Copolymers, star shape synthesis

In these two decades remarkable progress has been made in the development of excellent catalysts for living and stereospecific acetylene polymerizations (10,26-28). The r-conjugated polymers prepared by the sequential polsrmerization are strictly limited to polyacetylenes, except for only a few examples. Thus, synthesis of tailor-made conjugated macromolecules such as end-functionalized polymers, block copolymers, star-shaped polymers is possible only in the case of substituted acetylenes. 

Since there is no chain transfer involved, preparation through living polymer results in a narrow distribution of molecular weight. It is the best way to synthesize di- and triblock copolymers, star-shaped and comb-shaped polymers, and homopolymers with high molecular weight. The following is an example of the synthesis of di- and triblock copolymers ... 

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

Yijin X. and Caiyaun P., Block and star-hlock copolymers by mechanism transformation. 3. S-(PTHF-PSt)4 and S-(PTHF-PSt-PMMA)4 from living CROP to ATRP, Macromolecules, 33, 4750, 2000. Feldthusen J., Ivan B., and Mueller A.H.E., Synthesis of linear and star-shaped block copolymers of isobutylene and methacrylates hy combination of living cationic and anionic polymerizations. Macromolecules, 31, 578, 1998. 

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. 

In a different approach, Hedrick et al. have studied multifunctional dendritic initiators for the synthesis of multiarm star-shaped copolymers [102]. Several dendritic initiators with hydroxyl functionality ranging from 2 to 48 have been prepared according to the method developed by Hult et al. [120]. The bulk polymerization of sCL initiated by these multifunctional macroinitiators and acti-... 

Interestingly, Hedrick and coworkers reported a metal-free approach for the synthesis of star-shaped copolymers. They synthesized star-shaped PCLs by the ROP of eCL initiated by polyols in the presence of unencumbered N-heterocyclic carbenes [87]. 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

Inifer type reactions have been applied to a large number of cationic synthesis, yielding functional polymers 70 72), block copolymers 73), graft copolymers, and even star-shaped polymers73). As far as macromonomer synthesis is concerned three different methods have been used which are described below ... 

Synthesis of star-shaped copolymers by the arm-first method.. 865... 

Two general strategies are possible for the synthesis of star-shaped copolymers The arm-first method is based on the reaction of living chains with plurifunctional electrophiles carrying at least three reacting groups alternatively, polymerization can be initiated by a multifunctional initiator according to the core-first method. 

Star-shaped polymers have been of interest since the development of CRP methods. Matyjaszewski et al. showed that ATRP can be used to prepare star polymers [114], when a hexafunctional initiator was used for the ATRP of St using the CuCl/bpy catalyst system, resulting in a 6-arm star with a narrow molecular weight distribution. Hawker et al. reported the synthesis of star polymers using TEMPO-mediated polymerizations. They demonstrated that star polymers of pSt could be prepared using a trifunctional unimolecular TEMPO initiator [294]. Since then, many reports on star polymers with various types of copolymer arms have been reported. 

Ba-Gia H, Jerome R, Teyssie Ph. Star-shaped block copolymers. I. Synthesis of new A(B)2 star-shaped block copolymers based on vinyl or diene hydrocarbons (A) and oxirane (B). J Polym Sci Polym Chem Ed 1980 18 3483-3498. 

Smeets and co-workers reported the synthesis of amphiphilic HBP from the copolymerisation of a vinyl and divinyl monomers [11]. Grafting of HBP has also been reported by other researchers [12]. Hou and Yan reported the synthesis of a star-shaped copolymer using in situ grafting, which contained a hyperbranched poly(3-methyl-3-oxetanemethanol) core and tetrahydrofuran arms [13]. 

In the first case, the arms are grown from a single core with a given number of potentially active sites or a well-defined multifunctional initiator. In contrast to anionic multifunctional initiators, weU-defined soluble multifunctional cationic initiators are readily available. These multifunctional initiators with 3-8 initiating sites have been successfully applied for the synthesis of 3-8 arm star homo- and block copolymers of vinyl ethers, styrene and styrene derivatives, and IB. For example, six-arm star polystyrenes were prepared using initiator with six phenylethylchloride-type functions emanating from a central hexa-substituted benzene ring [250]. By subsequent end functionalization, a variety of end-functionaUzed A or (AB) (see above) star-shaped structures can also be obtained. 

Davaran, S., Rezaei, A., Ahmohammadi, S., Khandaghi, A.A., Nejati-Koshki, K., Nasrabadi, H.T., Akbarzadeh, A., 2014. Synthesis and physicochemical characterization of biodegradable star-shaped poly lactide-co-glycolide-p-cyclodextrin copolymer nanoparticles containing albumin. Advances in Nanoparticles 2014. 

Precision synthesis of microstructures in star-shaped copolymers of 8-caprolactone, LLA, and DXO was accomplished using a spirocyclic tin initiator and stannous(II) octoate (cocatalyst) together with pentaerythritol ethoxylate (coinitiator) [109]. 

Yuan et al. [165] reported the synthesis of well-defined dendrimer-star, block-comb polymers by the combination of living ring-opening polymerization and atom transfer radical polymerization on the basis of a dendrimer polyester. Star-shaped dendrimer PCLs were synthesized by the bulk polymerization of e-caprolactone with a dendrimer initiator. The dendrimer PCLs tiien were converted into macroinitiators via esterification with 2-bromopropionyl bromide. And the star-block copolymer dendrimer poly(s-caprolactone)-block-poly(2-hydroxyethyl... 

Figure 3.1 Synthesis of sequence-controlled pol3uners by living cationic polymerization. (A) Copolymers containing acid-degradable acetal units. (B) Block copolymers with an acid degradable segment. (C) and (D) Synthesis of star-shaped polymers with acid-degradable acetal units in the core.

S.Y Park, D.K. Han, S.C. Kim, Synthesis and characterization of star-shaped PLLA-PEO block copolymers with temperature-sensitive sol-gel transition behavior. Macromolecules, 34, 8821-4, 2001. 

Recent advances in controlled synthesis of macromolecules, in particular in coti-troUed radical polymerization, have allowed the cmistraction of polymers with various topologies and compositions. Successful design of star-shaped polymers, molecular polymer brashes, and amphiphilic block copolymers and terpolymers has... 

The norbornadiene dimer exo-trans-exo-pentacyc ote-tradeca-5,11-diene has been employed as a crosslinking agent for the controlled synthesis of these star-shaped copolymers in the catalytic presence of W(CHr-Bu)(NAr)(0/-Bu)2 or Mo(CHr-Bu)(NAr)Or-Bu)2 [66], Star polymers in which the arms are block copolymers of different polarities can be prepared by this method, as well as star polymers in which the composition of the arms is different, as are the solubility characteristics and stereochemical regularity. Heterostars thus obtained, in which the number of the different arms will be approximately equal, may exhibit interesting intramolecular phase separations. 

C. Synthesis of Star-Shaped Poly(e-caprolactone)-Poly(Ethylene Glycol) Block Copolymer PCL-PEG-FOL... 

Francis, R., Taton, D., Logan, J.L. et al. (2003) Synthesis and surface properties of amphiphilic star-shaped and dendrimer-like copolymers based on polystyrene core and polyfethylene oxide) corona. Macromolecules, 36,8253-8259. 

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