Polymerization conversion

We assume here that the concentrations of monomer and initiator remain essentially constant during polymerization and that any dependence of termination rate constants on polymer chain size and concentration or autoacceleration effects can be neglected. Molecular weight distributions derived under these conditions will not obviously apply to commercial polymers, whose polymerizations are often finished at high conversions. These high-conversion polymers may have distributions that differ from those calculated here. A later section discusses the size distributions of such polymers. 

A given monomer-ended radical may add a monomer molecule or undergo chain transfer or termination. The probability that it will add monomer is 

Note that the effects of any chain transfer agent are included in the term for solvent and that transfer to polymer is not included in the expression for Rtr because such transfer can be significant only at high conversion where sufficient polymer can be present. 

We first consider the polymerization where each kinetic chain yields one polymer molecule. This happens when the growth of microradical chains is terminated by disproportionation and/or chain transfer (i.e., ktc = 0). The situation here is completely analogous to that for linear, reversible step-growth polymerization described in Chapter 5. If we select an initiator fragment at the end of a macromolecule, the probability that the monomer molecule picked up by this initiator radical has added another monomer molecule is P. Continuing in this way the probability that x monomer molecules have been added one after another is (see p. 347). Since 

Substituting Eq. (6.199) into Eq. (6.205) and noting that for most addition polymerizations, Rp Rtr + Rt (or high-molecular-weight polymer would not be formed). 

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Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

So the results obtained by different groups and with different methods display the existence of a completely unusual chemical conversion, polymerization at very low temperatures. Similar effects have been found in y-irradiated acrylonitrile and acrolein [Gerasimov et al. 1980]. 

The proportion of useful radicals generated from common dialkyldiazenes is never quantitative typically it is the range 50-70% in media of low viscosity i.e. in low conversion polymerizations).3 88 89 The main cause of this inefficiency is loss of radicals through self-reaction within the solvent cage. 

Conflicting statements have appeared on the sensitivity of / to the nature of the monomer involved. Braun and Czerwinski91 reported, that for low conversion polymerizations, / is essentially the same in MMA, S, and NVP. Inkuda et a .92 reported that / varies between MMA and S. The solvent dependence of may account for this apparent conflict (Table 3.3). 

The radical concentration, when coupled with information on the rate of polymerization, allows k (and k,) to be calculated. The EPR methods have been applied to various polymerizations including those of B, DMA, MMA,361 566 S 67 368 and VAc.369 Values for kp are not always in complete agreement with those obtained by other methods (e.g. PLP, SIP) and this may reflect a calibration problem. Problems may also arise because of the heterogeneity of the polymerization reaction mixture,365 and insufficient sensitivity for the radical concentrations in low conversion polymerizations 63 or very low molecular weights. Some data must be treated with caution. However, the difficulties are now generally recognized and are being resolved. 60... 

The most used method is based on application of the Mayo equation (eq. 5). For low (zero) conversion polymerizations carried out in the presence of added transfer agent T, it follows from eq. 5 that a plot of 1/ Xn vs [T]0/[M]0 should yield a straight line with slope Clr.12 Thus, a typical experimental procedure involves evaluation of the degree of polymerization for low conversion polymerizations earned out in the presence of several concentrations of added transfer agent. The usual way of obtaining Xn values is by GPC analysis of the entire molecular weight distribution. 

The primary aim of most studies on Lewis acid controlled copolymerization has been the elucidation of mechanism and only low conversion polymerizations are reported. Sherrington et al.m studied the high conversion synthesis of alternating MMA-S copolymers in the presence of Lewis acids on a preparative scale. Many Lewis acids were found lo give poor control (i.e. deviation from 50 50 composition) and were further complicated by side reactions including cross-linking. They found that the use of catalytic BCI- as the Lewis acid and photoinitiation gave best results. 

For many situations, a simple total anthocyanin determination is inappropriate because of interference from polymeric anthocyanins, anthocyanin degradation products, or melanoidins from browning reactions. In those cases, the approach has been to measure the absorbance at two different pH values. The differential method measures the absorbance at two pH valnes and rehes on structural transformations of the anthocyanin chromophore as a function of pH. Anthocyanins switch from a saturated bright red-bluish color at pH 1 to colorless at pH 4.5. Conversely, polymeric anthocyanins and others retain their color at pH 4.5. Thus, measurement of anthocyanin samples at pH 1 and 4.5 can remove the interference of other materials that may show absorbance at the A is-max-... 

High-Conversion Polymerization Kinetic Modeling Utilizing Gel Permeation Chromatography... 

The purpose of this paper is to propose solutions to the GPC interpretation problems fitting the needs of high conversion polymerization kinetic modelling. 

BALKE AND PATEL High-ConVersion Polymerization Kinetic Modeling 169... 

Therefore, the classical polymerization model Is applicable only to those conversion trajectories that yield polydispersitles betwen 1.5 and 2 regardless of the mode of termination. Although this Is an expected result, It has not been Implemented, the high conversion polymerization models reported to date are based on the classical equations for which the constraint given by equation 24 Is applicable. The result has been piecewise continuous models, (1-6)... 

Upon mixing the metal halide and the monomer there is no reaction, or reaction is slow and stops at low conversion. Polymerization can be started, or re-started, by addition of water. 

Fig. 17 Top row. conversion (ln([M]o/[M]t) against time plots for 50mol% copolymerizations (a, c, e). Bottom row. relationship between the monomer feed (/i) and the actual monomer incorporation (Fi) at the initial ( 20% conversion) and final (>50% conversion) polymerization stages (b, d, f). Both conversion and monomer incorporation are shown for EtOx NonOx (a, b), MeOx NonOx (c, d), and MeOx EtOx (e, f) copolymerizations. (Reprinted with permission from [88]. Copyright (2006) American Chemical Society)...

What is the breadth of the size distribution to be expected for a low conversion polymerization where termination is entirely by coupling. Discuss the manner in which each of the following situations alters the size distribution ... 

Polymerized SUVs prepared from surfactants containing styrene or vinyl moieties retained their phase transition behavior. Conversely, polymerized SUVs prepared from diacetylenic surfactants lost their phase transitions and those of methacrylamide surfactants had lower phase transition temperatures... 

Fig. 8. Effect of photoirradiation on the polymerization of VCZ initiated by NaAuClj 2H20 at 30° C,in the presence of air. Solvent nitrobenzene. [VCZ] = 0.25 M, [AunlJ = 1.0 x 10- M. (1) (- -) time-conversion, polymerization in the dark (2) (-o-) time-conversion, a hard glass, cylindrical polymerization vessel was irradiated by a 300 W mercury lamp from a distance of 28 cm (3) (-O-) polymerization time-molecular weight, polymerization in dark...

Fig. 2. Number-average molecular weight and molecular weight distribution of the polyamide obtained by anionic polymerization of bicyclic lactam 58 as a function of conversion. Polymerization conditions [M]0, 2.11 mol P potassium pyrrolido-nate, 0.5 mol% activator, N-benzoyl derivative of 58, 2 mol% solvent, Me2SO temp., 25 °C...

No. Oligomer Initiator Temp. rc) Conversion (%) Polymerization Condensation ... 

HIGH MONOMER CONVERSION POLYMERIZATION 2.5.1 Free Radical... 

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