Polymers anionic chain ends

When the initial monomer supply is exhausted, the anionic chain ends retain their activity. Thus, these anionic chains have been termed living polymers. If more monomer is added, they resume propagation. If it is a second monomer, the result is a block copolymer. 

The liberated acid can then react with the anionic chain ends to inhibit the thermally activated decomposition of the polymer chain, outlined in Eq. 6. 

Functionalization of these reactive anionic chain ends can be achieved by a variety of methods all based on the general concepts of carbanion chemistry. For example, reaction with C02 or succinnic anhydride leads to the carboxy terminated derivatives [10], while hydroxy-terminated polymers can be easily obtained by reaction with ethylene oxide (Scheme 3) [11]. In select functionalization reactions, such as alkylation with p-vinyl benzyl chloride, the nucleophilicity of the carbanionic species may be necessary and this can be achieved by reaction of the chain end with 1,1-diphenylethene followed by functionalization [12,13]. 

The application of these functionalization reactions to polymers has been catalogued in the anionic polymer review literature (4-6). Unfortunately, many of the reported applications of these functionalization reactions to anionic chain-ended polymers have not been well characterized (7). In order to exploit these functionalization reactions to their potential, well-defined procedures for quantitative chain end functionalization must be available. 

Styrene-1,3-butadiene-styrene (SBS) or styrene-isoprene-styrene (SIS) triblock copolymers are manufactured by a three-stage sequential polymerization. One possible way of the synthesis is to start with the polymerization of styrene. Since all polystyrene chains have an active anionic chain end, adding butadiene to this reaction mixture resumes polymerization, leading to the formation of a polybutadiene block. The third block is formed after the addition of styrene again. The polymer thus produced contains glassy (or crystalline) polystyrene domains dispersed in a matrix of rubbery polybutadiene.120,481,486... 

The carbonation of dilithium reagents is complicated by the occurrence of gelation phenomena which produce severe mixing problems 145 146-323). In general, lithium derivatives of heteroatoms are highly associated in solution therefore, heteroatom functionalization of polymers with two active anionic chain ends will form an insoluble, three-dimensional network. The beneficial effect of decreasing the effects of association and gelation by solvents with solubility parameters <7.2 has been reported in the literature 140,324). 

SCBs play an important role in the formation of other block copolymers. For example, the relatively less nucleophilic poly(ethylene oxide) oxyanion cannot initiate the polymerization of styrene, which needs a more nucleophilic alkyllithium initiator. To enable the synthesis of multi-block copolymers from various combinations of monomers by anionic mechanisms, it is important to modify the reactivity of the growing anionic chain end of each polymer so as to attack the co-monomer. There have only been a few reports on the polymerization of styrene initiated by an oxyanion (see <2001MM4384> and references cited). Thus, there exists a need for a transitional species that is capable of converting oxyanions into carbanions. In 2000, Kawakami and co-workers came up with the concept of the carbanion pump , in which the ring-strain energy of the SCB is harnessed to convert an oxyanion into a carbanion (Scheme 13) <2000MI527>. 

After 1 week the anionic chain ends are terminated by the addition of methanol (2 mL) and the polymer is precipitated into an Erlenmeyer flask containing hexane (freshly distilled ca. 1 L). The polymer is then redissolved in THF and re-precipitated into hexane as before, it is then filtered and dried in the vacuum oven at 40°C for 24 h. 

We have also observed that, at elevated temperature (50°C), DPE could effectively cap the living anionic polyferrocenylsilane directly.30 Therefore, the addition of DMSB to pump up the activity of PFS anionic chain ends can potentially be omitted, but the yield of the isolated PFS- -PMMA is significantly reduced.31 The living DPE-capped PFS polymers also can react with chloromethyl functionalities of poly(styrene-co-chloromethylstyrene) (PS-co-PCMS), leading to the first PS-g-PFS graft copolymers (see Scheme 3.10).30... 

In producing a block copolymer with interesting applications in polymer blends, Chapiro produced a block copolymer of acrylonitrile and acrylonitrile-co-styrene by gamma irradiation of the mixed monomers in dimethylformamide/benzyl alcohol. It was reported that irradiation induced the formation of radical-anions. Acrylonitrile then added exclusively to the anionic chain ends, and acrylonitrile-styrene added randomly to the free radicals chain ends. The resultant polymer was found to be poly(acrylonitrile-b-acrylonitrile-co-styrene). 

Substituted DPEs have also been utilized to prepare carboxyl-functionalized polymers. The carboxyl functionality has been protected using the oxazoline group. The oxazoline-substituted DPE was not stable to the anionic chain end at room temperature, however. " It was necessary to effect this functionalization reaction in toIuene/THF mixtures (4/1, v/v) at -78 ° C to produce the carboxyl-functionalized polystyrene (Ain = 2.4 X 14.6 X lO gmoh ) in quantitative yield after acid hydrolysis as shown in eqn [42]. Quantitative formation of the oxazolyl-ftmctionalized polystyrene was determined by elemental analysis of the polymer. 

POLYMER AND CHAIN END STRUCTURE IN ANIONIC DIENE POLYMERIZATION... 

In order to optimize specific anionic functionalization reactions such as carbonation with carbon dioxide, the effect of chain end structure (stability and steric requirements) has often been investigated. The steric and electronic nature of the anionic chain end and the chain-end aggregation can be modified by reaction with 1,1-diphenylethylene as shown in Eq. (13). When the direct carbonation is effected in benzene at room temperature with the diphe-nylalkyllithium species formed by addition of poly(styryl)lithium to 1,1-diphenylethylene (Eq. 31), the carboxylated polymer can be isolated in 98% yield compared to only a 47% yield for poly(styryl)lithium without end-capping under the same conditions [141] ... 

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