Monomer observables

Ic. Initiator Efficiencies.—The strict adherence to first-order kinetics (with respect to monomer) observed in several instances indicates an efficiency of utilization of primary radicals which is independent of dilution. It may be inferred from this observation that the efficiency probably is near unity. In other cases where the kinetics point to a decrease in / with dilution this decrease is rather small, and efficiencies near unity for the undiluted monomer are by no means precluded. 

S-Methacryloyloxy-ljl -biadamantane, MBA, (Scheme 4) was efficiently polymerized anionically using [ l,l-bis(4/-trimethylsilylphenyl)-3-methylpen-lyl]lithium as the initiator, prepared in situ by the reaction of s-BuLi with l,l-bis(4-trimethylsilylphenyl)ethylene [17]. The polymerization took place at - 50 °C in order to avoid the solubility problems of the monomer, observed at - 78 °C. Narrow molecular weight distribution block copolymers of rather low molecular weights of PMBA with fBuMA and (2,2-dimethyl-... 

Figure 5 Transient absorption spectra of irradiated additive-free solid PMMA containing MMA monomers observed by the authors improved nanosecond pulse radiolysis system over a wide range of temperatures and with the monitoring wavelength region from 300 to 1600 nm.

Butyllithium initiation of methylmethacrylate has been studied by Korotkov (55) and by Wiles and Bywater (118). Korotkov s scheme involves four reactions 1) attack of butyllithium on the vinyl double bond to produce an active centre, 2) attack of butyllithium at the ester group of the monomer to give inactive products, 3) chain propagation, and 4) chain termination by attack of the polymer anion on the monomer ester function. On the basis of this reaction scheme an expression could be derived for the rate of monomer consumption which is unfortunately too complex for use directly and requires drastic simplification. The final expression derived is therefore only valid for low conversions and slow termination, and if propagation is rapid compared to initiation. The mechanism does not explain the initial rapid uptake of monomer observed, nor the period of anomalous propagation often observed with this initiator. The assumption that kv > kt is hardly likely to be true even after allowance is made for the fact that the concentration of active species is much smaller than that of the added initiator. Butyllithium disappears almost instantaneously but propagation proceeds over periods from tens to hundreds of minutes. The rate constants finally derived therefore cannot be taken seriously (the estimated A is 2 x 105 that of k ) nor can the mechanism be regarded as confirmed. 

Table 7. Growing spedes in cationic polymerization of heterocyclic monomers observed directly by H-NMR spectroscopy ...

As Is typical for polymeric systems, the fluorescence decay of the monomer and exclmer Is non-exponentlal. The fluorescence decay was fit satisfactorily to a blexponentlal function (see eqn. (5)) and the parameters for the monomer (observed at 340nm) and the exclmer (observed at wavelengths longer than l(00nm) are presented In Table 2. Also presented are the "quantum yield weighted average lifetimes" (eqn. (6)) and the average lifetime (eqn. (7)), defined by... 

Excimer fluorescence of pyrene attached to synthetic polymer chains can be used to study polymer conformation in solution and on particle surfaces. In this case, fluorescence spectroscopy involves the measurement of the emission intensity of monomer (/ , observed at 375 mn) and excimer (/ at 480 nm). The ratio of f to I is related to coiling/stretching behavior of a labeled polymer and we have called it the coiling index. In the absence of intermolecular interactions (the polymer concentration used is usually below this limit), a high value of IJI can be considered the result of a coiled conformation whereas a low value is associated with a stretched conformation. 

Table III. Growing species in Cationic Polymerization Heterocyclic Monomers Observed Directly by Spectroscopy H-NMR...

Figure 1. Mosaic texture of F10 monomer observed at 20 °C under a polarizing microscope.

The lithium enolate of t-amyl acetate exists as a doubly chelated dimer in the presence of TMEDA (A,A,A, At -tetramethylethylenediamine). Reaction with a simple aldimine such as pflra-F-C6H4-CH=N-Ph gives an iV-lithiated -amino ester as a monomer, observed by Li- and i N-NMR. Kinetic studies by i F-NMR give a reaction order consistent with a TS of stoichiometry [(ROLi)2(TMEDA)2(imine)], supported by DPT calculations. That such aza-aldol condensations involve dimeric mechanistic routes runs counter to many claims that monomers are more reactive. 

The ligand effects on monomers observed in the nucleation stage was suggested to be divided into three different classes [70] ... 

There are several explanations for the large concentration of monomers present under neutral and basic conditions. From Fig, 21 we see that the rate of hydrolysis of siloxane bonds increases by over three orders of magnitude between pH 4 and 7. Because hydrolysis occurs preferentially at less highly condensed Q sites [1], monomers are the primary by-product of siloxane bond hydrolysis. In a related study, Klemperer and Ramamurthi [93] have shown that siloxane bonds are broken by redistribution reactions under basic conditions (Eq. 42) that produce unhydrolyzed monomers as a by-product. In addition, from the pH-dependence of the hydrolysis reaction (Fig. 9), we see that the hydrolysis rate is minimized at neutral pH. Because the rate constant of the alcohol-producing condensation reaction is less than that of the water-producing reaction [63,95], unhydrolyzed or partially hydrolyzed monomers may persist in solution past the gel point. Presumably these combined factors contribute to the large concentrations of monomers observed under neutral and basic conditions. 

It is possible, then, to produce crystalline, stereospecific poly(propylene oxide) from optically pure isomer and racemic monomer with a variety of initiators (77) and to determine the product distribution of crystalline and amorphous fractions as well as the optical activity of the products. Further, in a penetrating series of experiments, Furukawa, Tsuruta, and co-workers used optically active coinitiators, including menthol and borneol, and optically active alkylzinc, di-2-methylbutylzinc/H20, to prepare crystalline polymer with optical activity from d,l-monomer, observing enrichment of unreacted monomer with the unreacted enantiomer monomeric species (79, 89-92). 

Polymerization and copolymerization with vinyl monomers observed [89]... 

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