Monofunctional initiation

These unsaturated alcohols act as monofunctional initiators, giving rise to terminally unsaturated PO-EO diblock impurities, which may be quantified by determining the degree of unsaturation in the final product. 

Initiators where the radical generating functions are sufficiently remote from each other break-down in a non-concerted fashion. Examples include the azoperoxide (68) and the bis-diazene (67).261 Their chemistry is often understandable in terms of the chemistry of analogous monofunctional initiators.260 This class also includes the dialkyl peroxyketaLs (see 3.3.2.4) and hydroperoxyketals (see 3.3.2.5). 

Tri- and higher block copolymers, such as ABA, ABC, ABCB, can be synthesized by a continuation of the processes with the successive additions of the appropriate monomers. A symmetric block copolymer such as ABA or CABAC can be made efficiently by using a difunctional initiator, such as, a, a-dichloro-p-xylene or dimethyl 2,6-dibromoheptanedioate, instead of the monofunctional initiator. For the ABA block copolymer only two, instead of three, monomer charges are needed ... 

Warzelhan et al. (23,24) ascribed structure V to these associates) lead to distinctly different tacticities than the mono functionally growing species (cf. Table I). These tacticities are intermediate between those produced in polar solvents by. monofunctional initiators and those produced in non-polar solvents, where association of the growing ends with methoxide or with the monomer has been proposed. 

It is the process forming the reactive centers from which macromolecules evolve. It may result from two different mechanisms a nucleophilic attack of the monomer by an organometallic initiator (butyllithium, cumylpotassium, benzylsodium, etc.), or the transfer to the monomer of the counterion and the extra electron of an electron-transfer initiator (lithium or sodium naphtalene, biphenyl.) later the ion-radical monomer having an extra electron in its lowest antibonding n orbital becomes a dicarbanion by dimerization of two activated monomer molecules. The use of a monofunctional initiator leads to a biblock copolymer AB, while that of a bifunctional initiator leads to a triblock copolymer ABA. 

The main advantage of this type of initiator is the production of significantly longer polymer chains without reduction of the polymerization rate (compared with monofunctional initiators) [104]. When both functional groups are part of the same peroxide, they are both decomposed to the initiating radicals at practically the same rate. When the difunctional initiator is composed of peroxides decomposing at various rates, this property may be utilized for the synthesis of block copolymers. 

Macromonomers were prepared by polymerizing oxazolines with monofunctional initiators (e.g., methyl p-toluenosulfonate) and terminating the polymerization with salts of acrylic or methacrylic acid. Macromonomers with M varying from M — 500 to —2500 and MJM - 1.2-1.4 were obtained functionalities, however, depended strongly on reaction conditions and the values between 0.99 down to —0.5 and lower were reported [280]. 

The success of obtaining polystyrene products by free radical processes is affected to a significant extent by the quality and performance of initiators. Monofunctional initiators such as benzoyl peroxide or azobisisobutyronitrile have been utilized in bulk and solution polymerizations for theoretical studies... 

This relationship holds for monofunctional initiators. If an initiator starts n chains, Xn = [M]/ i[I]. 

Monofunctional Initiators. AB, ABA, and multiblock copolymers can be synthesized by initiation of one monomer with a monofunctional initiator like n-butyl lithium. When the first monomer has been reacted, a second monomer can be added and polymerized. This monomer addition sequence can be reversed and repealed if the anion of each monomer sequence can initiate polymerization of the other monomer. The length of each block is determined by the amount of the corresponding monomer which was provided. Styrene-isoprene-styrene block copolymers can be made by this method by polymerizing in benzene solution and adding the styrene first. Addition of a small amount of ether accelerates the slow attack of dienyl lithium on styrene. 

Coupling Reactions. In this technique, a living AB block copolymer is made by monofunctional initiation and is then terminated with a bifunctional coupling agent likeadihaloalkane. ABA copolymers can be madeby joining AB polymeric anions ... 

A series of ABA- and BAB-type triblock copolymers (B-22 to B-24) were obtained by two-step block copolymerization with bifunctional initiators202 or by three-step block copolymerization with monofunctional initiators.202 366 368 ABC-type block copolymers B-25203 and B-26202 consisting of styrene, MA, and tBA can be obtained by the latter method the tBA segment may be hydrolyzed to give amphiphilic triblock copolymers. 

Functional groups can also be introduced in the spacer units. Bifunctional initiators with bipyridine units such as MI-17 and MI-18 induced the living radical polymerizations of styrene and MMA, respectively, with copper catalysts to give polymers that carry a coordination site at the middle of the chain.87,333 These polymers can be connected together into star polymers with a ruthenium cation at the core, where the arm numbers are varied among three, four, five, and six in combination with the polymers obtained from the monofunctional initiator with a bipyridine unit (FI-21 and FI-22 Figure 13).416 A... 

The monofunctional initiator [(oi-methylstyryD-lithium (a-MeSt"Li + )] is prepared by reacting sec-BuLi with a slight molar excess of a-MeSt at room temperature in dry THF. 

This type of initiation is known as monofunctional initiation as it produces one active (ionic) site for propagation. Propagation takes place by the addition of monomer to the ionic site ... 

The average degree of polymerization is given by the number of monomer molecules polymerized per polymer molecule formed. In the case of monofunctional initiation [cf. Eq. (8.7)], the number of polymer molecules... 

With monofunctional initiators (e.g., n-butyllithium), AB, ABA, and multiblock copolymers can be formed. For example, the synthesis of an AB block copolymer can be shown schematically as... 

The order of monomer addition is important. For example, to prepare an AB type block copolymer of styrene and methyl methacrylate, st ene must be polymerized first using a monofunctional initiator and when styrene is completely reacted, the other monomer MMA must be added. The copolymer would not form if MMA were polymerized first, because living poly(methyl methacrylate) is not basic enough to add to styrene. The length of each block is determined by the amount of corresponding monomer which was provided. To produce ABA type copolymer by monofunctional initiation, B can be added when A is consumed, and A added again when B is consumed. This procedure is possible if the anion of each monomer sequence can initiate polymerization of the other monomer. Multiblock copolymers can also be made in this way. 

PEO prepared by anionic polymerization using the monofunctional initiator ((diphenylmethyl)potassium) has one -OH end group leading to the formation of linear-dendritic diblock copolymers (Scheme 102). 

Unfortunately, this kind of discrimination is unfeasible in the polymethyl methacrylate system. However, the tactidty of the polymer provides a useful way of differentiating between these two alternatives. The polymethyl methacrylate formed in THF by monofunctional initiators is, to a high degree, syndiotactic. The tacticity should have remained unaltered had the association-dissociation mechanism been operative, whereas the polymer formed in the early stages of a reaction initiated by bifunctional initiators shows a larger proportion of isotactic diads. This demonstrates that the slow addition involves a different center, i.e. it takes place on the associated end-groups. A further discussion of tacticity is postponed here and will be continued later. 

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