Dendritic copolymer

Keywords polysiloxanes, cyclotrisiloxanes, anionic polymerization siloxane copolymers, block copolymers, dendritic polymers... 

Among the many unusual properties that the arborescent architecture leads to, most notable is the discovery that block copolymers with a high MW dendritic (arborescent) polyisobutylene core and poly(para-methylstyrene) end blocks can manifest themselves either as a rubber, or as a plastic, depending on their environment (Figures 7.16 and 7.17). The behavior is thermally irreversible. 

Tian, L. and Hammond, P.T. Comb-dendritic block copolymers as tree-shaped macromolecular amphi-philes for nanoparticle self-assembly, Chem. Mater., 18, 3976, 2006. 

Puskas, J.E. Dendritic (arborescent) pol3tisobutylene-polystyrene block copolymers DMTA analysis and swelling studies, Polym. Mater. Sci. Eng., 91, 875-876, 2004. 

Puskas, J.E. BiocompatibiUty studies of novel dendritic polyisobutylene-based block copolymers, Polym. Prepr., 45, 412- 13, 2004. 

Polymers and copolymers are among the most beneficial materials produced by synthetic chemistry. The invention and commercialization of new polymeric materials with radical new properties provides an opportunity to monopolize the market and justify the expense involved in the research and development. The commercialization of new polymers or copolymers always presents scale-up and design challenges. Scientists have recently developed new polymeric materials whose commercial impact has yet to be realized. Examples are semiconductive and conductive polymers and amphiphilic dendritic block copolymers. Other promising materials, such as polymers for (targeted) drug delivery and... 

More recently Frechet and Gitsov [130] used a similar approach as above and synthesized a novel series of dendritic copolymers derived from a central penta-erythritol core unit. These hybrid star molecules behaved as unimolecular micelles with different core-shell conformational-structures as a response to the polarity of the solvent used. 

Polystyrene/polyethylene oxide dendrimers were prepared by ATRP using tri- and tetra (bromomethyl) benzene as the initiators [207]. Each bromine end-group of the resulting stars was transformed first to two - OH groups and subsequently to potassium alcholate, as shown in Scheme 114. These - OK sites served to initiate the anionic polymerization of EO. The synthesized dendritic copolymers were found to display monomodal and narrow molecular weight distribution. 

E. Femandez-Megia, J. Correa, and R. Riguera, Clickable PEG-dendritic block copolymers, Biomacromolecules, 7 (2006) 3104-3111. 

Figure 3.3 Top row H-polymer and super H-polymer bottom row 7r-block copolymer and dendritic polymer...

Hybridization of Architectural States Dendritic-linear Copolymer Hybrids... 

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

Regardless of the desired architecture, there are three main synthetic approaches to the preparation of diblock, dumbbell or dendronized hybrid copolymers. These involve grafting, polymerization or stepwise dendritic growth... 

The properties of the hybrid diblock structures can be altered drastically by simply taking advantage of the high terminal functionality of the dendritic block. For example unusual diblock structures useful for the modification of surfaces have been prepared by ATRP of polystyrene (PS) initiated from the benzylic halide focal point of Frechet-type dendrons with terminal isophthalate ester groups [9b], Well-defined copolymers with narrow molecular weight distributions were obtained and excellent agreement was observed between calculated... 

JH NMR) studies confirmed the presence of the AB diblocks in the product. This determination was facilitated by the fact that the dendritic nitroxide could be differentiated from the nonnitroxide-bearing dendron by NMR spectroscopy. Careful analytical studies confirmed that the pure ABA copolymers could be separated by column chromatography and that the undesired diblock impurity resulted mainly from the loss of the dendritic nitroxide during the course of the reaction. Obviously, this approach to ABA triblocks has rather limited practical value since the thermal stability of the final product is quite low. 

The concept of making brush-type polymers in which a linear polymer is funtionalized with dendritic side-chains was first suggested by Tomalia in a 1987 patent, though actual experimental work on his approach was only reported recently recently [15]. Hawker and Frechet were first to document the preparation of a vinyl copolymer containing a few pendant Frechet-type dendrons (Figure 7.9). 

These were obtained by copolymerization of styrenic macromonomers containing dendritic pendant groups with styrene [16a] with a later extension to methacrylate-type copolymers [16b]. As these studies were carried out with... 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

In order to explore the properties that may be obtained by hybridizing the linear and dendritic architectural states, both diblock and triblock copolymer... 

As this brief overview demonstrates, novel copolymers obtained by hybridization of the linear and globular architectural states are readily prepared through a variety of synthetic approaches. In general the dendritic components of the hybrid copolymers are well defined, with unique molecular and structural characteristics. In contrast, all the linear components prepared polymerization are less precisely defined and are polydisperse. Only the very short linear components, themselves prepared by stepwise synthesis just like the dendrons, are monodisperse and can be used to prepare well-defined, monodisperse hybrids. While architectural and structural precision may be of great importance for the determination of ultimate properties, some degree of structural variation is quite acceptable for practical applications in many areas including, for example, surface modification, sensing, or encapsulated delivery. 

Controlling the size, shape and ordering of synthetic organic materials at the macromolecular and supramolecular levels is an important objective in chemistry. Such control may be used to improve specific advanced material properties. Initial efforts to control dendrimer shapes involved the use of appropriately shaped core templates upon which to amplify dendritic shells to produce either dendrimer spheroids or cylinders (rods). The first examples of covalent dendrimer rods were reported by Tomalia et al. [43] and Schluter et al. [44], These examples involved the reiterative growth of dendritic shells around a preformed linear polymeric backbone or the polymerization of a dendronized monomer to produce cylinders possessing substantial aspect ratios (i.e. 15-100) as observed by TEM and AFM. These architectural copolymers consisting of linear random... 

In this section the emphasis will be on assemblies prepared from amphiphilic dendrimers. Amphiphilic dendrimers, carrying both hydrophobic and hydrophilic regions within one molecule, tend to self-assemble into a large variety of different aggregates depending on their structure. The dendritic amphiphiles investigated so far include unimolecular micelles, bolaamphiphiles, superam-phiphiles and various other AB and ABA block copolymers. 

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