Branching step-growth

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

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 ... 

Monomers that participate in step growth polymerization may contain more or fewer than two functional groups. Difunctional monomers create linear polymers. Trifiinctional or polyfunctional monomers introduce branches which may lead to crosslinking when they are present in sufficiently high concentrations. Monofunctional monomers terminate polymerization by capping off the reactive end of the chain. Figure 2.12 illustrates the effect of functionality on molecular structure. 

We make copolymers by incorporating two or more different monomers into a single polymer. We can make copolymers via either chain growth or step growth polymerization methods. Copolymers are characterized based on the ordering of their monomers in the final chain. Figure 2.16 illustrates several of the more common classes of copolymer random, alternating, block, and branched block. 

The formation of synthetic polymers is a process which occurs via chemical connection of many hundreds up to many thousands of monomer molecules. As a result, macromolecular chains are formed. They are, in general, linear, but can be branched, hyperbranched, or crosslinked as well. However, depending on the number of different monomers and how they are connected, homo- or one of the various kinds of copolymers can result. The chemical process of chain formation may be subdivided roughly into two classes, depending on whether it proceeds as a chain-growth or as a step-growth reaction. 

Independently of each other, Lambert et al. [69] and Suzuki et al. [70] both gained access to low-generation silane dendrimers (Gl, G2) in 1995. The latter prepared a first-generation polysilane dendrimer 3 by a stepped growth polymerisation technique. Coupling of methyl[tris(chlorodimethylsilyl)]silane (1) to tris-(trimethylsilyl)silyllithiurn (2) led to the first-generation branched dendrimer 3 (Fig. 4.38). 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

The two principal in-situ syntheses of branched copolymers are by step growth or radical chemistry. It should be noted that crosslinking of the same phase can also occur in addition to branching. This crosslinking is the basic principle of IPN formation. Hence, in this section, we will only refer to reports where crosslinking of the same phase appears to be a side reaction and not the expected one. 

Spindler, R., and Frechet, J. M. J. 1993. Synthesis and characterization of hyper-branched polyurethanes prepared from blocked isocyanate monomers by step-growth polymerization. Macromolecules, 26, 4809 1813. 

Time sequenced, step growth of branch cells and dendrimer structure Self replication of branch cells throughout dendrimer construction Structural proliferation of dendrimers with exponential amplification of branch cells" and surface functionality as a function of generation... 

The following paragraphs include sample calculations which illustrate the practical application of the Carothers equation to step-growth polymerizations which yield branched polymers. 

In step-growth polymerizations with unfavorable values of K, it is therefore standard practice to operate at high temperatures and reduced pressures to remove the condensation products. This is typical of the manufacture of linear polyesters where the final stages of the polymerization are at pressures near I mm Hg and temperatures near 280 C. Alkyds (Section 5.4.2) are branched polyesters produced by esterification reactions of mixtures of polyhydric alcohols and acids with varying functionalities. They are used primarily in surface coatings. Alkyd syntheses are completed at temperatures near 240°C. It is not necessary to reduce the pressure to pull residual water out of the reaction mixture, because the final products are relatively low-molecular-weight fluids that are diluted with organic solvents before further use. In one process variation, a small amount of a solvent like xylene is added to the reactants to facilitate water removal by azeotropic... 

Thus a step-polymerization system synthesized from an AB2 monomer should be highly branched but never reach gelation even at full conversion of the available functional groups. This is the basis of the formation of hyperbranched polymers by step-growth polymerization (Jikei and Kakimoto, 2001) and a reaction scheme for AB2 hyperbranching is shown in Scheme 1.11. 

Polyethylenimine prepared in the usual way, i.e. with protonic acids in water solution at elevated temperatures is highly branched and has Mn < 100000. Under anhydrous conditions Mn < 3000. The kinetics of polymerization resembles a step-growth process. At 90% monomer conversion the main reaction product (80%) is the linear dimer [l-(2-aminoethyl) aziridine] 6I,62) ... 

---