing the polymerization at a toluene/ water interface with tetrabutylphosphonium bromide as a phase transfer catalyst. The mechanism is shown in Figure 10.7. 

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

Cyano and alkoxycarbonyl groups are favorable in this respect and propeneni-trile and methyl 2-methylpropenoate can be polymerized with sodium amide in liquid ammonia. Ethenylbenzene and 2-methyl-1,3-butadiene undergo anionic polymerization under the influence of organolithium and organosodium compounds, such as butyllithium and phenylsodium. 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

There are fewer reports on the preparation of block copolymers via the combination of anionic polymerization with nitroxide-mediated syntheses [132,133]. As shown in Scheme 3.30, the reaction product of sodium with 4-hydroxyl-TEMPO initiated the anionic polymerization of ethylene oxide at 60 °C in THF solution. After treatment with methanol, TEMPO-terminated PEO was obtained, and then used in the nitroxide-mediated radical polymerization of St at 120 °C resulting in block copolymers of type CLB-17 [133]. Another method is the transformation of anionic polymerization into nitroxide-mediated radical polymerization. A poly(butadienyl)lithium solution in... 

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

When undiluted 2-vinylfuran was added to metallic sodium (mirror or particles) an orange colour developed and some resinous material was deposited on the metal surface. On prolonged contact much of the monomer was converted into a partly-insoluble reddish resin with spectra unrelated to those of standard poly(2-vinyl-furan). Reaction of diluted monomer with sodium gave a milder interaction, but no evidence of living anionic polymerization. 

Fig. 3. Arrhenius plots of the propagation rate constants kp of the anionic polymerization of methyl methacrylate in THF for different ion pairs including the propagation rate constant at —98 °C with cryptated sodium and of the free PMMA-anion (H. Jeuck, A. H. E. Muller, Ref. 34 )-...

Preparation and Reactions of S-b-MM. As mentioned in the introduction, we were interested in block copolymers of styrene and alkali metal methacrylates with overall molecular weights of about 20,000 and methacrylate contents on the order of 10 mol%. The preparation of such copolymers by the usual anionic techniques is not feasible. An alternative is to prepare block copolymers of styrene and methacrylic esters by sequential anionic polymerization, followed by a post-polymerization reaction to produce the desired block copolymers. The obvious first choice of methacrylic esters is methyl methacrylate. It is inexpensive, readily available, and its block copolymers with styrene are well-known. In fact, Brown and White have reported the preparation and hydrolyses of a series of S-b-MM copolymers of varying MM content using p-toluenesulfonic acid (TsOH) (6). The resulting methacrylic acid copolymers were easily converted to their sodium carboxylates by neutralization with sodium hydroxide. 

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

The details of the anionic polymerization of nylon 6 have been extensively reviewed (1-8) and will only be discussed briefly as they affect the star-polymerization of nylon 6. Nylon 6 is polymerized anionically in a two-step process (Figure 1). The first step, creation of the activated species 3, is the slow step. The e-caprolactam monomer reacts in the presence of a strong base (such as sodium hydride) to form the caprolactam anion 2. This anion reacts with more caprolactam monomer to form 3. The reaction of this activated species with lactam anions occurs rapidly to form the nylon 6 polymer 4. 

Another way to initiate anionic polymerization is by electron transfer. The reaction of sodium with naphthalene gives sodium naphthalene (sodium dihydro-naphthylide) in which the sodium has not replaced a hydrogen atom, but has transferred an electron to the electronic levels of the naphthalene this electron can be transferred to styrene or a-methylstyrene, forming a radical anion ... 

---