Polymers, linear/hyperbranched

Fig. 7.1 Schematic representation of linear versus dendritic polymers linear (left) and hyperbranched (middle) polymers, perfect dendrimer (right). The amount of terminal groups is indicated below each structure. These architectures can also be attached to a cross-linked polymer bead to obtain a high-loading hybrid material.

Decomplexation of the P(III) moiety of the boronhydride complex (10) by the base l,4-diazabicyclo[2.2.2]octane (DABCO) followed by an internal Staudin-ger reaction has been reported to yield the linear polymer (12). Hyperbranched polymers are obtained by using the same procedure but starting from the diphosphine (13). Treatment of (11) with NaNs leads to an intramolecular Staudinger polycondensation affording polymer (12) as well. Compound (13) has proven to be a useful starting material for the synthesis of dendrimers with phosphine end groups. ... 

Gelation processes, such as crosslinking linear chains or condensation of /-functional monomers A/ (where A reacts with A) with /> 2 are quite different from either linear condensation polymers or hyperbranched polymers. Linear condensation polymers (made from AB monomers, -where A only reacts with B) and hyperbranched polymers (made from... 

Linear, Hyperbranched Polymers and Dendrimers Constituted by the Same Repeating Unit... 

Fig. 4 Polymer architectures (a) linear polymers, (b) ring polymers, (c-f) branched polymers (c) graft polymers, (d) star-shaped polymers, (e) hyperbranched polymers, and (1) dendrimers...

For the synthesis of linear hyperbranched block copolymers an analogy may be drawn to the linear dendrimer block copolymers. In a pioneering paper, the group of Kricheldorf suggested the use of terminally functional linear polymers as monoftmctional terminating agents or macro-cores for the synthesis of linear hyperbranched block copolymers. However, since a major side reaction is the well-known cydiza-tion of the focal A-group in the synthesis of hb polymers by polycondensation (see above), preparation of a block copolymer with narrow polydispersity cannot be achieved in this manner. 

Samperi, F., Battiato, S., Puglisi, C., Scamporrino, A., Ambrogi, V., et al. (2011) Combined techniques for the characterization of linear-hyperbranched hybrid poly (Butylene Adipate) copolymers. J. Polym. Science Part a-Polym. Chem., 49, 3615-3630. 

Tang and coworkers have reported that pyrolyzed hyperbranched polyferro-cenylsilanes have greater ceramic yields than their linear polymeric counterparts. Manners has reported that thermally crosslinked polyferrocenylsilanes (28) possessed greater thermal stability than their linear analogs. The swelling properties of these crosslinked polymers were examined, and the solubility parameter of the corresponding linear homopolymer was determined. The pyrolysis of linear, hyperbranched, and crosslinked polyferrocenylsilanes has resulted in the production of ceramics that possess magnetic properties. " ... 

The spherical architectures of highly branched macromolecules, such as dendrimers, star-shaped polymers, and hyperbranched polymers, have attracted much attention from the viewpoint of nanotechnology, because their numerous terminal units can be converted into various functional groups leading to novel nanomaterials (Zeng and Zimmerman, 1997 Hirao etal., 2007 Satoh, 2009). Thus, various types of dendrimers, star polymers, and hyperbranched polymers have been synthesized and their properties were compared to the linear analogues (Stiriba et al., 2002 Hirao et al., 2005). 

While A-B-A with composed alkylene dioxythiophene monomeric A units can be polymerized oxidatively to the corresponding polymer, linear (AB) and star branched polymers have to be made by organometallic reactions due to a low reactivity of the B unit. (AB) polymers with EDOT as the A unit and phenylene (Vlf) as well as fluorene (Vlg) structures as the B unit were synthesized by Suzuki polycondensation and show electrochromic behavior without any conspicuous properties. On the other hand, hyperbranched polymers with an additional triarylaminic core unit and similar composed branches made from 3,4-ethylenedioxythiophene and didodecyloxybenzene structures exhibited multicolor electrochromism to produce the three basic colors in the RGB system red, green, and blue. 

Baek JB, Tan LS (2003) Linear-hyperbranched copolymerization as a tool to modulate thermal properties and crystallinity of a para-poly(ether-ketone). Polymer 44 3451-3459... 

Possum E, Tan LS (2005) Geometrical influence of AB monomer structure on the thermal properties of linear-hyperbranched ether-ketone copolymers prepared via an AB + AB route. Polymer 46 9686-9693... 

Figure 4.2 Glycopolymer-based gene delivery vectors a) Linear polymer b) Hyperbranched polymer c) NP d) CNT and e) Nanogels. EDC l-ethyl-3-(3-dimethylaminopropyl) carbodiimide NHS N-hydroxysuccinimide P(APMA- -LAEMA) poly(3-aminopropyl methacrylamide-block-2-lactobionamidoethyl methacrylamide) and SWCNT single-walled carbon nanotube(s).

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. 

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). 

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... 

Figure 5.17 (a) Linear, (b) hyperbranched, and (c) dendritic aromatic polymers. 

Hyperbranched and dendronized polymers such as 40, 41, and 42 have also been synthesized using the transition metal coupling strategies in recent years.32 These polymers are fundamentally different from those traditional linear polymers. They possess dendritic arms within die polymer or along the polymer backbone. It is believed that they possess interesting properties and have potential applications in many fields such as nanotechnology and catalysis ... 

The DB obtainable in SCVP is DB=0.465 for r=kjk =l and reaches its maximum, DB=0.5, for r=2.6 [70,78]. This value is identical to that obtained in AB2 polycondensation when both B functions have the same reactivity [70,78]. Thus, hyperbranched polymers prepared by bulk polycondensation or polymerization contain at least 50% linear units, making this approach less efficient than the synthesis of dendrimers. 

Hyperbranched polymers are generally composed of branched (dendritic), Hn-ear, and terminal units. In contrast to AB2 systems, there are two different types of linear units in SCVP one resembles a repeat unit of a polycondensate (----A -b----) and one a monomer unit of a vinyl polymer (--a(B )---). 

Relationships between dilute solution viscosity and MW have been determined for many hyperbranched systems and the Mark-Houwink constant typically varies between 0.5 and 0.2, depending on the DB. In contrast, the exponent is typically in the region of 0.6-0.8 for linear homopolymers in a good solvent with a random coil conformation. The contraction factors [84], g=< g >branched/ <-Rg >iinear. =[ l]branched/[ l]iinear. are another Way of cxprcssing the compact structure of branched polymers. Experimentally, g is computed from the intrinsic viscosity ratio at constant MW. The contraction factor can be expressed as the averaged value over the MWD or as a continuous fraction of MW. 

Hyperbranched poly(ethyl methacrylate)s prepared by the photo-initiated radical polymerization of the inimer 13 were characterized by GPC with a lightscattering detector [51]. The hydrodynamic volume and radius of gyration (i g) of the resulting hyperbranched polymers were determined by DLS and SAXS, respectively. The ratios of Rg/R are in the range of 0.75-0.84, which are comparable to the value of hard spheres (0.775) and significantly lower than that of the linear unperturbed polymer coils (1.25-1.37). The compact nature of the hyperbranched poly(ethyl methacrylate)s is demonstrated by solution properties which are different from those of the linear analogs. 

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