Block from bifunctional initiators

The ABA-type block copolymers B-86 to B-88 were synthesized via termination of telechelic living poly-(THF) with sodium 2-bromoisopropionate followed by the copper-catalyzed radical polymerizations.387 A similar method has also been utilized for the synthesis of 4-arm star block polymers (arm B-82), where the transformation is done with /3-bromoacyl chloride and the hydroxyl terminal of poly(THF).388 The BAB-type block copolymers where polystyrene is the midsegment were prepared by copper-catalyzed radical polymerization of styrene from bifunctional initiators, followed by the transformation of the halogen terminal into a cationic species with silver perchlorate the resulting cation was for living cationic polymerization of THF.389 A similar transformation with Ph2I+PF6- was carried out for halogen-capped polystyrene and poly(/>methoxystyrene), and the resultant cationic species subsequently initiated cationic polymerization of cyclohexene oxide to produce... 

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and allyl methacrylate [Hertler, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate-acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytri methyl si loxymethylene)cyclohexane (XXXIII) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

Bifunctional Initiators. One notable omission from lithium alkyl initiators has been a bifunctional species soluble in hydrocarbon solvents. This would allow the production of living polydianions from isoprene and butadiene with predominantly a cw-1,4 microstructure in the backbone, the latter in turn being a convenient source for the production of ABA thermoplastic block copolymer... 

Living anionic polymerization can be used for the synthesis of AB block copolymers in which (a) or (b) is added to a living chain of the comonomer. The alternative procedure is limited due to the low reactivity of a chain end formed from (a) or (b). Block copolymers from butadiene and (a) or (b) show remarkable reduction of the cold flow [613,614]. ABA block copolymers are the result of the polymerization of butadiene (B) with a bifunctional initiator followed by the addition of (a) or (b) (A) [615]. AB block copolymers from... 

Block copolymers and also star-shaped [171] block copolymers from styrene and dienes are generally produced batch-wise with the monomers added in appropriate sequences. These sequences depend on the functionality of the initiator (mono- or bifunctional initiators) [172-175], on the use of coupling agents [176,177], and on the sharpness of the transition between the various blocks. Another reason for apportioning the monomer feed is the limited heat removal capacity of the reactor. 

Alternatively, block polymers may be prepared by the sequential polymerization of two monomers using a bifunctional initiator, as exemplified by the preparation of diblock peptide-polylactide copolymers. tert-Butoxycarbonylaminopropanol is first engaged in the zinc-promoted ROP of lactide and, after deprotection of the terminal amine chain-end, the polyamide block is obtained by ROP of the N-carboxyanhydride (NCA) derived from y-benzyl glutamate (Scheme 10.15) [63], Such diblock peptide-polylactide copolymers also have interesting self-assembling properties [63]. 

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

Copolymers. Copolymers from mixtures of different bisphenols or from mixtures of dichlorosulfone and dichlorobenzophenone have been reported in the patent literature. Bifunctional hydroxyl-terminated polyethersulfone oligomers are prepared readily by the polyetherification reaction simply by providing a suitable excess of the bisphenol. Block copolymers are obtained by reaction of the oligomers with other polymers having end groups capable of reacting with the phenol. Multiblock copolymers of BPA-polysulfone with polysiloxane have been made in this way by reaction with dimethyl amino-terminated polydimethylsiloxane the products are effective impact modifiers for the polyethersulfone (79). Block copolymers with nylon-6 are obtained when chlorine-terminated oligomers, which are prepared by polyetherification with excess dihalosulfone, are used as initiators for polymerization of caprolactam (80). 

Haloester-type trifunctional initiators are obtained from triols by a method similar to those for bifunctional haloesters. 3-arm star polymers of MMA are obtained with dichloroacetates MI-28 and MI-29, for which Ru-1 and Al(acac)3 are employed.228 The polymers have controlled molecular weights and narrow MWDs. Similarly, MI-30 and MI-31 with copper catalysts gave 3-arm star polymers of styrene, acrylates, and methacrylates suitable copper catalysts vary with each monomer.199,326 358 368 The obtained star polymers can be further transformed into star block copolymers comprised of hydrophilic/hydropho-bic368 or organic/inorganic326 segments by block copolymerizations of other monomers. 

Thus, it may be concluded that under the above conditions, polymerization of DME, initiated with living, bifunctional polyDXP, leads to the formation of ABA block copolymers with relatively high yields (50-60%). In addition to copolymer, low-molecular-weight polyDME is also formed, but it can be easily removed from copolymer by precipitation with methanol. 

It is also possible to carry out living cationic polymerization of isobutylene, initiated by a difiinctional initiator." This results in a formation of bifunctional living segments of polyis-obutylene that are soft and rubbery. Upon completion of the polymerization, another monomer, one that yields stiff segments and has a high Tg value, like indene, is introduced into the living charge. Polymerization of the second monomer is initiated from both ends of the formed polyisobutylene. When the reaction is complete, the polymerization is quenched. Preparations of a variety of such triblock and star block polymers have been described." ... 

Cyclic Amines. The three-membered cyclic amines and the four-membered cyclic amines can be polymerized only by cationic mechanism (324). Monofunctional initiators, such as methyl triflate (325), produce monofiinctional telechelics from f-butylaziridine (TEA). Addition of the monomer to a bifunctional living PTHF solution (326) gives bifimctional poly(TBA). This is a method of making ABA block copolymers. The aziridinium end groups react with acrylic acids to form the corresponding esters (327). 

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