Star block-copolymers

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

Generally, the number of the shell chains in a microsphere ranges from a few hundred to a few thousand. The range of the diameter of the core is from 10-100 nm. Such a core-shell structure is very similar to the (AB)n type star block copolymers, which have many arms and spherical polymer micelles of the block or graft copolymers formed in selective solvents that are good for the corona sequence and bad for the core sequence. In fact, many theoretical investigations of the chain con-... 

The feature of the core-shell type polymer microspheres that differentiates them the most from the (AB)n type star block copolymers is size. The external diameters of the core-shell type polymer microspheres are generally from about 20-200 nm in the good solvents instead... 

AB star block copolymer Figure 1 Types of block copolymers. 

Polycondensation of ACPC with triphenol gave a multibranched MAI with which a star block copolymer could be derived [13]. 

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). 

I he method of polymerization needs to be chosen for compatibility with functionality in the cores and the monomers to be used. Star block copolymers have also been reported. Mulli(bromo-compounds) may be used directly as ATRP initiators or they can be converted to RAFT agents. One of the most common... 

Star molecules containing branches made of two blocks have also been prepared by these methods102 103. Recently it was shown that such star-block copolymers exhibit very interesting so-called double-diamond structures in the bulk owing to segregation due to incompatibility between chemically unlike blocks 104. ... 

Asathana S., Majoros I., and Kennedy J.P., TPEs Star-block comprising multiple polystyrene-b-PIB arms radiating from a crossUnked polydivinylbengene core. Rubber Chem. TechnoL, 71, 949, 1998. Shim J.S. and Kennedy J.P., Novel thermoplastic elastomers. II. Properties of star-block copolymers of PST-b-PlB arms emanating from cyclosiloxane cores, J. Polym. Set, Part A, Polym. Chem., 37, 815, 1999. 

Youk J.H., Park M.K., Locklin J., Advincula R., Yang J., and Mays J. Preparation of aggregation stable gold nanoparticles using star-block copolymers, Langmuir, 18, 2455, 2002. 

Consider a polystyrene-( )-polybutadiene star block copolymer with four arms coupled by a central Si-atom. Or consider a metal catalyst (e.g., Au) supported in activated carbon. Then the scattering of only the selected element (Si, Au, respectively) can be extracted [242], Even the distribution of the elements in the material can be mapped based on ASAXS data. A concise review of the ASAXS method in combination with AXRD and AWAXS has been published by Goerigk et al. [243]. 

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. 

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. 

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). 

The oxocarbenium perchlorate C(CH20CH2CH2C0+C104 )4 was employed as a tetrafunctional initiator for the synthesis of PTHF 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 (PTHF-fc-PS)4 star-block copolymers. Further addition of MMA provided the (PTHF-fr-PS-fc-PMMA)4 star-block terpolymers. Relatively narrow molecular weight distributions were obtained with this synthetic procedure. 

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). 

PS-b-PEO) , n = 3, 4 star-block copolymers were synthesized by ATRP and anionic polymerization techniques [149]. Three- or four-arm PS stars were prepared using tri- or tetrafunctional benzylbromide initiators in the presence of CuBr/bipy. The polymerization was conducted in bulk at 110 °C. The end bromine groups were reacted with ethanolamine in order to generate the PS stars with hydroxyl end groups. These functions were then activated by DPMK to promote the polymerization of ethylene oxide and afford the desired well-defined products (Scheme 73). 

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... 

PS-fr-PBd) star-block copolymers were synthesized by the macromonomer technique in combination with anionic polymerization and ROMP [ 158], following the procedure outlined in Scheme 83. The macromonomers were prepared with two different methods. In the first the living diblock copolymer was reacted with ethylene oxide to reduce the nucleophihcity of the living end followed by termination with 5-carbonyl chloride bicycle (2.2.1) hept-2-ene, while in the second method the functional initiator 5-lithiomethyl bicycle... 

In order to probe the effect of junction point functionality on chain conformation and morphology of miktoarm star block copolymer architectures, a series of PI PS (n = 2, 4, 16) was synthesized [166]. A single batch of both living PS and PI arms have been used, in order to ensure that all chemically identical arms (either A or B) have the same molecular weights. The living A and B chains were reacted with the appropriate chlorosilane, under appropriate experimental conditions, to produce the corresponding //-stars, as shown in Scheme 88. 

Hyperbranched polymers have also been prepared via living anionic polymerization. The reaction of poly(4-methylstyrene)-fo-polystyrene lithium with a small amount of divinylbenzene, afforded a star-block copolymer with 4-methylstyrene units in the periphery [200]. The methyl groups were subsequently metalated with s-butyllithium/tetramethylethylenediamine. The produced anions initiated the polymerization of a-methylstyrene (Scheme 109). From the radius of gyration to hydrodynamic radius ratio (0.96-1.1) it was concluded that the second generation polymers behaved like soft spheres. 

The term star-block copolymer is used for a star architecture in which each arm is a diblock. The influence of chain topology on mechanical and morphological properties was investigated for copolymers composed of PS and PB with a constant styrene content of = 0.74 by Michler s group (Fig. 32) [101,102], While hexagonally packed cylinders of PB in a PS matrix were observed in a symmetric PS-fo-PB-fr-PS triblock copolymer, an L phase... 

Fig. 32 Schematic representation of molecular structure and morphology observed in PS-fo-PB-fc-PS linear and star-block copolymers. Oblique lines between blocks for LN2 and ST2 indicate tapered transition of dissimilar blocks. From [102], Copyright 2003 Wiley...

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