Chain polymerization and

Recent work on the synthesis, structure and some properties of macromolecules bearing furan rings is discussed. Two basic sources of monomers are considered, viz. furfural for monomers apt to undergo chain polymerization and hydroxymethylfurfural for monomers suitable for step polymerization.Within the first context, free radical, catiomc and anionic systems are reviewed and the peculiarities arising from the presence of furan moieties in the monomer and/or the polymer examined in detail. As for the second context, the polymers considered are polyesters, polyethers, polyamides and polyurethanes. Finally, the chemical modification of aU these oligomers, polymers and copolymers is envisaged on the basis of the unique reactivity of the furan heterocycle. 

Fig. l-l Variation of molecular weight with conversion (a) chain polymerization (b) step polymerization (c) nonterminating chain polymerization and protein synthesis. 

The rate of polymeric chain generation used to help in the classification of polymerization processes, and to this day chain polymerizations and step polyadditions are distinguished. This classification was introduced long ago and is mainly based on the observation that the molar mass of the product is sometines roughly independent of time (conversion) whereas in other cases it increases considerably. Strictly speaking, every polymerization is a chain polymerization and, at the same time, each is a polyaddition. 

Heavy-Chain Disease. Rarely, unusual forms of IgG, IgA, or light chains polymerize and cause a similar syndrome with high blood viscosity. Heavy-chain diseases in which the paraprotein consists only of a heavy chain, usually incomplete, are rare conditions associated with lymphoid infiltration. The most common of these is a-chain disease, in which the intestine is infiltrated and severe malabsorption may be seen. 

The second step of initiation [Eq. (8.83)], being slower than the first [Eq. (8.82)], is rate-determining for initiation (unlike in the case of free-radical chain polymerization) and so though the amide ion produced upon chain transfer to ammonia can initiate polymerization it is but only at a rate controlled by the rate constant, ki, for initiation. Therefore, this chain transfer reaction may be considered as a true kinetic-chain termination step and the application of steady-state condition gives Eq. (8.90). 

The diluted polymer solution of concentration c is considered let m be the degree of chain polymerization and / be its degree of ionization. 

Figure 1.2 Variation of molecular weight with conversion in (a) step polymerization, (b) free-radical chain polymerization, and (c) ionic chain polymerization.. (Adapted from Odian, 1991.)...

A host of crosslinkers have been employed, see Table 6.2. The mixed functionality crosslinkers often serve as grafting sites between networks of quite different nature, perhaps one network chain polymerized, and the other step polymerized. Then, each type of functionality reacts with each type of monomer during polymerization. 

Free-radical polymerization is the most widely practised method of chain polymerization and is used almost exclusively for the preparation of polymers from olefinic monomers of the general structure CH2=CR R, where R and R are two substituent groups which may be identical, but more often are different. The structures of some common monomers and the homopolymers derived from them are shown in Table 1.1. 

There are many mechanisms describing the process of polymerization (two of which were already discussed—addition and condensation). These terms describe only the chemical process, not the major engineering aspect— the rate of the reaction (kinetics). Therefore, it is beneficial to differentiate between two major polymerization routes—chain polymerization and stepwise polymerization. 

Lim et al. used a palladium complex for the cationic polymerization of TH F and the ROMP of NB [10]. The same group also showed that even condensation and chain polymerization could be performed simultaneously in one step (Scheme 11.45). This was achieved by the use of unimolecular compounds which can simultaneously act both as an initiator for chain polymerization, and as an end-capper for condensation polymerization. The method provides a simple means of combining NMRP with a condensation polymerization to yield interesting and useful block copolymers [207]. Another interesting new system for the combination of chain (AROP of CL) and step (dehydration polycondensation) polymerizations for polyester-based new material, in which scandium trifluoromethane sulfonate catalyzed both polymerization modes, was reported by Takasu et al. (Scheme 11.46) [208]. 

Distinguish between step polymerizations and chain polymerizations and give the three reactions that all chain polymerizations are based upon. 

IX. Stereospecific Chain Polymerization and Copolymerization by Coordination Catalysts... 

IX. STEREOSPECIFIC CHAIN POLYMERIZATION AND COPOLYMERIZATION BY COORDINATION CATALYSTS... 

High-mass macromolecules are produced from the start of chain polymerization, and the molar mass and its distribution are controlled by the rates of initiation and propagation, the mode and rate of termination and by chain-transfer reactions. 

Several groups have conducted graft copolymerizations on starch or its purified components, amylose and amylopectin. The syntheses are based on attack of the anhydroglucose unit of starch by cerium (+4) ion. The free radical produced from this attack is then immersed in a monomer solution polymerizable by free radical, chain polymerization and a graft copolymer is formed. A typical synthesis procedure (10,16) is as follows. 

The key difference between poly( styrene) and styrene is the presence of a double bond in the latter. In poly(styrene), there are two sigma bonds cormecting neighboring styrene units as opposed to the one pi bond in monomeric styrene. The polymerization of alkenes like styrene represents one of the most common polymer synthesis techniques and leads to ubiquitous materials like poly(ethylene), poly(propylene), and poly(vinyl chloride) in addition to poly(styrene). There are many ways to convert alkenes into polymers by chain polymerization and while they are all different in detail, there are some common features. As a starting point, let us first consider the anionic polymerization of styrene to prepare the poly(styr-ene) molecule depicted in Figure 1(a). 

---