Substituent effects, monomer/polymer

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

Different Substituent Effects Between Monomer and Polymer... 

Even general AB-type monomers, affording polyamide, polyester, polyether, and so on, undergo chain-growth condensation polymerization if the polymer end group becomes more reactive than the monomer by virtue of the change of substituent effects between the monomer and polymer. Both the resonance effect and inductive effect of the nucleophilic site on the reactivity of the electrophilic site at the para and meta positions of the monomer are applicable, respectively. 

Random hydrocarbon copolymers can also be produced by this new equilibrium polymerization method. Copolymers containing octenylene and butenylene linkages in a statistical array based on feed ratio result from the cocondensation of the two respective monomers or by the reaction of diene with unsaturated polymer. More controlled polymer stmctures have also been prepared by the slow addition of a diene solution to an unsaturated polymer containing active catalyst. Substituent effects were shown to dictate the polymerizability of monomers and in some cases selective polymerization of speciflc aUcenes in the monomer resulted in what appears as perfectly alternating copolymers. ... 

Fig. 6. Effect of chain length of substituent on the monomer—polymer equilibrium of seven- and eight-membered lactams. Linear alkyl groups in position e( ) and at 260°C and in the 7-position (O) at 254°C [28, 29, 63—68].

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

The rates of radical-monomer reactions are also dependent on considerations of steric effects. It is observed that most common 1,1-disubstituted monomers — for example, isobutylene, methyl methacrylate and methacrylo-nitrile—react quite readily in both homo- and copolymerizations. On the other hand, 1,2-disubstituted vinyl monomers exhibit a reluctance to ho-mopolymerize, but they do, however, add quite readily to monosubstituted, and perhaps 1,1-disubstituted monomers. A well-known example is styrene (Ml) and maleic anhydride (M2), which copolymerize with r — 0.01 and T2 = 0 at 60°C, forming a 50/50 alternating copolymer over a wide range of monomer feed compositions. This behavior seems to be a consequence of steric hindrance. Calculation of A i2 values for the reactions of various chloroethylenes with radicals of monosubstituted monomers such as styrene, acrylonitrile, and vinyl acetate shows that the effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or cr-position, but a second substituent when in the 2- or /3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the polymer radical to which it is adding. 

Couturier 1992). With 27 and 28 most catalysts give polymers with randomly oriented substituents (Ivin 1980b, 1981a). Thus the further away the substituent is from the double bond, the less effect it has on the head or tail orientation of the incoming monomer. In polymers of single enantiomers the orientation of the substituents in the polymer is correlated with the ring tacticity. Details are given in Ch. 11. 

Cqpolymerization of MCM with traditional monomers is the main technique of metal insertion into a polymer chain, and it is more widely used than their homopolymerization. However, ocpolymerization laws in such systems are difficult to analyze because of their raultiparameter dependence of the kinetics and copolymerization characteristics on the process, parameters such as pH, solvent nature and even concentration ratio (30). The metal-containing giroup in MCM is, as a rule, an electron-donor substituent (scheme Q-e). The copolymerization yields complexes of different comonomers, effecting the polymer composition and structure. In our view, the most remarkable one is cqpolymerization of transition metal diacrylates with MMA, styrene, etc. (37), as well as vinylpyridine and vinylimidazole MX complexes and formation of ternary copolymers of the following composition (38) ... 

To solve this problem, the most effective and useful way to immobilize a specific substituent into a polymer matrix is to covalently bind it to the monomer before polymerization. This requires developing synthesis routes but remains massively employed. 

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