Copolymer , graft ideal

Enzymes are perfectly equipped to convert substrates into products in high enantio-, regio-, or chemoselectivity, a property that is commonly used in industry to prepare optically active fine-chemical intermediates [5]. More specifically, lipases appeared as ideal catalysts as a result of their high enantioselectivity, broad substrate scope and stability. In addition, lipases are powerful catalysts for the preparation of polyesters, polycarbonates and even polyamides, as is reviewed in Chapters 4 and 5 of this book. Moreover, a variety of different polymer architectures such as block copolymers, graft copolymers etc have been prepared using lipases as the catalyst (see Chapter 12). 

Because of the incompatibility (i.e. non-miscibility) of the various segments, the interchange forces rearrangement into blocks. These block copolymers often have more desirable properties than the parent random copolymers. An idealized structure for a graft copolymer is... 

The results of several copolymerization experiments with 7 are given in Table 3.8, from which it is clear that the assumption of similarity between 7 and benzyl methacrylate is reasonable. From Fig. 3.4. it is predicted that both MMA and MAN should copolymerize with 7 almost ideally , whereas styrene will deviate considerably from ideality . These predictions are verified by the results in Table 3.8. If the azo monomer is incorporated into the polymer in the same proportion as it is present in the initial monomer mixture, then it is possible to convert the, relatively valuable, azo monomer essentially 100% into polymer without changing the composition of the copolymer with conversion an important consideration for the technical utilisation of such products as the starting materials for graft copolymers. If the poly-... 

Multiarm star polymers have recently emerged as ideal model polymer-colloids, with properties interpolating between those of polymers and hard spheres [62-64]. They are representatives of a large class of soft colloids encompassing grafted particles and block copolymer micelles. Star polymers consist of f polymer chains attached to a solid core, which plays the role of a topological constraint (Fig. Ic). When fire functionality f is large, stars are virtually spherical objects, and for f = oo the hard sphere limit is recovered. A considerable literature describes the synthesis, structure, and dynamics of star polymers both in melt and in solution (for a review see [2]). 

The graft copolymers (25-26) have well-defined hydrophobic-hydrophilic regions, and are therefore ideally suitable for surface modification (coating) in aqueous media. Table 12 shows the result of coating of clean polystyrene latex particles (nanospheres) by the graft copolymers (25a-25d) in aqueous solutioa The coat layer thickness measured by Photon Correlation Spectroscopy (PCS) and Laser Doppler Anemometry (LDA) are in good agreement. They also correlate well with the zeta potential data [47]. 

FIGURE 4.42 Ideal location of block and graft copolymers at the interface between polymer phases A and B in a polyblend. 

This technique is probably one of the simplest ways to form graft copolymers. It consists of carrying out free-radical polymerizations of monomers in the presence of polymers preformed from different monomers. A prerequisite for this synthesis is that the active sites must form on the polymeric backbones during the course of the reactions. Ideally, this occurs if the steps of initiations consist only of attacks by the initiating radicals on the backbones ... 

Gmix is positive and the interaction is repulsive. The G,tux value increases very rapidly with a decrease in h when the latter is less than 18. This explains whey graft copolymers, such as Atlox 4913 or Hypermer CG-6(a) are ideal for the stabilization of suspensions in aqueous media. For the stabilization of dispersions in non-aqueous media, such as water-in-oil (W/0) emulsions, the stabilizing chain has to be soluble in the oil phase (normally a hydrocarbon oil). In this case, poly(hydroxystearic acid) (PHS) chains are ideal. An ABA block of PHS-PEO-PHS (Arlacel PI35, from UNIQEMA) is ideal for the stabilization of W/0 emulsions. 

Classical synthetic polyesters such as poly(E-caprolactone) (PCL), poly(L-lactic acid) (PLLA), poly(glycolic acid) (PGA), and their copolymers can provide the necessary strength for structural stability, and their scaffolds have been explored for the regeneration of blood vessel substitutes [51-53]. However, they are relatively stiff, nonelastomeric materials and not ideally suited for engineering of soft flexible tissues such as vascular grafts. 

---