Step-growth mechanism

In spite of the assortment of things discussed in this chapter, there are also a variety of topics that could be included but which are not owing to space limitations. We do not discuss copolymers formed by the step-growth mechanism, for example, or the use of Ziegler-Natta catalysts to regulate geometrical isomerism in, say, butadiene polymerization. Some other important omissions are noted in passing in the body of the chapter. 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

The generic condensation polymerization begins with monomers AMA and BMB and produces molecules of the forms A A, B B, and A B. Each step of the reaction generates a longer polymer by the step-growth mechanism of Equation (13.2) and produces 1 mol of condensation product AB. 

Direct transformation from 2,5-DSP to poly-2,5-DSP through 2,5-DSP oligomer in the crystal is shown in Scheme 1. This new reaction was named a four-centre-type photopolymerization. As well as being the first example of a topochemical reaction in a pure sense, the four-centre-type photopolymerization of 2,5-DSP crystals was the first example of photopolymerization via a step-growth mechanism. 

After the discovery of the photopolymerization of 2,5-DSP crystals, several types of photoproducts were found, not only the linear polymers, but some other derivatives, e.g. the V-shaped dimer or cyclophane (Hasegawa and Hashimoto, 1992). The photopolymerization occurs in a step-growth mechanism by cyclobutane formation between the excited olefin and the olefin in the ground state. 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

These are normally prepared by a step growth mechanism. They can be thermoplastic or thermoset and are often used in composites. Composites are materials that contain two or more phases, one of which acts as a reinforcement... 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

Conducting polymers are quite different systems to the electroinitiated chain polymerisations discussed above being formed by a step-growth mechanism involving stoichiometric transfer of electrons. 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

If one of the species is monomeric oxirane, then J = 1. Likewise, if one of the polymeric species supplied the oxirane, then J > 1. The molecule with the acid group is at degree of polymerization k. The degree of polymerization indexes the number of oxirane residuals within the macromolecule. Though the reaction sequence is simplified, it retains the essence of one molecule reacting with every other molecule. This step-growth mechanism (13) develops the thermoset resin microstructure. 

In addition to post-functionalizing polymers by bonding the macrocycle to the preformed polymer backbone, macrocycles can be incorporated into polymer matrices by direct polymerization of the macrocycle, either by a step-growth mechanism or a chain-growth mechanism. [46] Polymeric crown ether stationary phases were pioneered by Blasius et al. [34, 59-62] These resins were used to separate both cations (including protonated amines) with a common anion, and anions with a common cation in high... 

Individual oligomeric BPA/DC cyclotrimerizates were separated by the HPLC gradient-elution technique in THF/H20 [13], The step-growth mechanism of the polycyclotrimerization was supported by quantitative analysis of the results. 

Figure 5-1. Polymer molar mass as polymerization proceeds by step-growth mechanism.

Other polymers are prepared by a step-growth mechanism. Many of these are condensation reactions in which two different functional groups react, forming a new functional group (e.g., ester, amide) and a small-molecule byproduct. Often these reactions can be carried out neat, meaning no solvents are used. These reactions account for millions of tonnes each... 

Explain the differences between addition and condensation polymers. Show the differences between the chain-growth and step-growth mechanisms of polymerization. 

The linear polymerization of DVB was accomplished with Pd(PPh3)2(BF4)2 in acetonitrile-chloroform solvent at temperatures ranging from 40 to 70° C. The polymer contained structural units derived from both head-tail and head-head coupling. The polymerization proceeded by a step-growth mechanism. The molecular weight increased exponentially as a function of time. This reaction was also used for a one-pot synthesis... 

The last step in this second model is to relate the extent of reaction at vitrification to time. For the step growth mechanism, for both linear and nonlinear systons, nth order kinetics were assumed... 

Thus far, we have considered addition polymerization routes - either catalyzed or uncatalyzed. Although this is sufficient to describe the synthesis of common packaging materials such as polyethylene, polypropylene, polystyrene, etc., other classes of polymers such as nylon, PETE, and polyacrylamide are generated through step-growth mechanisms. Although the synthetic pathway for these polymers is more straightforward than addition polymerization, there are many intricate considerations that affect overall polymer properties. 

FIGURE 20.7 Schematic illustrating the step growth mechanism, considering a 100 surface of a simple cubic crystal as an example and each atom as a cnbe with a coordination number of six (six chemical bonds) in bnUc crystal. 

A large body of polymerization reactions following step growth mechanism are carbonyl addition reactions followed by elimination. The general reaction mechanism of the carbonyl addition-elimination reaction is well understood (6). The nucleophilic reagent attacks approximately perpendicular to the sp -orbitals of the carbonyl and forms a bond with the electropoative carbonyl carbon. The metastable intermediate has the ji electron pair of the C=0 bond localized on the oxygen. Furtha reaction leads to the loss of either substiuent X or Y. In the latter case reaction leads to the desired product ... 

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

Figure 9.2 Three-step growth mechanism of CNFs (a) carbon dissolution through metal particles (b) carbon diffusion inside the metal (c) carbon atom precipitation and ejection of the particle from the support.

---