Monomers, cyclic terms

Irrespective of the reaction mechanism, the polymerization of lactams leads to an equilibrium between monomer, cyclic oligomers and polymer. Tobolsky and Eisenberg [9] showed that the thermodynamic parameters are independent of the reaction mechanism, so that the polymerizability may be rationalized in terms of the ease of formation of the cyclic monomer, or, its opening into a linear chain unit. The simple relation between the equilibrium monomer concentration [L]e, temperature, and standard heat and entropy of polymerization. 

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

Bifunctional monomers capable of forming six- or seven-membered rings condense variably, depending upon the particular monomer. The products normally obtained in the absence of diluent in various representative bifunctional condensations are listed in Table IX for unit lengths of six and seven members. The term interconvertibility refers to the reversible transformation between the ring and the linear polymer. Several of the six-membered units (Table IX) prefer the ring form exclusively, but most of them yield both products, or at any rate the ring and chain products are readily interconvertible. Seven-membered units either yield linear polymers exclusively, or, if the cyclic monomer is formed under ordinary conditions, it is convertible to the linear polymer. 

The remarkably high molecular weights of copolymers of dioxolan and styrene, which were achieved by Yamashita, are also easily intelligible in terms of the insertion mechanisms shown in schemes (D) and (E) the fact that cationically produced polystyrenes have low molecular weights is mainly due to proton transfer to monomer from the P-carbon at the growing end of a chain. If there is no growing end but a cyclic oxonium ion, this reaction cannot occur, and thus the principal chainbreaking reaction is frustrated. 

In the case of some cyclics, such as the thionylphosphazene six-membered ring [NS(0)Cl(NPCl2)2], the small degree of ring-strain makes the favourable A/Zrop and unfavourable TAArop terms finely balanced. In the presence of a catalyst such as GaCls, ROP can be favoured at high concentrations and depolymerisation to the cyclic monomer at low concentrations [eqn (8.2)]. 

A rare exception to the dominance of enthalpic driving force is provided by the unstrained eight-membered ring (Me2SiO)4, which undergoes ROP to give poly(dimethylsiloxane). For this process, A//rop is approximately zero but the remarkable skeletal flexibility of the polysiloxane backbone makes the TAArop term favourable and entropy provides the driving force for ROP. Exceptional behaviour occurs in the extremely rare cases of monomers where the polymerisation is driven by entropy and A7/rop is unfavourable. In these cases, the formation of polymer is favoured at elevated temperatures. For example, cyclo-Sg will polymerise above 150 °C (where the favourable TAArop term dominates), but slowly depolymerises back to the cyclic Sg monomer at room temperature. ... 

The most important feature of ionizing radiations is, as the term implies, ionization to give ionic intermediates in irradiated systems. Though radiation-induced radical polymerization had long been studied, it is only a decade since radiation-induced ionic polymerization was first found. In 1957, Davison et al. obtained polymer from isobutene, which is known not to be polymerized by radical catalysts, by irradiating at low temperature with y-rays (7). Before long, the radiation-induced polymerization of styrene was proved to proceed as an ionic mechanism in suitable solvents (2,3,4). Since these pioneering researches, the study of the chemical kinetics of radiation-induced ionic polymerization has been extended to several vinyl, diene and cyclic monomers. 

Compounds In(Me)3 and Tl(Me)3 are monomeric in solution and the gas phase. In the solid state, they also exist as monomers essentially, but close intermolecular contacts become important. In crystalline In(Me)3, significant In- -C intermolecular interactions are observed, suggesting that the structure can be described in terms of cyclic tetramers, as shown in Fig. 13.6.1(e), with interatomic distances of In-C 218 pm and In- -C 308 pm. The corresponding bond distances in isostructural Tl(Me)3 areTl-C = 230pmandTl- C = 316pm. 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

Spiro orthoesters (92, R = Me, Ph, and H) show typical equilibrium polymerization behavior at or below ambient temperature. [92] The poly(cyclic orthoester)s derived from 92 depolymerize to the monomers, although they have sufficient strains to be able to undergo ring-opening polymerization. The polymerization enthalpies and entropies for these three monomers were evaluated from the temperature dependence of equilibrium monomer concentrations (Table 5). The enthalpy became less negative as the size of the substituent at the 2-position in 92 was increased H < Me < Ph. This behavior can be explained in terms of the polymer state being made less stable by steric repulsion between the bulky substituents and/or between the substituent and the polymer main chain. The entropy also changed in a similar manner with the size of the substituents. 

Note that the polymerizations of lactones and lactams give polyesters and polyamides, respectively, polymers that can also be synthesized by conventional step-growth polymerization from noncyclic monomers. Although the polymer prepared from cyclic monomers looks like a condensation polymery in strict terms it is not, because no small-molecule byproduct was pro-... 

According to Eq. (25), a cyclic phosphite monomer (MN) 38 is oxidized to a phosphate unit yielding copolymer 40 whereas the a-keto acid monomer (ME) 39 is reduced to the corresponding a-hydroxy acid ester. Thus, the term redox copolymerization has been proposed to designate this type of copolymerization in which one monomer is reduced and the other monomer oxidized. The redox copolymerization clearly differs from the so-called redox polymerization in classical polymer chemistry where the redox reaction between the two catalyst components (oxidant and reductant) is responsible for the production of free radicals. 

The most important monomers for the production of polyolefins, in terms of industrial capacity, are ethylene, propylene and butene, followed by isobutene and 4-methyl-1-pentene. Higher a-olefins, such as 1-hexene, and cyclic monomers, such as norbornene, are used together with the monomers mentioned above, to produce copolymer materials. Another monomer with wide application in the polymer industry is styrene. The main sources presently used and conceivably usable for olefin monomer production are petroleum (see also Chapters 1 and 3), natural gas (largely methane plus some ethane, etc.), coal (a composite of polymerized and cross-linked hydrocarbons containing many impurities), biomass (organic wastes from plants or animals), and vegetable oils (see Chapter 3). 

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