Star homopolymer

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 recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

Gi2> distance between the star center and the center of mass of the i homopolymer branch in a symmetric star homopolymer with the same number of branches as the miktoarm star... 

For the diblock copolymer, which exhibits a flow region at longer times than pure polyisoprene, the relaxation of the isoprene sequence is treated like the relaxation of the arm of a star polymer. We have followed the description proposed by McLeish [17, 18] for star homopolymers. The distribution of relaxation times is given by Eq. (9), where Mb is the molecular weight of one branch (here the molecular weight of the polyisoprene sequence), s ranges be-... 

In addition, dihlock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol. 

A series of star DHBCs were evaluated for their ability to transfect hmnan cervical HeLa cancer cells with the modified plasmid pRLSV40, bearing the enhanced green fluorescent protein as the reporter gene [55]. The copolymers utilized were composed of PDMAEMA and PHEGMA blocks (where PDMAEMA is an ionizable block, while PHEGMA is a non-ionic water soluble block). The experimental data indicate a decreased toxicity for the star copolymer, compared to a reference PDMAEMA star homopolymer, for the same amounts of star polymer tested. Moreover, it has been found that the architecture of the star copolymer, i.e. star block, miktoarm star etc, plays a decisive role on the transfection efficiency. The best performance, for all star copolymers tested, was observed for a star block copolymer with... 

The use of click chemistry has also influenced the construction of more sophisticated star polymers, such as those with block copolymer arms. Maty-jaszewski has eloquently demonstrated the preparation of three-arm star block copolymers by again combining ATRP with CuAAC click couphng [109]. In these studies the ATRP of styrene, starting from a trifunctional initiator, yielded the three-arm star homopolymer bearing bromide end groups that subsequently were transformed by substitution with sodium azide. CuAAC reaction with PEO-alkyne... 

Numerous studies have been reported for the synthesis of star homopolymers with different functionalities. However, more complex stmctures have appeared in the literature. The most important of them are presented as follows. 

One of the first works reported on the synthesis of stats using atom transfer radical polymerization (ATRP) was the preparation of 3-arm PMMA star homopolymers with a ttifunctional didilor-oacetate initiator using RuCl2(PPh3)3 in the presence of either A1 (OiPr)3 or Al(acac)3 (acac acetylacetonate) as catalyst. Both aromatic and aliphatic analogues of the initiators were examined. The stars were characterized by SEC and NMR spectroscopy. The polydispersity indices obtained were rather low (1.2-1.3). It was found that when AI(OiPr)3 was used, deviation from the theoretical molecular weight was observed. Reactions in the presence of Al(acac)s avoided this problem. NMR smdies determined that the more basic Al(OiPr)3 promoted a transestetification reaction with the initiator, which altered the acmal monomer-to-initiator ratio and poisoned the catalyst. 

The difunctional monomer methodology has been used for the synthesis of PS ° and poly(tert-butyl acrylate) (PtBuA) star homopolymers.The difunctional monomers used were divinyl benzene and 1,4-butanediol diacrylate,... 

Arm PS star homopolymers have also been synthesized using a dendritic compound with 12 terminal benzyl dithiobenzoate groups in bulk at 110 °C in the presence of AIBN (Scheme 38). The reaction was monitored by SEC-UV. It was found that conversions higher than 69% produced a bimodal distribution. The high-molecular-weight peak corresponded to the star molecule, whereas the other peak was attributed to terminate linear chains formed by irreversible terminations. After fractionation, the stars exhibited rather low PDIs (<1.2). 

Figure 12.15 Schematic representations showing the differences between various types of star (co)polymers (a) star homopolymers (b) star block copolymers (c) and (d) miktoarm star copolymers. Solid lines and dotted lines represent polymer chains differing in composition and/or molecular weight.

Georgiou TK, Vamvakaki M, Patrickios CS (2004) Nanoscopic cationic methacrylate star homopolymers synthesis by group transfer polymerization, characterization and evaluation as transfection reagents. Biomacromolecules 5 2221-2229... 

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