Coupling triblocks

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

The gel permeation chromatogram shown in Fig. 6 illustrates the purity of a block copolymer obtained by ion coupling. It is seen that about 5% of uncoupled block copolymer contaminates a triblock copolymer of narrow molecular weight distribution. The synthesis of star block polymers owes its recent development to the use of new coupling agents412. ... 

Polystyrene-polytetrahydrofuran block copolymers121122 are an interesting case of coupling between functional polymers The mutual deactivation of living anionic polystyrene and living cationic polyoxolane occurs quantitatively to yield polystyrene-polyoxolane block copolymers. Since either of the initial polymer species can be mono- or difunctional, diblock, triblock or multiblock copolymers can be obtained. 

Poly(styrene-/7-isobutylene-/ -styrene) (PS-PIB-PS), triblock copolymers can be prepared via coupling of living PS-PIB diblock copolymers in a one-pot procedure [12]. 

In the past several years, we have used the SAAP method to prepare different long multiblock copolymers. Generalities about the preparation and characterization of different end-functionalized triblock copolymers are first outlined. Then, the micellization of triblock copolymer as well as coupling efficiency with and without self-assembly method are discussed. 

The peak in /(Rh) located at 3-4 nm represents individual triblock copolymer chains. At 29 °C, an additional peak appears indicating the self-assembly of the triblock copolymer chains. Pentanediol (H0(CH2)50H) was added as the linking agent to couple each two functional ends of the triblock copolymer chains in the presence of pyridine. The resultant multiblock heteropolymer chains have a structure like (PMMA-/)-PS-/)-PMMA-c-)n, where c denotes the linking agent, pentanediol. The structure can also be written as (PMMA-Z>-PS)n, in which the PMMA block is twice longer than that in the initial triblock PMMA-Z>-PS-Z>-PMMA copolymer chain because each two PMMA blocks are connected together in the resultant multiblock copolymer. 

Narrowly distributed Pl-ft-PS-i-PI triblock copolymer chains with both of their ends capped with bromobutyl groups were prepared by sequential addition of living anionic polymerization and terminated by excess of 1,4-dibromobutane (PS block Mw = 3.5 x 103 g/mol PI blocks Mw = 3.1 x 103 g/mol Mw/Mn = 1.12 The degree of end-functionalization was 92% characterized by HNMR). Figure 6 shows the SEC profile of such prepared triblock copolymer chains. The small but a detectable amount ( 5%) of Pl-i-PS-i-PI dimers, PI-Z>-PS-Z>-PI-c-PI-Z>-PS-Z>-PI, is presumably formed via the Wurtz-type coupling reaction. 

Before the coupling reaction, the self-assembly of PI-Z>-PS-Z>-PI triblock copolymer chains in w-hcxane was investigated by LLS. Figure 7 shows typical hydrodynamic radius distributions (/(Rh)) of individual PI-Z>-PS-Z>-PI triblock chains in THF, a good solvent for both the PI and PS blocks, and the core-shell micelles formed via the self-assembly of the triblock copolymer chains in -hexane, a solvent selectively good for the PI block. The shifting of the peak from... 

Figure 6. SEC profiles of PI-6-PS-6-PI triblock copolymer chains end-capped with butyl bromide group (SI44) before and after the coupling reaction in n-hexane, with the self-assembly as well as in THF without the self-assembly.

In contrast, when the coupling reaction was performed in THF without the self-assembly, the SEC profile contains only two peaks, namely, a dimer peak with Mw 1.2 x 104 and precursor triblock copolymer peak. It shows that the coupling reaction stops after two triblock copolymer chains are linked together. This is because for longer copolymer chains (Mw > 104 g/mol), the chain ends are likely wrapped and hidden inside the chains coiled in a good... 

Figure 8. SEC profiles of the triblock copolymer precursor ended with carboxyl acid group coupled by SAAP with 1,6-hexamethylenediamine (HMDA) catalyzed by 1,3-dicyclohexylcarbodiimide (DCC) as well as coupled directly in THF as a reference.

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

The annual worlwide production of triblock thermoplastic elastomers, clear impact-resistant polystyrene, and other styrene-diene products produced by anionic polymerization exceeds a couple of billion pounds. (Commercial utilization of anionic polymerization also includes the polymerization of 1,3-butadiene alone.)... 

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