Copolymers, graft amphiphilic type

A polymer is considered to be a copolymer when more than one type of repeat unit is present within the chain. There are a variety of copolymers, depending on the relative placement of the different types of repeat units. These are broadly classified as random, block, graft, and alternating copolymers (see Fig. 2.1 for structural details Cheremisinoff 1997 Ravve 2000 Odian 2004). Among these stmctures, block copolymers have attracted particular attention, because of their versatility to form well-defined supramolecular assemblies. When a block copolymer contains two blocks (hydrophobic and hydrophilic), it is called an amphiphilic diblock copolymer. The immiscibility of the hydrophilic and lipophilic blocks in the polymers provides the ability to form a variety of assemblies, the stmctures and morphologies of which can be controlled by tuning the overall molecular weight and molar ratios of the different blocks (Alexandridis et al. 2000). 

Block copolymers that consist of hydrophilic and hydrophobic segments are typical amphiphilic polymers, a variety of which have been synthesized by living cationic polymerization. Figure 9 schematically illustrates the structures of some of these amphiphilic polymers thus far obtained though the examples therein are based on poly(vinyl ether) segments, any other appropriate segments may be incorporated. As seen in the illustrations, macromolecular amphiphiles are not necessarily linear AB- and ABA-type block copolymers but may be graft, multiarmed, and network polymers, where the basic components are amphiphilic block copolymers. 

Essential details of four graft copolymer samples (25a-25d) thus obtained are given in Table 11. The samples have been briefly examined by sedimentation equilibrium analysis. The molecular weights (M ) obtained by this method are consistent with, but lower than, the expected values recorded in Table 11. Another series of amphiphilic graft copolymers with a double comb structure has also been synthesized (26, in Fig. 19) [47]. Similarly, type 27 graft copolymers have been prepared by the reaction of the activated copolymer with hydroxy-terminated polyurethanes, but these have not so far been characterized. 

Siloxane amphiphilic copolymers are a class of silicone surfactants consisting of a methylated siloxane hydrophobe attached to one or more polar chains such as poly(oxyalkylene). The blocks in the copolymer can be arranged in a linear fashion (as in A-B-type block copolymer) or, as in graft copolymers, one type of blocks can be attached (usually randomly) to a linear, backbone chain of the other type of blocks. Siloxane amphiphiUc copolymers are of special interest since their hydrophile-hydrophobe balance can be easily tailored by changing the length of the corresponding blocks [26-28]. 

Amphiphilic copolymers have also been employed for the hydrophilization of PDMS surfaces. Two types of copolymers have been employed poly(ethylene oxide)-h/ock-poly(propylene oxide)-htock-poly(ethylene oxide) (PEO-h-PPO-h-PEO) and poly(L-lysine)-graft-poly(ethy-lene glycol) (PLL-g-PEG). PEO-h-PPO-I -PEO (Plutonic ) was a commercially available, standard material that was kindly supplied by BASE (Mt Olive, NJ, USA). The molecular weight of the PEG and PPO blocks of the PEO-I -PPO-h-PEO used in this work were ca. 3250 (1625 for each PEO side) and 3250 g/mol, respectively (denoted as P105 according to the manufacmrer [13,14]). The PEO-h-PPO-1)-PEO was dissolved in distilled water at 2 mg/ml concentration and used as an aqueous lubricant. Since PEO-h-PPO-h-PEO is well known to adsorb onto various hydrophobic surfaces through hydrophobic interactions between the PPO block and the surface, no treatment was carried out other than immersing the tribopair into the polymer solution (15 min prior to the friction measurements). 

Nomura and coworkers used a similar approach for the formation of micelles from NBE-based copolymer backbones [102]. In this work, however, a molybdenum catalyst (Mol) allowed the termination of the polymer chain with a silyl ether-modified aldehyde moiety which subsequently could be modified to be made amphiphilic by grafting-to the polymer terminus with a mesylated PEG chain (Figure 6.27). The resulting polymers formed either spherical or cylindrical micelles depending on whether the PEG chain was grafted to the copolymer backbone after one round of ROMP (linear ABA-type polymer) or after the second round of ROMP (brush-type polymer). 

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