Synthesis of Star-Block Copolymers

Other less common methods for the synthesis of star-block copolymers have also been reported. Recent characteristic examples will be given in the following paragraphs. 

The oxocarbenium perchlorate C(CH2OCH2CH2CO C1O4 )4 was employed as a tetrafunctional initiator for the synthesis of PTHE 4-arm stars [146]. The living ends were subsequently reacted either with sodium bromoacetate or bromoisobutyryl chloride. The end-capping reaction was not efficient in the first case (lower than 45%). Therefore, the second procedure was the method of choice for the synthesis of the bromoisobutyryl star-shaped macroinitiators. In the presence of CuCl/bpy the ATRP of styrene was initiated in bulk, leading to the formation of (PTHP-fo-PS)4 star-block copolymers. Further addition of MMA provided the (PTHF-h-PS-h-PMMA)4 star-block terpolymers. Relatively narrow molecular weight distributions were obtained with this synthetic procedure. 

The relatively shght broadening of the molecular weight distribution during the polymerization of the MMA blocks showed that the macroinitiator (PTHF)4 is highly efficient in promoting the block copolymerization. 

These double hydrophilic block copolymers exhibit stimuli-responsive properties and have potential biotech applications. 

4-(dimethylamino)pyridine, DMAP, to provide the desired star-block copolymers. NMR measurements showed that the reaction sequence was successful. Moderately broad molecular weight distributions (1.18 Mw/Mn 1.32) were obtained. 

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Star-block copolymers are star polymers in which each arm is a block (diblock or triblock) copolymer. There are several methods used for the synthesis of star-block copolymers [142], and the most commonly used strategies are given in Scheme 67. 

Star-block copolymers can be envisioned as star polymers where each arm is a diblock or a triblock copolymer (Figure 2). These stmctures combine the properties of diblock or ttiblock copolymers along with those of star polymers. The synthesis of star-block copolymers can be achieved by all the previously mentioned methods using a single living polymerization technique or a combination of techniques. 

Cgo was also used for the synthesis of star-block copolymers. Living PS-b-P2VP diblocks, having short P2VP chains, were prepared by sequential anionic polymerization in THF. The living diblocks were reacted with a... 

H) Atom transfer radical polymerization Numerous reports have been published regarding the synthesis of star polymers using multifunctional initiators capable of initiating the atom transfer radical polymerization (ATRP) of certain monomers, mainly styrene, methacrylates, and acrylates. The living character of the growing chain ends provides the possibility for the synthesis of star-block copolymers. 

Diels-Alder click reaction for the synthesis of star block copolymers consisting of PSt-PMMA arms. This so-called double click strategy is summarized in Scheme 71. 

Figure 8.11 Synthesis of star block copolymers by the combination of ATRP and CuAAC.

The method described above was applied to the synthesis of star block copolymers (Figure 4), in which each branch was a polystyrene-Z)-polydiene copolymer. Under selected conditions of composition, branch length, and concentration, they exhibit bicontinuous mesomorphic phases referred to as double-diamond structures [29], which had never been observed before. 

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. 

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. ... 

ATRP allows the synthesis of di-block copolymers by sequential (one-pot) or separated steps (two-pot) methods (26). To synthesize di-block, tri-block, 3-and 4-arm star-block copolymers by the two-pot method, a typical ATRP procedure was performed. First, a homopolymer was synthesized as mentioned above and then this was used as a macroinitiator. In addition to the two-pot procedure, one of the tri-block copolymers (P2 in Table 1) was also synthesized by the one-pot ATRP procedure. Once first monomer polymerized to complete conversion, the second monomer was added to the flask under nitrogen to obtain the block copolymers. In both cases, the samples were taken periodically via a syringe to follow the molecular weight of the polymer by GPC and the conversion of polymerization by GC measurements. 

Li, Y. and Kissel, T., Synthesis characteristics and in vitro degradation of star-block copolymers consisting of Z-lactide, glycolide and branched multi-arm poly(ethylene oxide). Polymer, 39(18), 4421,1998. 

Scheme 26.5 Living coupling reaction of living PIB with BDTEP and chain ramification reaction of MeVE for the synthesis ofA2B2 star-block copolymer.

When traditional sequential monomer addition is employed in the preparation of star-block copolymers, the reaction is usually stopped before complete conversion of the first monomer to circumvent excessive terminations. The A block is usually isolated by precipitation, and used as macroinitiator for the polymerization of monomer B. If the second monomer is added before complete conversion of the first monomer, a gradient copolymer is formed with characteristics similar to those of the block [156]. The synthesis of gradient copolymers simplifies the process with some loss of control over the polymer structure. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Hexaepoxy squalene, HES (Scheme 70) was used as a multifunctional initiator in the presence of TiCU as a coinitiator, di-f-butylpyridine as a proton trap, and N,N-dimethylacetamide as an electron pair donor in methylcy-clohexane/methyl chloride solvent mixtures at - 80 °C for the synthesis of (PIB-fc-PS)n star-block copolymers [145]. IB was polymerized first followed by the addition of styrene. The efficiency and the functionality of the initiator were greatly influenced by both the HES/IB ratio and the concentration ofTiCL, thus indicating that all epoxy initiation sites were not equivalent for polymerization. Depending on the reaction conditions stars with 3 to 10 arms were synthesized. The molecular weight distribution of the initial PIB stars was fairly narrow (Mw/Mn < 1.2), but it was sufficiently increased after the polymerization of styrene (1.32 < Mw/Mn < 1.88). 

Benzenetricarbonyl trichloride and l,2,4,5-tetrakis(bromomethyl) benzene were employed as multifunctional initiators for the synthesis of 3-and 4-arm PTHF stars, respectively [147]. The living ends were reacted with sodium 2-bromoisobutyrate followed by reduction with Sml2. The samarium enolates, thus formed were efficient initiators for the polymerization of MMA to give the (PTHF-fo-PMMA) , n = 3 or 4 star-block copolymers, according to Scheme 71. 

A combination of anionic and ATRP was employed for the synthesis of (PEO-b-PS) , n = 3, 4 star-block copolymers [148]. 2-Hydroxymethyl-l,3-propanediol was used as the initiator for the synthesis of the 3-arm PEO star. The hydroxyl functions were activated by diphenylmethyl potassium, DPMK in DMSO as the solvent. Only 20% of the stoichiometric quantity of DPMK was used to prevent a very fast polymerization of EO. Employing pentaerythritol as the multifunctional initiator a 4-arm PEO star was obtained. Well-defined products were provided in both cases. The hydroxyl end groups of the star polymers were activated with D PM K and reacted with an excess of 2-bromopropionylbro-mide at room temperature. Using these 2-bromopropionate-ended PEO stars in the presence of CuBr/bpy the ATRP of styrene was conducted in bulk at 100 °C, leading to the synthesis of the star-block copolymers with relatively narrow molecular weight distributions (Scheme 72). 

PLLA-b-PEO)3 star-block copolymers have been synthesized by a combination of ROP and post-polymerization reactions [152], as depicted in Scheme 76. Glycerol was employed for the synthesis of a 3-arm PLLA star... 

Anionic polymerization and suitable Unking chemistry were employed for the synthesis of 3-arm PCHD-fc-PS star-block copolymers with PCHD either as the inner or the outer block (Scheme 77) [153]. The block copolymers were prepared by sequential addition of monomers. It was shown that the crossover reaction of either PSIi or PCHDLi was efficient and led to well-defined block copolymers. However, in the case of the PCHD-fc-PSLi copolymers, longer polymerization times were needed for long PCHD... 

The fullerene C o was used as the Unking agent for the synthesis of (PCHD-fc-PS)6 and (PS-fc-PCHD)6 star-block copolymers [154], The polymers were then aromatized with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, DDQ, in 1,2-dichlorobenzene to yield the corresponding copolymers containing poly(l,4-phenylene) blocks. In order to achieve high 1,4-isomer contents and to avoid termination reactions, the polymerization of CHD was conducted in toluene at 10 °C without the presence of any additive to yield products with low molecular weights. Coupling of the PCHD-fo-PSLi to C60... 

The facility to introduce well-defined chain ends has been used to prepare star polymers557 and diblocks via reaction with macromolecular aldehydes.558 The synthesis of amphiphilic star block copolymers has also been described using a cross-linking agent.559 560 A similar strategy has recently... 

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