Anionic polymerization difunctional initiators

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 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... 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Anionic polymerization has also been used to make telechelic polymers (Greek telos, end, and chele, claw), i.e., polymers with reactive terminal groups.We coined the term, telechelic in 1957 and it has been accepted ever since in technical as well as patent literature. Liquid carboxy- and hydroxy telechelic polybutadienes initiated with difunctional organolithium initiators are commercially produced since 1962. Some of the physical properties,production details and uses as in solid rockets... 

Since soluble multifunctional initiators are more readily available in cationic polymerization than in the anionic counterpart, ABA type linear triblock copolymers have been almost exclusively prepared using difunctional initiation followed by sequential monomer addition. The preparation and properties of ABA type block copolymer thermoplastic elastomers (TPEs), where the middle segment is PIB, have been reviewed recently [47]. 

The value of Fn was determined by 1H NMR spectroscopy and found to be close to unity. By essentially the same method, bifunctional and trifunctional cyanoacrylate functional PIBs have also been prepared. Anionic polymerization of CA-PIB with N,N-dimethyl-p-toluidine as initiator in solution resulted in high MW product (Mn 35,000 gmol ) [107]. Anionic copolymerization of difunctional and trifunctional PIB yielded clear flexible films with low sol fraction. The... 

Most interesting from the standpoint of commercial development is the formation of block copolymers by the living polymer method. Sequential addition of monomers to a living anionic polymerization system is at present the most useful method of synthesizing well-defined block copolymers. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types. 

Polymer Preparation. Two bifunctional (telechelic) polymers were used in this study. Carboxy-telechelic polybutadiene (PB) is commercially available from B. F. Goodrich (Hycar CTB 2000X156) with molecular characteristics of Mn=4,600, Mw/Mn= 1.8, functionality 2.00 and cis/trans/vinyl ratio of 20/65/15. Carboxy-telechelic polyisoprene (PIP) was prepared by anionic polymerization in THF at -78°C with a-methylstyrene tetramer as a difunctional initiator. The living macrodianions were deactivated by anhydrous carbon dioxide. Five polymers werejjrepared with Mn=6,000 10,000, 24,000, 30,000 and 37,000 having Mw/Mn=sl.l5 a microstructure ratio of 3, 4/1, 2 of 65/35, respectively, and a functionality >1.95. 

Anionic polymerization of 4-vinylpyridine by Ba[CMe2Ph]2 occurs in THF, giving a non-stereospecific polymer 2-vinylpyridine is also polymerized by Ba[CMe2Ph]2 or difunctional initiator Ba[Ph2CCH2CH2CPh2], giving >50% isotactic linkages. ... 

Living cationic polymerization techniques are also capable of producing well defined star-block copolymers. An approach similar to the DVB method described above for the case of anionic polymerization was employed in order to prepare amphiphilic star-block copolymers [20]. In one case, living diblock copolymers of vinyl ethers and ester-containing vinyl ethers, prepared by the initiating system Hl/Znh in toluene, were reacted with a small amount of a difunctional vinyl ether to produce star shaped block copolymers (Scheme 5). 

Ring-opening metathesis polymerization was also used recently for the preparation of amphiphilic star-block copolymers [25]. Mo (CH-f-Bu) (NAr) (0-f-Bu)2 was used as the initiator for sequential polymerization of norbornene-type, unfunctionalized and functionalized, monomers. The living diblocks were reacted with endo-ris-endo-hexacyclo- [ 10.2.1.1.3/115>8.02>l 1. O 1-9] heptadeca-6,13-diene, a difunctional monomer in a scheme analogous to the use of DVB in anionic polymerization, to form the central core of the star (Scheme 10). 

In addition, dihlock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol. 

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. 

---