Sequential anionic copolymerization

The polymerization of D3 was followed by GC/MS. After 36 h, the reaction was quenched by the introduction of trimethylchlorosilane. About 92% of the D3 had been converted, while the amount of unconverted D/ had not changed significantly. Si NMR analyses allowed the determination of the sequence distribution of repeat units, which showed no random copolymerization of D/ and D3 as in the case of diblock copolymers prepared by sequential anionic copolymerization of D3 extended with D/. Because the polymerization of the first monomer could not be carried out to completion (<100% conversion) without increasing the molecular weight distribution, the second monomer (with faster propagation rates) had to be introduced before the equilibration reaction became established. Therefore, unreacted monomer from the first step was still in solution when the second monomer was added. The risk of random copolymerization can be suppressed if the second monomer has far higher reactivity towards polymerization than the first monomer. The block formed in the second step contained only a few methylvinylsiloxane units, i.e. the block purity was very high. 

Star polymers having several PS branches and only one poly(2-vinyl naphthalene), PVN branch were prepared by Takano et al. using anionic polymerization techniques [31]. Sequential anionic block copolymerization of (4-vinyl-phenyl) dimethylvinylsilane (VS) and VN was employed. The double bonds attached to silicon have to remain unaffected during the polymerization of VS. This was ac-... 

The statistical anionic copolymerization of acrylates and methacrylates is also controlled in the presence of LiOEEM (30), as testified by the copolymerization of MMA and tBuA in THF at —78°C. Block copolymers were also prepared by the sequential polymerization of at least two methacrylates and acrylates. For instance, PMMA- >/c cA -PbBuA and PMMA-fcZocA -PnNonA were synthesized . The addition order of the comonomers is important. Indeed, when living PnBuA is the macroinitiator of the MMA polymerization, the expected block copolymer is contaminated by homo-PnBuA, which is not the case when the polymerization sequence is reversed. A fuUy acrylic-based thermoplastic elastomer, PMMA-fcZocA -P(2EtHA)-fcZocA -PMMA, was prepared by the sequential LiOEEM-ligated polymerization of MMA, 2-EtHA and MMA. ... 

Polymers and copolymers were laboratory-prepared samples. Samples W4 and W7 of the diblock copolymer AB poly(styrene-fo-tetramethylene oxide) (PS—PT) were synthesized by producing a polystyrene prepolymer whose terminal group was transformed to a macroinitiator for the polymerization of THF. Samples B13 and B16 of the diblock copolymer AB poly[styrene-h-(dimethyl siloxane)] (PS-PDMS) were prepared by sequential anionic polymerization. Samples of statistical copolymers of styrene and n-butyl methacrylate (PSBMA) were produced by radical copolymerization. Details of synthetic and characterization methods have been reported elsewhere (15, 17-19). 

The anionic copolymerization (AROCP) of different MSCBs with the monomer of another type capable of anionic polymerization in polar or nonpolar medium initiated by AlkLi mostly yielded random copolymers. The sequential polymerization (addition of the second monomer after polymerization of the first monomer was completed) enabled one to synthesize various block copolymers. As MSCBs, various symmetrically and unsymmetrically substituted derivatives were used. As monomers of other types, styrene, butadiene, isoprene, and 2,4-dimethylstyrene were tested [49]. 

The sequential block copolymerization of la with styrene efficiently proceeded to afford an objective AB diblock copolymer, poly(la)- 7/ocfe-polystyrene, with well-defined structures. This success further confirms the living nature of the anionic polymerization of la. Similarly, a well-defined BA diblock copolymer, polystyrene-l7/ocfe-poly(la), was synthesized by reversing the sequence of monomer addition, namely styrene followed by la. Thus, the possible crossover copolymerization indicates that the electrophilicities of la and styrene as well as the nucleophilicities of both living polymers are very similar. 

Sterically bulkier 2,6-diisopropylphenyl (16c), 2,6-di(tert-butyl)-4-methylphenyl (16d), and 2,6-di(tert-butyl)-4-methoxyphenyl esters (16e) were more effective to protect the carboxylic acid of 16. These ester-protected styrene monomers, 16c-16e, underwent living anionic polymerization without any side reaction in THF even at -78 °C. The resulting living polymers were stable for 24 h under such conditions. Polymers with predictable molecular weights and narrow molecular weight distributions (A4w/A4n< 11) were quantitatively obtained. The success of postpolymerization and the sequential block copolymerization with tert-butyl methacrylate (tBMA) further supports the living nature of the polymerization of 16c-16e. Thus, tert-butyl and sterically bulkier phenyl esters satisfactorily protect the carboxylic acid function of 16 to enable the living anionic polymerization of their ester-protected monomers. 

The living nature of the anionic polymerization of 28a and 28b was also demonstrated by the success of postpolymerization and the sequential block copolymerization of 28a or 28b... 

As a matter of fact, anionic copolymerization is essentially utilized for the preparation of block copolymers (see Section 9.2) in general, one operates by sequential addition of the comonomers in the order of increasing electroafiinity. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Industrially the most significant route to block copolymerization of st5Tene and butadiene is through anionic polymerization. The living nature of the anionic polymerization makes it possible to produce block copolymers of styrene and butadiene mostly through sequential addition of monomers. 

Here, we report on the synthesis and characterization of PMMA-poly(/i-butylacrylate)-PMMA (MBuM) symmetric triblock copolymers, as a new family of thermoplastic elastomers. These compounds have be prepared by a novel route based on controlled radical polymerization [4]. Compared to classical anionic living polymerization [5], this new route, sketched in Scheme la, appears particularly appealing since the triblock copolymers are prepared in a two-step process instead of the usual three steps required in anionic polymerization. In the latter case, as butylacrylate cannot be polymerized via a living process, the MBuM symmetric triblocks have to be synthesized by sequential copolymerization of er butylacrylate (tBuA) and MMA followed by the transalcoholysis of the tBu esters with n-butanol... 

Block copolymers can be obtained by copolymerization of cycloolefins of entirely different reactivities or by applying adequate sequential addition of the monomer. They also arise from cycloolefins and vinylic monomers, including linear olefins, in the presence of Ziegler-Natta catalysts [5] [Eq. (3)] or of metathesis catalysts. In the latter case it is usual to change the reaction mechanism to Ziegler Natta [6] and group transfer polymerization [7] or from anionic-coordina-tive to metathesis polymerization [8] [Eq. (4)]. 

The anionic arm-first methods can also be applied to the synthesis of star block copolymers [59]. The procedure is identical except that living diblock copolymers (arising from sequential copolymerization of two appropriate monomers, added in the order of increasing nucleophilicity) are used as living precursor chains. The active sites subsequently initiate the polymerization of a small amount of a bis-unsaturated monomer (DVB in most cases) to generate the cores. If polystyrene and polyisoprene (or polybutadiene) are selected, the resulting star block copol)miers behave as thermoplastic elastomers because of their different glass transition temperatures. 

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