Polymers, hyperbranched

In fact, many systems can produce a hyperbranched topology. An ABj monomer will produce a hyperbranch, as will an A2B3 monomer. Yet, the A2B3 monomer also leads to a network (a highly cross-linked macromolecule). An alternative strategy is to react two different monomers, such as an A with a B2 or B3 monomer. Again, a hyperbranched system will result. 

What features can we expect from a hyperbranched polymer It will certainly not be monodisperse. This is a typical polymerization, and significant PDIs are expected. However, some of the desirable features of dendrimers are retained. For example, hyperbranched polymers tend to be quite soluble, an apparent consequence of their dendritic topology. This is much more evident for systems made from a single monomer. That is, the product from an AB3 polymerization wilt tend to be more soluble than that which results from the reaction of an A with a B3. In addition, for their molecular weight, the undesirable low viscosity associated with a dendrimer is retained, and hyperbranched polymers have poor mechanical properties. 

Many methods have been reported to synthesize hyperbranched polymers. These materials were first reported in the late 1980s and early 1990s by Odian and Tomalia [9], Kim and Webster [10], and Hawker and Frechet [11]. As early as 1952, Hory actually developed a model for the polymerization of AB -type monomers and the branched structures that would result, identified as random AB polycondensates [46], Condensation step-growth polymerization is likely the most commonly used approach however, it is not the only method reported for the synthesis of statistically branched dendritic polymers chain growth and ringopening polymerization methods have also been applied. 

Frdchet proposed to use the ratio of the number of fully branched monomer units to the total number of monomer units contained within the polymer N ) to describe the degree of structural perfection of hyperbranched polymers 

The relative amounts of each of the three types of monomer units can, in some cases, be determined by NMR analysis or by other spectroscopic methods. In situations where the units cannot be differentiated by these methods, selective labeling and/or degradation can be performed, followed by spectroscopic analysis. It should be noted that Equation 30.6 does not tend toward zero for linear polymers as it should. This discrepancy prompted the development 

In his original 1952 article [46], Hory also predicted the influence of branching on the DP and molecular weight dispersity, as shown in the following equation, which is derived from Equation 30.5  

It is apparent that the breadth of the MWD is highly dependent on the extent of reaction (conversion) attained in these reactions, as B increases with the conversion. At low conversions, the MWD for an AB system corresponds to a Flory distribution (B 2) however, B trends toward infinity as full conversion is approached. Eor trifunctional monomers, including equally reactive A3 monomers and ABC monomers with reactivity differentials, the MWD also depends on the DP but in a different way B is proportional to DP in an A3 system, while for an ABC system it is proportional to (DP) [49, 50]. 

Preparation of dendrimers requires a high degree of purity of the starting materials and high yields of the individual synthetic steps, all of which generally increases the effort involved. Polydisperse, hyperbranched compounds, which admittedly show defects yet often display properties similar to their ideally perfect dendritic relations, can readily be synthesised. 

Ihe literature describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites.  

Heavily branched polysiloxanes can be prepared by a variety of techniques, some of which generate hybrid mesostructures. Branched and linear chains have been compared with regard to their interactions with solvents.  

The partially soluble polymer hb- l (Table 1, entry 5) was decomposed by a similar procedure. 50.4mg of 15 and 36.5 mg of ohgomer of 1 were obtained from 125.8 mg of polymer. 

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A hyperbranched polymer 42 comprising oxadiazole subunits has been synthesized, but defect formation in such a structure appears to limit its use as a holeblocking material [74]. 

With appropriate choice of reaction conditions, hyperbranched polymers can be formed by sclf-condcnsing vinyl polymerization of monomers that additionally contain the appropriate initiator (NMP, ATRP), when the compounds are called inimers, or RAFT agent functionality. Monomers used in this process include 340,716 341717 and 34204 (for NMP), 108714,714 and 344 and related monomers720 723 (for ATRP) and 343408 (for RAFT). Careful control of reaction conditions is required to avoid network formation. 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch polymers. 

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

Assuming that no intramolecular or side reactions take place and that all groups are equireactive, the polydispersity index, 7P, of hyperbranched polymers obtained by step-growth polymerization of ABX monomers is given by Eq. (2.2), where pA is die conversion in A groups.196 Note that the classical Flory relationship DPn = 1/(1 — pa) holds for ABX monomer polymerizations ... 

The molar mass distribution of hyperbranched polymers is, therefore, always larger than diat of titeir linear homologues and tends toward infinity when conversion becomes close to 1. The use of a B3, comonomer, acting as a chain limiter and core molecule, helps in reducing polydispersity and controlling the molar mass of the final polymer.197... 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

AB2 reacts selectively with only one antagonist function of a second polyfunctional molecule, the other ones being protected81 (Fig. 5.16). The perfect hyperbranched molecules obtained according to that step-by-step process are called dendrimers. The degree of branching characterizes the structure of a hyperbranched polymer and has been defined by Hawker et al.82 as... 

Hyperbranched polymers generally have very low melt and indinsic viscosities. The large number of chain-end functional groups present in hyperbranched macromolecules have also been shown to dramatically affect physical properties... 

Recently, Fossum et al. prepared several phosphine-oxide-containing monomers (Scheme 6.23).163 These monomers were used to prepare hyperbranched polymers in a typical aromatic nucleophilic substitution. However, only oligomers with M lower than 2500 g/mol were obtained. These results did not surprise us, since our previous work demonstrated that the para-hydroxyl group of the phosphonyl group is not very reactive and would require higher reaction temperatures.11... 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Hyperbranched polyesters, 18, 32, 55-58 bulk synthesis of, 64 synthesis of, 114-118 Hyperbranched polyimides, 307-309 Hyperbranched polymers, 8-10, 348-350, 475-476, 481, 519-520 degree of branching in, 57 Hyperbranched polyphenylquinoxalines, 312-314... 

Fig. 21 —Friction force and topography images of the L-B films of Cgo-Pst hyperbranched polymer under load 2 nN using AFM/FFM, image (a) and (b) are friction force images and image (c) is the topography.

Diagram of friction versus load of multilayer L-B film of Pst hyperbranched polymer molecules (two layers). 

The molecular structure of a Cgo-Pst hyperbranched polymer can be described as in Fig. 20. Cgo-Pst hyperbranched polymer molecules were transferred onto mica chips by the vertical dipping method of L-B him techniques as introduced above. 

Figures 27-30 are diagrams of friction force versus load for different multilayer L-B hlms of the C5o-Pst hyperbranched polymer molecules obtained using AFM/FFM. The...

Hyperbranched Polymers Grafted from Planar Surfaces. 28... 

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