Branched block copolymer

Polymerization of styrene or methyl methacrylate by macroazoinimers having two vinyl groups (MIM-2v) resulted in crosslinked block copolymers, while macroazoinimers with one vinyl end (MIM-1 v) group to polymerize vinyl monomers yielded branched block copolymers. 

Solprene Branched block copolymer of styrene and butadiene Phillips Petroleum... 

To obtain improved ionic mobility, and thus high conductivity, alternative polymer structures have been developed, for example comb-branched block copolymers such as poly[bis(methoxy ethoxy ethoxide)], usually known as MEEP. Room temperature conductivities of MEEP-based polymer electrolytes of the order of 10-5 S/cm have been achieved, values... 

Fig. 60. Representation of a branched block-copolymer as a full rooted tree left) and as a reduced rooted tree (rightf5 ...

Equation (E.42) to (E.44) have the great advantage that all properties of the homoblock can be calculated separately, and the properties of the branched block-copolymer are obtained by using the reduced transition matrix P which contains only the coupling probabilities. Note that these coupling appear only as off-diagonal elements. 

ABA tribiock, or all three can be different, as in an ABC triblock copolymer. Obviously, the number of possible block sequences increases rapidly with the number of blocks and the number of different types of block in the chain. One can also synthesize block copolymers with branched architecture, such as star-branched block copolymers, in which each of the arms of the star contains either the same or different block sequences (see Fig. 13-1). One or more of the blocks could also be stiff or liquid crystalline (Chiellini et al. 1994 Chen et al. 1996 Radzilowski et al. 1997 Jenekhe and Chen 1998). For a given type of block copolymer, the degree of polymerization N of the whole molecule, or the degree of polymerization Ni of one or more of the blocks, can be varied. Thus, the number of different types of block copolymers is practically endless. 

Block copolymers may also be possible by fadle, cost effective routes and they will contain some compositional heterogeneity. Taking a triblock copolymer as an example, this compositional heterogeneity may result from some block polydispersity (1.2-1.5) or minor fractions of non-targeted molecules such as homopolymer, diblocks or tetrablocks. Branched block copolymers could also be possible, where non-linear architeaures are produced with... 

Figure 8 Schematic representation of (a) branched-block copolymers which could be available by statisticallpragmatic modification of free-radical polymerisation. (b) represents a controlled structure available from living free-radical polymerisation...

Pistel KF, Bittner B, Koll H, Winter G, Kissel T. Biodegradable recombinant human erythropoietin loaded microspheres prepared from linear and star-branched block copolymers Influence of encapsulation technique and polymer composition on particle characteristics. J Control Release 1999 59 309-325. 

Graft copolymers A nonlinear or branched block copolymer is formed by attaching chains of one monomer to the main chain of another homopolymer. 

The symbol -g- may be employed, as in poly(butadiene-g-styrene). The first polymer mentioned forms the backbone chain, and the second the branches. Block copolymers of the type... 

Liu and coworkers [52] have described the synthesis of two different types of branched block copolymers, namely A2BA2 and A4BA4. In both cases, the B block was poly(propylene oxide) (PPO) while the A block was PDEAEMA. The synthesis was realized via ATRP using a commercially available diamine terminated PPO as the backbone of the copolymer. The initial PPO central block was functionalized in order to introduce multi-ATRP initiator fimctionalities at both fiee ends of the polymer. Subsequently the polymerization of DEAEMA monomer was performed following typical ATRP procedures. The followed synthesis scheme is shown in Figure 7. 

Thermoplastic graft copolymers can be considered as branched block copolymers with many branch points. A graft copolymer of isobutylene on poly (ethylene) and a graft copolymer of styrene/acrylonitrile on acrylic rubber is used in industry. 

FIGURE 3.8 Illustration of different types of copolymers. Sections of polymer chains are shown and each circle represents a repeat unit, (a) Alternating copolymer, (b) random copolymer, (c) block copolymer, and (d) branched block copolymer. 

The morphology and structural architecture of polymers has significant effect on drug release. The release rates of 5-FU and paclitaxel, widely used chemotherapeutics, have been investigated in di-, tri-, and four-arm (star-branched) block copolymers of PLA and PEO. Micellar aggregates were prepared from these block copolymers and release rates were studied over 3 weeks. More complete dmg release was observed in star-shaped polymers [154]. 

Li and Kissel [195] prepared linear and 4-arm/8-arm branched block copolymers of LLA and GA coinitiated with branched PEO and showed that the branching affected the NHD as traced by weight loss and molecular weight change. 

Fig. 10.1 Nomenclature for linear and branched block copolymer architectures of diblock and...

Holappa, S., Raresoja, M., Shan, J. and Tenhu, H. (2002) Solution properties of linear and branched block copolymers consisting of acidic and PEO blocks. Macromolecules, 35, 4733-4738. 

Riess and coworkers [214, 215] have also developed a unique method to synthesize heteroarm, ABC-type, three-armed, star-branched block copolymers as shown in Scheme 30. Poly(styryl)potassium was reacted with a tert-butyldimethylsiloxy-protected, hydroxyethylated 1,1-diphenylethylene (85) to form a siloxy-functionalized polymeric 1,1-diphenylalkylpotassium (86), that was then used to initiate polymerization of ethylene oxide as shown in Scheme30 [214]. Samples of the diblock copolymers, 87, exhibited rather broad molecular weight distributions (Mw/M =1.15-1.16). After removal of the protecting group with tetrabutylammonium fluoride the corresponding... 

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