Exact graft copolymer

In 2000, Hadjichristidis et al. reported the first successful synthesis of an exact graft copolymer composed of a PI main chain and two PS branches by a stepwise iterative methodology [229]. This involved the addition of PILi to l,4-bis(phenylethenyl)benzene, in order to introduce the DPE moiety at the chain-end and create a subsequent linking reaction of PSLi with the resulting... 

More recently, the same group proposed a new methodology for the synthesis of exact graft copolymers composed of PMMA and five PS branches [231]. For this synthesis, a specially designed hving AB diblock copolymer (PS-b-PMMA), in-chain-functionahzed with a 3-tert-butyldimethylsilyloxymethylphenyl (SiOMP) group, is used as the building block unit (Scheme 5.23). In this case, three reaction steps are employed in an iterative synthetic sequence ... 

A series of well-defined exact graft copolymers with up to five PS branches was successfully synthesized by repeating the above synthetic sequence five times (Mn = 55.0 kgmor Mw/Mn = 1.04, [PS]/[PMMA] = 50/50). 

The complex architectural polymers that have been introduced in the preceding sections include multiblock copolymers, exact graft copolymers, high-density... 

Recently, Hirao et and Paraskeva and Hadjichristidis succeeded in the synthesis of exact graft copolymer with two, three, four, and five branches via a new iterative methodology based on living anionic polymerization. The reaction involves three steps site transformation, linking, and addition (Figure 19). 

Fig. 1. Graft Copolymers (1) random graft copolymer (identical branches randomly distributed along the backbone) (2) regular graft copolymer (identical branches equally spaced along the backbone) (3) simple graft copolymer (3-miktoarm star copolymer) and (4) graft copolymer with two trifunctional branch points. Exact graft copolymers.

Exact Graft Copolymers. The term exact graft copolymers refers to molecules where all the molecular and structural characteristics, such as the backbones and branches molecular weights and molecular weights distribution, the number of branches, and the specific grafting points on the backbone, can be controlled and varied at will. The parameters that are most difficult to control are the number and the spacing distribution of branches along the backbone chain. 

A new promising methodology was reported for the synthesis of exact graft copolymers with PI backbone and PS branches (132) (Fig. 11). The method is... 

Fig. 11. Synthesis of exact graft copolymers of styrene and isoprene.

The "comb" dispersing agent was a graft copolymer of polymethylmethacrylate-methacrylic acid (methoxypolyethylene oxide methacrylate) supplied by ICI Paints Division (Slough) and used as received. The exact molecular weight of the polymer is not known, but it is ejqpected to be in the region of 20-30,000 (as indicated by ICI Paints Division). The of the polyethylene chains was 750. 

The structure of a multi block copolymer does not seem suitable for the purpose. On the other hand, the structure of graft copolymers with short polysiloxane branches is considered to be the exact structure for the purpose, since one may maintain film forming properties, flexibility of slloxane chain (high permeation rate), and high selectivity. The flexibility of the slloxane chain can be maintained in side chains of graft copolymers and film forming properties and selectivity of permeation can be provided by backbone For this purpose, polystyrene was chosen as the backbone material... 

An alternate route to graft copolymers is by synthesis of macromolecules possessing exactly one polymerizable group in the chain and their subsequent polymerization, i.e. grafting through. Such reactive polymers are macromolecular monomers, i.e. Macromers as abbreviated by Milkovitch 159), who was the first to call the attention of the scientific community to the importance of this field although macromonomers have been prepared and copolymerized as early as 1962 160). 

Comparison of the data in Tables 14 and 19 of graft and block copolymers, respectively, based on methyl methacrylate and a mesogenic methacrylate confirm that block copolymers phase separate more easily than graft copolymers. Although not exactly comparable due to the different mesogenic methacrylates, the block copolymers phase separate at shorter block lengths than the graft copolymers. In addition, the distribu-... 

Figure 19 Synthesis of a comb-shaped PS with two branches. This can be extended to graft copolymers with exact spacing of side chains. Reproduced from Hirao, A. Watanabe, T. Kurokawa, R. Macromolecules 42, 3973. " ...

Exact graft copolsrmers having rather simple structures, ie one or two branches, have been prepared so far. Among these are the A2B and AA B single graft copolymers (121-128) and the H- and -shaped copolymers (129-131) (Fig. 10). Several techniques have been used for the ssmthesis of these structures and mainly anionic polymerization and controlled chlorosilane chemistry. 

Many important advances have been made in the nomenclature of polymer blends, grafts, blocks and IPN s. These include a nomenclature document recently published by the lUPAC Nomenclature Committee and one now under consideration. Briefly, two advances were made that relate to IPN s. The first was the use of the prefix cross- to indicate a crosslinked polymer. Thus, cross-poly-butadiene is distinguished from the linear product, written polybutadiene. The second advance was the introduction of the symbol -inter-, which means interpenetrating. Thus, cross-poly-(ethyl acrylate)-mtcr-cross-polystyrene (1) represents the IPN based on poly(ethyl acrylate) and polystyrene. The symbol -inter- has exactly the equivalent meaning as -block- and -graft- possess for block and graft copolymers, respectively. 

Although the exact structure of the copolymer is not known, it is convenient to consider that the reaction leads to a crosslinked copolymer. This is based on the assumption that the acrylate epoxide groups are truly grafted onto the PP chains and are not all at terminal sites. The latter structure could result from PP decomposition in the presence of radical initiator, followed by trapping the terminal radical by monomeric acrylate epoxide. 

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