Difunctional initiators

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

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). 

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). 

This product is a difunctional initiator and can polymerize styrene monomer ... 

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... 

The first reported initiators were polyhalogeno-compounds (e.g. CC14, CHC13, CCl3CH2OH, CCl3Br). Trichloromcthanc derivatives and tctrachloromethanc appear effective initiators. Mono- and dichloromethane derivatives are inefficient initiators. Tetrachloromethane may act as a difunctional initiator. 

Successive polymerization of monomers starting from a difunctional initiator... 

The nitrated model compound, 9, proved even more resistant to reduction than the polymeric analog the dissolving metal technique used to reduce 15 failed on 9, but finally the amino model, 10, was produced by treatment of with a 25-fold excess of sodium borohydride. Compound 1 serves as a difunctional initiator for NCA polymerization. 

Synthesis of a PB sequence with two living ends, using a difunctional initiator, followed by the addition of styrene to build two sequences of PS (two steps). 

Symmetric triblock copolymers P4VP-fr-PBd-fo-P4VP were prepared using a difunctional initiator derived from the reaction of m-diisopropenylbenzene with f-butyllithium at - 20 °C (Scheme 9) [27]. The synthesis was conducted in a mixture of toluene and THF at temperatures higher than room temperature for the polymerization of Bd, followed by a lowering of the temperature at - 78 °C and finally addition of an extra quantity of THF and 4VP. The 4VP content was kept lower than 30% to avoid problems arising from the poor sol-... 

Triblock copolymers PCHD-b-PBd-b-PCHD were also prepared by polymerizing Bd with a difunctional initiator, prepared by the reaction of 1,3-... 

Symmetric triblock copolymers of the ABA type, where B was PTHF and A poly(2-methyl-2-oxazoline), PMeOx, were prepared by cationic polymerization with trifluoromethanesulfonic anhydride as a difunctional initiator [58]. Subsequent hydrolysis of the PMeOx blocks with HC1 in a methanol/ water mixture resulted in the formation of the corresponding polyethylen-imine blocks (Scheme 20). Samples with relatively low molecular weight distributions were obtained. 

Diflubenzuron, registered for use in aquaculture in Europe, 3 220t Difluorobromine tetrafluoroborate, 4 144t 1,1-Difluoroethane, 13 722 physical properties of, l 778t Difluoromaleic anhydride, 15 492 Difunctional initiators, 14 252-254 23 380, 381 24 706... 

Homogeneous copolymerization, of acrylonitrile, 2 2 202-203 Homogeneous difunctional initiators, 24 246... 

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