Comb-grafted polymers

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

These inerts are mostly polymeric or oligomeric in nature and can be both nonionic and anionic. They have multiple binding / interaction sites to the interface, so that a displacement becomes improbable. Several materials have been made available to the industry [6d,ej. The chemistries range from sulfonic condensates (Morwet) or lignin sul-phonates (Reaxx) to EO-PO-block copolymers (Pluronics), comb graft polymers, acrylate based copolymers or various other block copolymers. 

Formation of hydrophobic clusters in a comb-grafted polymer gel leads to additional junction points and to an abrupt increase in the cross-Unking density (Yoshida et al. 1995). 

Additional examples of comb-grafted polymers via GTP have been reported by Heitz and Webster [43], Hertler et al. [44], and Jenkins et al. [45]. A ladder-lype polymer can be prepared fi-om GTP using a bis-methacrylate monomer and a diftmctional initiator using dilute solutions [46]. If a mono-ftmctional initiator is used, crosslinked polymer is obtained. The ladder polymer had a = 5100 and a = 1.89 and was fi-ee of gel (Scheme 8). The... 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

Anionic grafting methods (vide infra) can be applied to the synthesis of comb-shaped polymers. As an example, a polystyrene backbone is partially chloromethylated (under mild conditions) and used as an electrophilic deactivator for a living polystyrene 89). The grafting onto process yields well defined species that have been characterized accurately. The branches are distributed randomly along the backbone 90). 

The same procedure can be employed to make well defined comb-like polymers Living polystyrene can be grafted onto a partially chloromethylated polystyrene89 146), or onto a random copolymer of styrene and methyl methacrylate containing less than 10% of the latter monomer I48). 

Graft copolymers are comb-shaped polymers consisting of a backbone and two or more branches which differ chemically from the backbone. Branches are usually distributed randomly along the backbone, although recent advances in synthetic methods have allowed for the preparation of better-... 

Monomer Iniferter C=C-R-X > -C M) R-fM X Comb-like polymer, Graft copolymer... 

Note 1 The homopolymerization or copolymerization of a macromonomer yields a comb or graft polymer. 

Macromers have been used to produce thermoplastic elastomers. Generally, the backbone serves as the elastomeric phase while the branches serve as the hard phases. These structures are often referred to as comb -shaped because of the similarity between the rigid part of the comb and its teeth and the structure of these graft polymers. 

Special interest has been devoted in recent years to comb shaped polymers. This architecture consists of a backbone carrying p randomly distributed grafts of known length. 

The simultaneous hydrosilylation and ring-opening polymerization process provides a powerful method for the construction of a variety of novel polymer networks including comb, block and graft polymers, e.g. 272 (equation 102)271. 

The graft copolymers were already used for preparation and stabilization of polymer particles by Barrett [1]. He synthesized a poly(12-hydrostearic acid) macromonomer with a methacrylate end group. This macromonomer was copolymerized with MMA to obtain a preformed comb-graft copolymer, which was successfully used as stabilizer in nonaqueous dispersions of MMA. 

Benns, J.M., Choi, J.S., Mahato, R.I., Park, J.S. and Kim, S.W. (2000) pH-sensitive cationic polymer gene delivery vehicle V-Ac-poly(l- histidine)-graft-poly(l- lysine) comb shaped polymer. Bioconjug. Chem., 11, 637-645. 

A similar evaluation was made for the other series of CA-g-PHAs to establish a general relationship between their molecular architecture and thermal transition behavior [24]. Of particular interest is the finding that the composition dependence of the Tg of the cellulosic graft copolymers was represented well in terms of a formulation based on a comb-like polymer model [29], when CAs of acetyl DS 2 were employed as a trunk polymer. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

In the next section, therefore, we review recent studies of simpler cases, i.e., homopoly(macromonomers), star- and comb-shaped polymers, followed by some interesting properties of the graft copolymers to be used as polymeric surfactants, surface modifiers, and compatibilizers for blends. 

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