Copolymerization block

Block Copolymerization —The two principal modes of cationic block copolymerization involve either the direct sequential cationic polymerization of two monomers or the preparation of a macro-initiator, which is then used to induce the cationic polymerization of a second monomer. The second of these alternatives can be executed with macro-initiators prepared by either cationic or non-cationic polymerizations. 

An example of the first technique is provided by the polymerization of isobutyl vinyl ether (IBVE) on poly(p-methoxystyrene) (p-MOS). Iodine-initiated polymerizations of p-MOS at —15 conducted in solvents of low polarity or in the presence of excess common anions produce non-dissociated active centres that can 

When a difunctional trityl ion salt was used to initiate the living cationic polymerization of cyclopentadiene, block copolymers were formed through charge neutralization with living anionic ct-MS. By varying the functionality of the a-MS polymers, controlled block sequences were generated in the copolymers. 

To produce block copolymers by free-radical polymerization, a radical center must be produced at the end of the chain from where fresh chain growth may take place. Two of the ways by which such terminal radicals can be produced are 

To produce block copolymers by free-radical polymerization, a radical center must be produced at the end of the chain from where fresh chain growth may take place. Two of the ways by which such terminal radicals can be produced are (a) decomposition of peroxide groups introduced as an internal part of a polymer chain backbone or as an end group and (b) breaking of C-C bonds in the polymer chain by mechanical means. More recently, the advent of hving or controlled free radical polymerization has opened up a more versatile route to block copolymers by the free radical process. 

Block and graft copolymerizations involve initiating polymerization reactions through active sites bound on the parent polymer molecule. Block copolymerization involves terminal active sites, whereas graft copolymerization involves active sites attached either to the backbone or to pendant side groups. Copolymerizations only by free-radical processes are discussed in this section those involving ionic mechanisms are described in Chapter 8. 

Block and graft copolymerizations by free-radical mechanism are usually conducted in a mixture of the parent polymer, the monomer(s) to be grown on the parent polymer, and fresh initiator. However, the product obtained in this case is likely to be a mixture. Thus, in addition to the desired block or graft copolymer, it may contain homopolymer of fresh monomer and parent homopolymer molecules that did not take part in the copolymerization. 

Block copolymerization by free-radical processes calls for generation of terminal radicals (that is, a radical center at the end of the chain). Terminal radicals can be produced by (a) Decomposition of peroxide groups that are introduced into the polymer by special means. These peroxide groups may 

It decomposes by random cleavage to form shorter diradical species, which become incorporated into the parent polymer backbone. When fresh monomer is added and additional heating is supplied, the remaining peroxide linkages in the polymer backbone decompose to form terminal radicals and polymerization resumes leading to a block copolymer. 

Using such an initiator to produce a polymer, followed by conversion of the carboxylic acid end group to the acid chloride and then reaction of this with t-butyl hydroperoxide results in polymer molecules containing t-butyl perester end groups to initiate polymerization of a second monomer. 

Similarly, polymerizations of THF have been initiated by anionicaUy-poly- 

The obvious, but experimentally very difficult route to block polymer formation by charge neutralization between living anionic and cationic polymers was successful for polystyrene and poly(THF).  

In many cases, the polarities of the monomers or the macroions are very different for example, in the anionic copolymerization of styrene (1) and methyl methacrylate (2). A styryl anion can add methyl methacrylate therefore /ci2 7 0 but styrene will not add on to the methyl methacrylate anion (/c2i = 0). All the styrene incorporated into the polymer must therefore come from the starting step and the propagation step immediately following it. Thus, for the unipolymerization. 

Since /C21 = 0 only one of the two possible copolymerization cross-reactions need be considered  

For the conditions at the start, the change in M2 concentration with /C21 = 0 is given by 

For the propagation reactions, on the other hand, the following equations can be established  

Since equations (22-63) and (22-56) represent expressions for limiting cases, they can be generalized to 

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Thus, BP-aniline may serve as the photoinitiator in the establishment of some new methods of block copolymerization. 

Formula Abbreviation Reference synthesis Reference block copolymerization... 

Apart from poly(ethylene glycol), other hydroxyl-terminated polymers and low-molecular weight compounds were condensed with ACPC. An interesting example is the reaction of ACPC with preformed poly(bu-tadiene) possessing terminal OH groups [26]. The reaction was carried out in chloroform solution and (CH3CH2)3N was used as a catalyst. MAIs based on butadiene thus obtained were used for the thermally induced block copolymerization with styrene [26] and dimethyl itaconate [27]. 

By means of a ring-opening polymerization of the condensation type Vlasov et al. [50] synthesized polypeptide based MAIs with azo groups in the polymeric backbone. The method is based on the reaction of a hydracide derivative of AIBN and a N-carboxy anhydride. Containing one central azo group in the polymer main chain, the polymeric azo initiator was used for initiating block copolymerizations of styrene and various methacrylamides. 

Polyaddition reactions based on isocyanate-terminated poly(ethylene glycol)s and subsequent block copolymerization with styrene monomer were utilized for the impregnation of wood [54]. Hazer [55] prepared block copolymers containing poly(ethylene adipate) and po-ly(peroxy carbamate) by an addition of the respective isocyanate-terminated prepolymers to polyazoesters. By both bulk and solution polymerization and subsequent thermal polymerization in the presence of a vinyl monomer, multiblock copolymers could be formed. 

Moreover, free radical block copolymerization has been performed by means of low-molecular initiators containing two azo groups of different thermal reactivity. The first thermal treatment at a relatively low temperature in the presence of a monomer A results in a polymeric azo initiator. The more stable azo functions being situated at the end of A blocks can be subjected to a second thermal treatment at a higher temperature in the presence of monomer B. 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

Many research papers, patents, and books have been published on this kind of block copolymerization. Recent topics will be reviewed in this chapter. 

For achieving improved effective initiation for each step of block copolymerization, type I MAI having dual... 

Block copolymerization is carried out by thermolysis of the macroinitiator in bulk, solution, suspension, or emulsion system. Further, it is possible to apply photolysis of azo group. In another case, an ionic active site coupled with an azo group is utilized [3]. 

An MAI having a cyclodiborazane group was synthesized by the reaction of thexyl borane, adipoyldini-trile, and AIBN. It was block copolymerized with St [69]. 

Keli, T., Doi, Y. Synthesis of Living Polyolefins with Soluble Ziegler-Natta Catalysts and Application to Block Copolymerization. Vol. 73/74, pp. 201 —248. 

The values of rx and r2 found in the block copolymerization for this pair are equal to rr = 0,03 0,03 and r2 = 0,45 0,05 respectively35. The difference in these values indicates that DMSO produces a noticeable effect on the reactivity of AN. 

As it was shown in73, 74), methods that can be used to synthesize these copolymers of PAN are those of radical AN block copolymerization in the presence of an oxidation-reduction system in which the hydroxyl end groups of polyethylene oxide) (PEO)73) and polypropylene oxide) (PPO)74- oligomers serve as the reducing agents and tetravalent cerium salts as the oxidizing agents. 

When PAN block copolymers are synthesized with the help of such method the reaction is conducted during a period of time (20—30 min) shorter than the induction period of AN polymerization (45 min) in the presence of cerium ions. When the mechanism and the laws governing the reaction of AN copolymerization with PEO and PPO were studied, it was established that the initiation of the block copolymerization proceeds in accordance with the following scheme ... 

Anionic polymerization of pivalolactone with the polystyrene anion produced only homopolymer mixtures, but the polystyrene carboxylate anion was able to give a block copolymer336. The block efficiency depends on catalyst ratio and conversion because the initiation step is slow compared with propagation337. Tough and elastic films were obtained by graft copolymerization or block copolymerization of pivalolactone onto elastomers containing tetrabutylammonium carboxylate groups338,339. ... 

The synthesis of tailor-made star-shaped polymers can be performed in several ways by means of a plurifunctional organometallic initiator, or by reacting a living precursor polymer with a plurifunctional reagent, to build the centra] body, or by block copolymerization involving a diunsaturated monomer (Scheme 3). 

By anionic block copolymerization of two monomers, thesecond being bi-unsaturated... 

There are few reports on block-copolymeric TPE (namely, polyurethane, EVA, SBS, poly (styrene-fo-butyl acrylate) (PSBA))-clay nanocomposites also [196-199]. Choi et al. [196] studied the effect of the silicate layers in the nanocomposites on the order-disorder transition temperature of... 

FIGURE S.1 Chemical structure of block copolymeric thermoplastic elastomers (TPEs) (a) styrenic, (b) COPE, (c) thermoplastic pol)oirethane, and (d) thermoplastic polyamide. 

After almost half a century of use in the health field, PU remains one of the most popular biomaterials for medical applications. Their segmented block copolymeric character endows them with a wide range of versatility in tailoring their physical properties, biodegradation character, and blood compatibility. The physical properties of urethanes can be varied from soft thermoplastic elastomers to hard, brittle, and highly cross-linked thermoset material. 

Anionic polymerization in suitable systems allows the preparation of polymers with controlled molecular weight, narrow molecular weight distributions and functional termination. The functional termination of a living anionic polymerization with a polymerizable group has been used frequently in the preparation of macromonomers (4). Our research has encompassed the anionic homo and block copolymerizations of D- or hexamethyl cyclotrisiloxane with organolithiums to prepare well defined polymers. As early as 1962 PSX macromonomers were reported in the literature by Greber (5) but the copolymerization of these macromonomers did not become accepted technique until their value was demonstrated by Milkovich and... 

Anionic polymerization of polystyrene takes place very rapidly- much faster than free radical polymerization. When practiced on a large scale, this gives rise to heat transfer problems and limits its commercial practice to special cases, such as block copolymerization by living reactions. We employ anionic polymerization to make tri-block copolymer rubbers such as polystyrene-polybutadiene-polystyrene. This type of synthetic rubber is widely used in the handles of power tools, the soft grips of pens, and the elastic side panels of disposable diapers. 

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