Synthesis of Block Copolymers by Cationic Polymerization

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instabihty of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52]. The most frequent procedure is the ehmination of the cation s J-prolon, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophilic stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive-. The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophilic anions, such as ammonium and phosphonium derivatives. 

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines,. V-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

The cationic ring opening polymerization of -caprolactone, CL, and 5-valerolactone, VL, was investigated using n-BuOH/HCl Et2O as the initiation system [56]. It was observed that narrow molecular weight distribution samples were obtained. These results were combined with those previously 

Well-defined phosphazene block copolymers were prepared by the cationic polymerization of phosphoranimines [57]. Block copolymers of the type [N = PCl2] [N = PR(R )]m were prepared using a wide variety of phos- 

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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. 

An interesting synthesis of block copolymers by cationic polymerization of vinyl compounds was described by Kennedy and Melby [277] who used... 

Intermolecular chain transfer to polymer is well documented in the cationic polymerization of cyclic acetals. In the polymerization of TOX, as will be discussed in Section 4.10.3, chain transfer to polymer is essential for the preparation of thermally stable polyacetal. Intermolecular chain transfer to polymer is detrimental to the synthesis of monofunctional polymers such as macromonomers because segment exchange (scrambling) leads to disproportionation and formation of products having two, one, and none of the functional groups (Scheme 21). " Intermolecular chain transfer to polymer prohibits also the synthesis of block copolymers by sequential polymerization of two cyclic acetals. Addition of DXP to a solution of living polyDXL resulted in further polymerization but the copolymer formed had a nearly statistical distribution of units." ... 

Scheme 4. Synthesis of block copolymer by combination of ATRP and Free Radical Promoted Cationic Polymerization.

Scheme 11.20 Synthesis of block copolymers by combination of living cationic polymerization and ATRP using macroinitiator technique.

Scheme 11.25 Synthesis of block copolymers by transformation of living anionic polymerization into living cationic polymerization.

Extensive transacetalization proceeding parallel to propagation essentially precludes the possibility of the synthesis of perfectly monoftmctional polymers or block copolymers by cationic polymerization of cyclic acetals. Transacetalization is not that detrimental to synthesis of diftmctional telechelic... 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

Due to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystalline block copolymers have not been prepared by living cationic sequential block copolymerization. Although site-transformation has been utilized extensively for the synthesis of block copolymers, only a few PIB/crystalline block copolymers such as poly(L-lactide-fc-IB-fc-L-lactide) [92], poly(IB-fr- -caprolactone( -CL)) [93] diblock and poly( -CL-fr-IB-fr- -CL) [94] triblock copolymers with relatively short PIB block segment (Mn< 10,000 g/mol) were reported. This is most likely due to difficulties in quantitative end-functionalization of high molecular weight PIB. 

The combination of living cationic and living anionic polymerizations provides a unique approach to the synthesis of block copolymers not available by a single method. Coupling of living anionic and cationic polymers is conceptually simple, but few examples have been reported so far. This is most likely due to the different reaction conditions required for living cationic and anionic polymerizations. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

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]. 

Living cationic sequential block copolymerization is one of the simplest and most convenient methods to provide well-defined block copolymers. The successful synthesis of block copolymers via sequential monomer addition relies on the rational selection of polymerization conditions, such as Lewis acid, solvent, additives, and temperature, and on the selection of the appropriate order of monomer addition. For a successful living cationic sequential block copolymerization, the rate of crossover to a second monomer ( ) must be faster than or at least equal to that of the homopolymerization of a second monomer (i p). In other words, efficient crossover could be achieved when the two monomers have similar reactivities or when crossover occurs from the more reactive to the less reactive monomer. When crossover is from the less reactive monomer to the more reactive one a mixture of block copolymer and homopolymer is invariably formed because of the unfavorable Rcr/Rp ratio. The nucleophilicity parameter (N) reported by Mayr s group might be used as the relative scale of monomer reactivity [171]. 

Owing to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystaUine block copolymers have not been prepared by living cationic sequential block copolymerization. Although site transformation has been utilized extensively for the synthesis of block copolymers, only a few... 

Several approaches involving a combination of cationic and anionic polymerizations have been reported for the synthesis of block copolymers. These approaches are based on the anionic-to-cationic transformation mechanism and vice versa. Regardless of the mechanism, well-defined block polymeric architectures can be prepared by living modes of both polymerization techniques. 

Reversible addition-fragmentation chain transfer polymerization (RAFT) polymerization of methyl acrylate was combined with cationic polymerization of THF to synthesize comb copolymers. Asymmetric star block copolymers based on polystyrene (PS), PTHF, and PMMA were synthesized by a combination of CROP and redox polymerization methods. Miktoarm star polymers containing poly(THF) and polystyrene arms were also obtained by combining CROP and ATRP methods. Another approach for the synthesis of block copolymers... 

Another method for the synthesis of block copolymers is the sequential monomer addition method. In this method, the polymerization of TBA is initiated by the cationic living chain end of another cationic polymerization. This has been carried out with perchlorate-terminated polystyrene and with cationic living polyTHF as the initiating polymer. The polyTHF can be mono-, di-, or trifunctionally living leading to AB, BAB, and three-arm star-shaped block copolymers, respectively. ... 

The main techniques for synthesis of block copolymers in research labs around the world are presently anionic polymerization and controlled radical polymerization methods. The older technique of anionic polymerization is still used widely in the industrial manufacture of block copolymers. Cationic polymerization may be used to polymerize monomers that eannot be polymerized anioni-cally, although it is used for only a limited range of monomers. A summary of block copolymer synthesis techniques has been provided by Hillmyer [5]. 

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