Thermal monomer conversion

Recently Biggs [74] has investigated the emulsion polymerisation of styrene using ultrasonic irradiation as the initiation source (i. e. in the absence of a chemical initiator). Similar to Lorimer and Mason using a thermally initiated system, Biggs found both a marked increase in monomer conversion rate as a function of time as the ultrasonic intensity was increased but remarkable constancy in the resultant latex particle... 

Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type monomers can be overcome by changing to heterofunctional AB-type monomers. Indeed, IIMU has been subjected to bulk polycondensation using lipases as catalyst in the presence of 4 A molecular sieves. At 70 °C, CALB showed 84% monomer conversion and a low molecular weight polymer (Mn 1.1 kDa, PDI 1.9). No significant polymerization was observed with other lipases (except R cepacia lipase, 47% conversion, oligomers only) and in reference reactions with thermally deactivated CALB or in the absence of enzyme. Further optimization of the reaction conditions (60wt% CALB, II0°C, 3 days, 4 A molecular sieves) gave a polymer with Mn of 14.8 kDa (PDI 2.3) in 86% yield after precipitation [42]. 

According to Scheme 13 a benzyl derivative of Nd is formed. At a polymerization temperature of 60 °C the benzyl Nd intermediate once formed decomposes rapidly as Nd(benzyl)3 is reported to be stable only below - 15 °C [425,426]. As a consequence of the low thermal stability of the Nd benzyl species proton transfer from toluene is irreversible and the overall rate of polymerization is reduced by the decrease of the amount of the active catalyst species. As TBB lacks benzyl protons it can only act as a 7r-donor. Therefore, TBB reduces the polymerization rate to a lower extent than toluene. Beside the interpretations given, the study also presents detailed investigations on the evolution of the MMDs with monomer conversion in the three solvents n-hexane, TBB, toluene [422]. In the two aromatic solvents a high molar mass fraction is more pronounced than in n-hexane. 

However, the probability for the reaction progression greatly depends on the monomer conversion. Because the viscosity of the dispersed phase, in the first stage, is fairly low and the quantity of styrene is sufficiently high, the decomposition process (Figure 9.4) occurs only up to the benzoyloxy radical, which can directly start the kinetic chain. The purely thermal start of chains with reactive dimers of styrene, as a result of Diels-Alder reaction, can be ignored at fairly low temperatures of suspension polymerization, in contrast to the conditions for the bulk styrene process [4-7]. 

Furthermore, the authors noticed that the DPn increased with the monomer-conversion-rate since the initiator decomposed very quickly. The products exhibited very good thermal properties actually at 300 °C the oligtaners v/eie still stable because of the absence of double bonds obtained in traditional radical polymerization ofMMA. 

The presence of the nitroxide radical was confirmed through EPR and XH NMR spectroscopic methods. The copolymer GPC trace (Mn=33,100, Mw/Mn=1.37) was symmetrical with no evidence of unreacted macroinitiator or homopolymer of St resulting from either thermal initiation or from disproportionation of the pSt from the TEMPO chain end [231]. The kinetic results showed a first-order relationship between monomer conversion and time and the molecular weights increased linearly with conversion, indicating the polymerization proceeded with minimal termination or chain transfer reactions. The presence of the pEAD block produces an amphiphilic copolymer with a biodegradable block that may be useful for biomedical applications. 

Fig. S.15 Experimental thermal and chemical monomer conversions and mean molecular weight at 400 rpm.

Fradet and co-workers reported on the thermal ROP of y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolac-tam (compare Scheme 7). Both monomers were polymerized in bulk at 250 °C. In both cases, the authors observed that monomer conversion was limited and did not exceed a plateau value of 0.53 (after a reaction time of 3 h) or 0.57 (after 30 min) for y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolactam, respectively. The limiting monomer conversion was ascribed to ring-chain equilibria in both cases. The polymerizations could be accelerated by the addition of polyamidation catalysts, such as phosphorous and hypo-phosphoric acids, but no change of the maximum monomer conversion was observed. In a control experiment, 4-aminoethyl-1,7-heptanedioic acid was polymerized via thermal polymerization however, this only resulted in a low molecular mass compound. This was attributed to the much faster rate of the intra- versus the intermolecular amidation reaction. Cross-linked material was obtained, when both monomers were heated for a prolonged time, and loss of NH3 was observed, which was ascribed to amidine formation and deamination. 

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