Dilithium initiator

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

Soluble analogues of these difunctional initiators have been prepared either by addition of small amounts of weakly basic additives such as triethylamine (73) or anisole (74) which have relatively minor effects on diene microstmcture (37). Another method to solubilize these initiators is to use a seeding technique, whereby small amounts of diene monomer are added to form a hydrocarbon-soluble, oligomeric dilithium-initiating species (69,75). 

The stoichiometric reaction of y -diisopropenylbenzene [3748-13-8] with two moles of j -butyUithium in the presence of triethylamine has been reported to produce a useful, hydrocarbon-soluble dilithium initiator because of the low ceiling temperature of the monomer (78,79) which is analogous in stmcture to a-methylstyrene however, other studies suggest that oligomerization occurs to form initiators with functionahties higher than two (80). 

Itarou H., Mays I.W., and Hadjichri-Stidis N., Regular comb polystyrene and graft polyisoprene/ polystyrene copolymers with double branches ( Centipedes ). Quality of (l,3-phenylene)bis(3-methyl-l-phenylpentylidene)dilithium initiator in the presence of polar additives. Macromolecules, 31,6697, 1998. 

The LPDE system is applied to several reactions in which the metal ions coordinate to the lone pairs of heteroatoms, thereby activating the substrate. Initially, the effectiveness was shown in Diels Alder reactions (Scheme 1). In a highly concentrated (5.0 M) LPDE solution, Diels- Alder reactions proceeded smoothly.6-7 Generally, a catalytic amount of LiC104 is not effective in this reaction. In some cases, a catalytic amount of an additional Bronsted acid, such as camphorsulphonic acid (CSA), gives better results.8 An interesting double activation of carbonyl moieties by using dilithium compounds has been reported (compound... 

In principle, mono-, di-, and polyfunctional terminal polymers are all available by electrophilic termination of living polymer chains. For example, difunctional polymers can be prepared by the use of dilithium initiation, followed by ditermination (25-35). However, the strict end-use requirements (e.g., linear chain extension) for difhnctional materials are especially demanding. 

Dilithium initiators must be soluble, stable,and purely difunctional. Many early dilithium initiators (25.29) failed all or some of these criteria (36). 

Mono and Dilithium Piperazinide. Both initiators were prepared by the reaction of n -butyllithium with piperazine. Monolithium piperazinide was prepared by the use of a 1 1 ratio of n-butyllithium to piperazine, while dilithium piperazinide was prepared with a 2 1 ratio of n-butyllithium to piperazine. Both salts are crystalline and insoluble in hexane. 

The methodulogy for preparation of hydrocarbon-soluble, dilithium initiators is generally based nn the reaction of an aromatic divinyl precursor with two moles of butyllilhium,... 

Reed 332) has reported that reaction of ethylene oxide with the a,(a-dilithiumpoly-butadiene in predominantly hydrocarbon media (some residual ether from the dilithium initiator preparation was present) produced telechelic polybutadienes with hydroxyl functionalities (determined by infrared spectroscopy) of 2.0 + 0.1 in most cases. A recent report by Morton, et al.146) confirms the efficiency of the ethylene oxide termination reaction for a,ta-dilithiumpolyisoprene functionalities of 1.99, 1.92 and 2.0j were reported (determined by titration using Method B of ASTM method E222-66). It should be noted, however, that term of a, co-dilithium-polymers with ethylene oxide resulted in gel formation which required 1-4 days for completion. In general, epoxides are not polymerized by lithium bases 333,334), presumably because of the unreactivity of the strongly associated lithium alkoxides641 which are formed. With counter ions such as sodium or potassium, reaction of the polymeric anions with ethylene oxide will effect polymerization to form block copolymers (Eq. (80) 334 336>). 

The use of a monolithium initiator and a three-stage sequential copolymerization, Eqs. (25)—(27), is straightforward and requires no further elaboration. However, for success, the dilithium, Eqs. (28) and (29), and coupling, Eqs. (30)-(32), approaches depend upon the proper choice of reagents. 

Foss and co-workers (88) reported ABA copolymers obtained from a new dilithium reagent this organolithium initiator was formed by the addition of sec-butyllithium to m-diisopropenylbenzene in the presence of a small proportion of triethylamine, followed by reaction with isoprene to improve the hydrocarbon solubility. Unfortunately, the starting ma-... 

Sigwalt and co-workers (91, 92), also, have described a dilithium initiator (21) for use in the preparation of block copolymers. However, a multistep synthesis of this initiator is required. 

Similarly, Tung and co-workers (92a) reported dilithium anionic initiators based upon double 1,1-diphenylethylene compounds, such as bis-[4-(l-phenylethenyl)phenyl] either (21a). 

Addition of sec-BuLi to 21a resulted in a hexane-insoluble dilithium initiator that could be solubilized with 1,3-butadiene, and subsequently used for block-copolymer synthesis. Once again, the starting material for the initiator based upon 21a is available only via special syntheses. 

Both the synthesis [38] and X-ray crystal structure [39] of dilithium pentalenediide 22 -2Li+ have been reported (Scheme 7, Figure 5). Reaction of the dihydropentalene 46 with n-butyllithium yielded the crystalline dilithium pentalenediide 22 -2Li+ (Scheme 7). The more recently reported flash vacuum thermolysis of 6-norbornenylfulvene (45), initially producing 6-ethenylfulvene, which immediately cyclizes to dihydropentalene (46), allows one to prepare this immediate precursor to 22 in gram quantities [40]. A similarly convenient... 

SiCU, the third and fourth substitution reactions are much slower than the first two, allowing the synthesis of A2SiB2 structures [35]. A2B2 polymers have also been prepared by adding l,3-bis(l-phenylethenyl)benzene at a 0.5 1 molar ratio on chain ends, and using the resulting dilithium species to initiate the growth of two more arms [36]. 

Ma et al.177 synthesized a,co-dilithium poly(styrene-b-butadiene-b-styrene)s (PS-b-PBd-b-PS) by using l,3-bis(l-phenylethylenyl)benzene activated with 2 mol of s-BuLi as initiator for the sequential polymerization of butadiene and styrene in the presence of s-BuOLi in benzene. The cyclization reaction was performed under high dilution in cyclohexane with either dichlorodimethylsilane or MDPPE (Scheme 87). The cyclic copolymer was isolated by fractional precipitation. The only indication of the formation of this architecture was the lower intrinsic viscosity. 

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