Cationic chain polymerization block copolymer

Back-biting reaction occurring during cationic polymerization of lactams is detrimental to preparation of block copolymers of two different lactams by sequential polymerization. Block copolymers can be obtained only in those systems in which the rate of polymerization of the second monomer is much higher than the rate of chain transfer to polymer resulting in transamidation [219]. 

In the same scheme, moreover, it is evident that, besides phosphazene homopolymers, the substitution of the chlorines with two (or more) different substituents leads to the preparation of substituent phosphazene copolymers [263] containing different homosubstituted and heterosubstituted monomeric units. Moreover, the cationic polymerization of phosphoranimines [215-217] produces polymers with hving reactive ends (vide supra) from which the preparation of chain phosphazene copolymers (block copolymers) [220,223,225, 229,232-235,239, 240] formed by different polymeric backbones linked together in a unique macromolecule could be obtained. 

In both anionic and cationic polymerization it is possible to create living polymers . In this process, we starve the reacting species of monomer. Once the monomer is exhausted, the terminal groups of the chains are still activated. If we add more monomer to the reaction vessel, chain groivth will restart. This technique provides us with a uniquely controllable system in which we can add different monomers to living chains to create block copolymers. 

Nitroxide attached to macromolecules also induces the living radical polymerization of St. Yoshida and Sugita [252] prepared a polymeric stable radical by the reaction of the living end of the polytetrahydrofuran prepared by cationic polymerization with 4-hydroxy-TEMPO and studied the living radical polymerization of St with the nitroxide-bearing polytetrahydrofuran chain. The nitroxides attached to the dendrimer have been synthesized (Eq. 69) to yield block copolymers consisting of a dendrimer and a linear polymer [250,253]. 

Inaki (1992) synthesized a wide range of nucleobase-functionalized random and homopolymers. In addition, Inaki et al. (1980) synthesized block copolymers containing thymine and uracil groups in the main chain through ring-opening cationic and anionic polymerization of cychc derivatives of the nucleobases. 

In recent years several attempts have been made to prepare polymers possessing chain end functions capable of giving rise to free radical or to cationic sltes i This research has been mostly aimed at extending the possibilities of synthesis of block copolymers, in which only one of the blocks is obtained anionlcally. The synthesis of -hydroperoxy polymers has already been mentioned. Peroxy-or peranhydride functions have also been introduced into polymer chainsSubsequent radical polymerization of a second monomer results in block copolymers. 

Anionic polymerization of conjugated dienes and olefins retains its lithium on the chain ends as being active moities and capable of propagating additional monomer. This distinguishing feature has an advantage over other methods of polymerization such as radical, cationic and Ziegler polymerization. Many attempts have been made to prepare block copolymers by the above methods, but they were not successful in preparing the clear characterized block copolymer produced by anionic technique. 

PS/PIB/PS block copolymers can be made by controlled-living cationic polymerization. In this polymerization process, the propagating chains are in equilibrium with the dormant species. A suit-... 

One of the most useful features of living polymerizations, which proceed in the absence of chain transfer to monomer and irreversible termination, is the ability to prepare block copolymers. Compared with living anionic polymerization the development of living cationic polymerization is rather re-... 

Polyphosphazene block copolymers were synthesized by these chain-growth polymerization methods. The successive anionic polymerization of N-silylphosphoranimines 19d and 19a at 133 °C yielded the block copolymer with Mw/Mn of 1.4-2.3 (Scheme 80) [278,279]. However, due to the presence of two possible leaving groups in 19d, this approach yielded block copolymers where one of the block segments contained a mixture of side groups. On the other hand, the cationic polymerization of 19b with PCI5 was carried out at ambient temperature, followed by addition of a second phosphoranimine to yield a block copolymer with Mw/Mn of 1.1-1.4, where each block segment had one kind of side chain (Scheme 81) [280]. 

In the 1980s, a synthetic method to produce AB block copolymers of propylene and tetrahydrofurane (THF) was proposed [29]. Polypropylene-fi/ock-poly(THF) was prepared by a combination of living polymerization of propylene with a V(acac)3 catalyst and the living polymerization of THF. Its synthesis was based on the transformation of living polypropylene chain ends to cationic ones, which initiated the living polymerization of THF. 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

Table III shows the increase of molecular weight of BCMO polymerization with conversion, although the polymer tends to precipitate. The monomer reactivity ratios of DOL-BCMO copolymerization were previously determined as rx (DOL) = 0.65 0.05, r2 (BCMO) = 1.5 0.1 at 0°C. by BF3 Et20 (8). Table IV shows a preparation of block copolymer of DOL, St, and BCMO. In the first step we polymerized DOL and St in the second step we added BCMO to this living system. The copolymer obtained showed an increase of molecular weight, and considerable BCMO was incorporated in the copolymer still remaining soluble in ethylene dichloride. The solubility behavior together with the increase of molecular weight with addition of BCMO shows that this polymer consists of block sequences of DOL-St and (St)-DOL-BCMO. This we call block and random copolymer of DOL-St—BCMO. We can deny the presence of BCMO, St, or DOL homopolymers in this system, but some chain-breaking reactions are unavoidable, leading to copolymer mixtures. Thus, the principle of formation of block copolymers by cationic system is partly substantiated.

An interesting synthesis of block copolymers by cationic polymerization of vinyl compounds was described by Kennedy and Melby [277] who used 2-chloro-6-bromo-2,6-dimethylheptane as coinitiator. Br- is eliminated by triethylaluminium, and styrene can be polymerized, without transfer, on the generated carbocation. After all the styrene has reacted, diethylaluminium chloride is added to eliminate Cl- from the coinitiator and thus produce new carbocations on the polymer chain. In the presence of 2-methylpropene, the two-block copolymer poly(styrene)-6/ock-poly(2-methylpropene) is formed. 

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

Diblock, triblock, and multiblock copolymers are typically prepared by sequential monomer addition to polymerization systems in which the chain-breaking reactions are sufficiently suppressed. Polymer properties can thereby be varied by manipulating the constituent blocks compatibilities, hydrophilicities/hydrophobicities, and hardness/softness. New and/ or novel topologies can also be prepared by controlled processes, including cyclic polymers and/or copolymers, comb-like macromolecules, and star polymers. The synthetic range of cationic vinyl polymerizations will be discussed in detail in Chapter 5. 

Along with block copolymers, polymers with terminal functions, or end-functionalized polymers, are another typical class of well-designed polymers that living polymerizations can provide. On the basis of the absence of chain transfer and termination, when coupled with the quantitative and selective initiation from a well-defined initiator, living polymerizations offer two basic methods to prepare end-functionalized polymers, as Scheme 4 illustrates for cationic processes ... 

The macromonomer method (C) has also been adopted in cationic polymerization. For instance, amphiphilic graft polymers of vinyl ethers are synthesized by the cationic polymerization of a vinyl ether-capped macromonomer (26) with a block copolymer chain consisting of IBVE and AcOVE segments, followed by alkaline hydrolysis of the latter part into the HOVE units [165], This graft polymer also undergoes a host-guest interaction similar to those with amphiphilic star block copolymers [220]. 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

The most characteristic feature of the cationic polymerization of cyclic acetals, however, is an excessive participation of the polymer chain in the polymerization processes. This is exemplified by the results of attempted synthesis of block copolymer containing segments of poly(l,3-dioxolane, DXL) and poly(l,3-dioxepane, DXP) [130]. 

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