Star polymers styrene

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Star polymers are a class of polymers with interesting rheological and physical properties. The tetra-functionalized adamantane cores (adamantyls) have been employed as initiators in the atom transfer radical polymerization (ATRP) method applied to styrene and various acrylate monomers (see Fig. 21). 

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). 

The deliberate introduction of multifunctional branching into anionically prepared polydiene and poly (diene-co-styrene) polymers produces materials with unique morphological and viscoelastic properties (1-3). Work has included synthesis of symmetric star polymers produced by reaction of living polyanionic "arms" with multi-functional chlorosilane (4-9),... 

Compounds containing two or more carbon-carbon double bonds also act as coupling agents and also as multifunctional initiators [Hadjichristidis et al., 2001 Quirk et al., 2000]. Such compounds can also be used to synthesize multifunctional initiators that subsequently produce star polymers. Consider l,3,5-tris(l-phenylethenyl)benzene (XL). Reaction with r-butyllithium produces a trifunctional initiator XLI, which initiates polymerization of a monomer such as styrene to form a 3-arm star polystyrene [Quirk and Tsai, 1998]. The 3-arm... 

The synthesis of extremely high molecular weight star polymers has been achieved by another method. First divlnyl-benzene is reacted at very low concentration with an anionic Initiator (sec. BuLi) to yield a suspension of poly-DVB nodules fitted with numerous initiating sites. Then styrene Is added, and each Initiating site should give yield to a branch. However, the polydispersity of the samples obtained is very high. 

Preparation of star polymers of p-methoxystyrene (p-MOS) and p-tert-butoxy-styrene (fBOS) using two different bifunctional vinyl compounds 1 and 5 was reported by Deng et al. [5]. 

Asymmetric PS stars of the type (PSA)n(PSB)n were also prepared by the divinyl-benzene (DVB) method [9]. Living PS chains, prepared by sec-BuLi initiation, were reacted with a small amount of DVB producing star homopolymers. The DVB core of the stars contains active anions which, if no accidental deactivation occurs, are equal to the number of the arms that have been linked to this core. These active sites are available for the polymerization of an additional quantity of monomer. Consequently further addition of styrene produced asymmetric star polymers... 

The exterior of carbosilane dendrimers can also serve as initiator sites for polymerization, in which case the dendrimers serve as cores for star polymers. Vasilenko and coworkers95 synthesized a multilithiated carbosilane dendrimer which they then used as an initiator for the ring-opening polymerization of hexamethylcyclotrisiloxane to yield star polymers of narrow weight distribution (Scheme 7). The researchers later extended this work to polymerize styrene, isoprene and ethylene oxide96,97. Star poly(ethylene oxides) were also prepared using hydroxy-terminated carbosilane dendrimers as the core98,99. 

Functional groups can also be introduced in the spacer units. Bifunctional initiators with bipyridine units such as MI-17 and MI-18 induced the living radical polymerizations of styrene and MMA, respectively, with copper catalysts to give polymers that carry a coordination site at the middle of the chain.87,333 These polymers can be connected together into star polymers with a ruthenium cation at the core, where the arm numbers are varied among three, four, five, and six in combination with the polymers obtained from the monofunctional initiator with a bipyridine unit (FI-21 and FI-22 Figure 13).416 A... 

Haloester-type trifunctional initiators are obtained from triols by a method similar to those for bifunctional haloesters. 3-arm star polymers of MMA are obtained with dichloroacetates MI-28 and MI-29, for which Ru-1 and Al(acac)3 are employed.228 The polymers have controlled molecular weights and narrow MWDs. Similarly, MI-30 and MI-31 with copper catalysts gave 3-arm star polymers of styrene, acrylates, and methacrylates suitable copper catalysts vary with each monomer.199,326 358 368 The obtained star polymers can be further transformed into star block copolymers comprised of hydrophilic/hydropho-bic368 or organic/inorganic326 segments by block copolymerizations of other monomers. 

The star polymers obtained with the bromoester-type calixarene-based initiators were analyzed by SEC equipped with a multiangle laser light scattering (MALLS) detector. The arm numbers were well controlled (close to the initiator s functionality), although the octafunctional initiator MI-51 induced star—star coupling in the styrene-polymerization at conversions higher than 20%.421 A similar series of tetra-, hexa-, and octafunctionalized initiators with calixarene cores were synthesized for sulfonyl chloride versions (MI-41, MI-48, and MI-52) and employed for copper-catalyzed MMA polymerizations.343... 

The self-condensing copper-catalyzed polymerization of macromonomer of poly(tBA) with a reactive C—Br bond (H-6) affords hyperbranched or highly branched poly(tBA).447 Copolymerization of H-1 and TV-cyclohexylmaleimide induced alternating and self-condensing vinyl polymerization.448 The residual C—Cl bond was further employed for the copper-catalyzed radical homopolymerization of styrene to give star polymers with hyperbranched structures. Hyperbranched polymers of H-1 further serve as a complex multifunctionalized macroinitiator for the copper-catalyzed polymerization of a functional monomer with polar chromophores to yield possible second-order nonlinear optical materials.325... 

An acetal-protected lithium initiator was used to polymerize styrene followed by linking with 1,3,5-triallyloxy-2,4,6-triazine to produce three-arm star polystyrenes.73 The protective acetal group was cleaved by weak acidic treatment in THF to give star polymers with terminal OH groups. These functional groups were coupled with toluene-2,4-diisocyanate to give randomly cross-linked products. Unfortunately, few characterization data were provided in this study. 

Using this method asymmetric stars of the type (PSa) (PSb)/7 were prepared.77 Living PS chains were obtained by s-BuLi initiation and reacted with a small amount of DVB to give a living star polymer. The anionic sites of the star core were subsequently used to initiate the polymerization of a new quantity of styrene. This initiation step was accelerated by the addition of a small quantity of THF. It was revealed by SEC analysis that high molecular weight species were also present, probably due to the formation of linked stars. These structures can be obtained when living anionic branches of one star react with the residual double bonds of the DVB-linked core of another star. 

These theoretical predictions are in good agreement with experiments [142] on the coUapse transition in dilute solutions of organosoluble star polymers, i.e., poly(styrene) stars in cyclohexane. In these experiments, the temperature was varied around the theta-point (ca. 34.5°C). Lowering the temperature corresponds to an inferior solvent strength of cyclohexane. 

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