Hb polymers

Highly non-linear homopolymers can even be synthesized exclusively from monomers of functionality higher than 2. Dendrimers (from ancient Greek "SavSpov" — tree) are obtained when all the reaction sites of each branch links to another one and when all branches exhibit the same length. Whereas, hyperbranched (HB) polymers result from incomplete reaction of each multifunctional monomer, see Figure 9. 

Understanding of the sickling process and of the structure of the HbS polymer provides a rational basis for ways of correcting the molecular defect. Thus, dilution of the HbS in the red cells, blockage of the interaction of the y86 valine with the hydrophobic pocket, and decrease of... 

Structure of hemoglobin S (HbS) polymer. The valine at the j66 position of the deoxy-HbS fits into the hydrophobic pocket formed by leucine and phenylalanine at jS85 and j688 of an adjacent chain. Since each chain has an acceptor pocket and a donor valine, the HbS polymer has a double-stranded, half-staggered structure. [Reproduced with permission from S. Charache, Advances in the understanding of sickle cell anemia. Hasp. Pract. 21(2), 173 (1986). J. E. Zupko, illustrator.]... 

Microstructure is a key aspect for polymers in general, and for polyurethanes (PUs) in particular. The morphology of PUs is governed by the formation of hard and soft domains and their intercalation. Consequently, new microstructures can be developed using dendritic and hyperbranched (HB) polymers in PU systems. 

Two HB PAMAMs were tested as primers The first (AD-102) was chosen because primary amines react with both epoxies and PUs. The second (IB-100) has a lower functionality and lower MW (Table 15.1). For both HB polymers, seven different concentrations were used 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, and 5.0% w/w in ethanol. 

The durability of joints with adherends treated with optimum concentrations of the HB polymer primer and bonded with epoxy or PU adhesives was tested on wedge joints (ASTM D-3762) with aging conditions of 50°C/95% RH, for 1,... 

It can be seen that the maximum shear and peel strengths was obtained with HB polymer Epox AD-102 at a concentration of 1%. Higher concentrations resulted in decreased shear and peel strengths. The same result was obtained with HB Epox lB-100. 

Elevated temperatures accelerate the diffusion of the bulky HB polymer. Furthermore, the functionahty of the HB polymer plays a major role in the crossHnk density obtained. The lower MW polymer, Epox lB-100, has a lower functionahty than the higher MW one, Epox AD-102. 

The use of hyperbranches as primers or as crosslinking agents for the epoxy and PU adhesives resulted in improved shear strength and higher adhesion durability (wedge test results). The modification of PU and Epoxy systems with the hyperbranched or dendritic polymers was effective using small amounts. HB polymers were more effective than the dendrimers for shear strength improvement. 

Epoxy adhesive showed better improvement with all HB polymer and dendri-mer primers. 

Today, hb polymers are used and discussed widely for different applications, with a major use of these highly branched materials as reactive components in coating and resin formulations. In addition, potential applications as additive compounds in linear polymers, especially for improving not only rheological, flow and surface properties but also thermal stabihty and modulus, represent the main reasons for the development of hb macromolecules as specialty polymers. These application fields relate to the major features of hb polymers a highly branched, dense but irregular structure which leads to excellent solubility a low solution viscosity and a modified melt rheological behavior in combination with the option to introduce a wide variety of reactive end functionalities [6-11). 

Besides these rather well-documented applications, new fields have recently been explored for hb polymers. The special features of the dendritic structure on the nanometer scale, with the option of a specific confinement of functional units, the formation of cavities, and an interesting molecular dynamics, play significant roles for apphcations, for example, as thin films in sensorics and diagnostics, as porogens for nanofoams, or as carriers for additives, catalytic species, or... 

The majority of the structures is prepared from AB2 monomers by polycondensation, to result in hb polyesters, polyamides, polyethers, poly(ester amide)s, polysulfones, poly(ether ketone)s, polyphenylenes (among others), and increasingly also by polyaddition leading to, for example, poly(carbosilane)s, poly(urea urethane)s, polyarylenes, poly(ether amide)s or polythioethers, and many others [6-11, 13, 17, 21]. In particular, cycloaddition reactions offer the advantage of an often very selective and clean, high-yield reaction that is not influenced by special functionalities [33]. The relatively easy synthesis of the hb polyphenylenes described by Mullen et al. [34]. is an excellent example of this. In addition, certain cycloaddition reactions form as Hnear units nonstable intermediates, which allows the preparation of hb polymers without any linear units, which therefore exhibit formally a DB of 100% [35]. 

As a typical example, Frey etal. [16, 27] described the anionic polymerization of glycidol, which was considered also as a latent AB2 (= ABB ) monomer (Scheme 24.3). The polymerization proved to be very versatile and led to hb polymers with a rather narrow molar mass distribution (Mu,/M = 1.1-1.4) due to a chain growth-hke character of the reaction when only partial deprotonation to the initiating alkoxide (initiating site, triol in Scheme 24.3) was performed. This led to a more or less simultaneous growth of all chain ends, and allowed control of both the molar mass and polydispersity. By use of the trifunctional initiator (core molecule) and slow monomer addition, cychzation was suppressed such that the molar mass and polydispersity could be controlled. 

Another example is the reaction of a diepoxy-substituted phenol involving a proton-transfer mechanism to a hb polymer [49]. Here, an important feature - in order to achieve in this case a hb polyether without undesired propagation through the nucleophihc center of the secondary alkoxide - was that phenolate formation occurred significantly faster than the nucleophihc propagation step. In addition, the... 

The main approaches toward hb polymers in one-pot procedures with the potential of branching in each repeating units as outlined in Section 24.2, lead to complex, polymeric structures with multiple structural units within one molecule, a high number of functional end groups, an irregular, highly branched structure, high variations in molar masses, and often with an extremely broad polydispersity. In order to elucidate the success of the synthetic approach, and also to be able to validate and understand the observed materials properties, two major questions must be addressed (i) the verification of the chemical structure in question in as much detail as possible and (ii) the reliable determination of molar mass and polydispersity. 

The method of choice for elucidating chemical stmctures is NMR spectroscopy, supplemented by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS). In addition, a multitude of chromatographic and light-scattering methods provide access to the molecular weights and solution properties of these macromolecules. Based on these methods, a detailed understanding of the structure of hb polymers has been acquired since their discovery however, there remain a number of open questions that must be answered, mainly by method adaptation and development. 

Despite the large variety of chemical structures realized in hb polymers, they can be clearly classified according to the general monomer type or monomer combination used. Therefore, before tackhng the structure characterization of hb polymer it would be helpful to outline which structural units constitute the overall structure of the hb polymer. 

The ratio of these units in a hb polymer determines the DB, which is a fundamental parameter of branched polymers moreover, its extent exerts a tremendous influence on the physical and chemical properties of branched polymer materials. Thus, the determination and control of DB is an important issue. 

Following an initial report by Kim etal. [20, 54] of a branching factor for a hb polyphenylene, Frechet et al. [55] later deflned for the first time the DB for describing hb polymers. By adopting the structure of an ideally branched dendrimer possessing only dendritic and terminal units, it was possible to relate the sum of dendritic and terminal units to the total sum of all repeating units, by considering linear units as units which decreased the DB (Equation 24.1) ... 

Another approach to describe the branching in hb polymers has been proposed by Frey et al. [57] and Yan et al. [58]. In the concept of Frey et al. [57], the number of actual growth directions of the polymer chain (D) is compared to the maximum number of possible growth directions (D k), finally resulting in Eq. (24.2) for AB2 polymers. 

Several approaches to enhance the D B of hb polymers, including a slow monomer addition procedure without or with the use of core molecules of type By [60, 62,... 

Typically, H and NMR spectroscopy have been apphed when studying the structure of hb polymers. Although the proton spectrum is the easier to acquire, it is often less informative than the carbon spectrum. In the case of monomers based on amines, sihcon, or phosphorus, valuable information has been obtained from [78, 79], Si [76, 80-88], and NMR [89] spectra. The acquisition of a NMR spectrum should be considered if fluorine is present in the subunits [56,... 

The synthesis of model compounds mimicking the structural characteristics of the possible subunits, and the comparison of their spectra with that of the hb polymer, represents a common method for assigning signals to subunits. This procedure, for the assignment of urea and urethane carbonyl carbon signals of a hb poly(urea urethane) synthesized from an AA (2,4-toluylene diisocyanate) and B2 B (diethanol amine) monomer [94], is shown in Figure 24.3. 

The DB of branching can be modified by special synthetic approaches, as demonstrated by the NMR quantification of subunits. Copolymerization - for example, the addition of bifunctional monomers AB - resulted in an increase in linear units and, therefore, in a decrease of the DB [109-112]. An enhancement of DB was realized, for example, by employing a slow monomer addition technique [113], the polymerization of prefabricated dendron macromonomers [56, 114], and by a stepwise addition of the monomer mixture for the (A2 + B3) approach [92]. Whereas dendritic and terminal units are essential for a dendritic structure, in the case of hb polymers the content of the linear units can vary greatly. To date, few examples of AB2 hb polymerizations have been reported were the linear unit is a chemically labile structure that either breaks down to the initial educts, or reacts immediately with a further terminal unit to form the stabile dendritic unit. Thus, a hb polymer containing only T and D units is formed, with 100% DB [35,115-119]. 

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