Normalized copolymer composition

Figure 17. Theoretical normalized copolymer composition distributions (ti 0.56, Tf = 0.40 letters correspond to sample in Table V)...

Alongside the radical distinction of the mechanism of this process from that of chain polymerization, linear polycondensation features a number of specific peculiarities. So, for instance, the theory of copolycondensation does not deal with the problem of the calculation of a copolymer composition which normally coincides with the initial monomer mixture composition. Conversely, unlike chain polymerization, of particular importance for the products of polycondensation processes with the participation of asymmetric monomers is structural isomerism, so that the fractions of the head-to-head and head-to-tail patterns of ar-... 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

The composition of the grafted side chain copolymer has also been determined by Sakurada (113) and found to be different from the normal copolymer formed with acrylonitrile and butadiene. With styrene the grafted copolymers were found to be richer in acrylonitrile than the normal copolymer. Similar differences were found by Resting (114) with methyl methacrylate and styrene grafted to cotton and by Odian et al. (115) with grafting mixed monomers to Teflon and to polyethylene. It is believed that one monomer may be preferentially sorbed or diffused faster than the other, leading to a different monomer ratio at the actual site of grafting. 

Considering that polymerization takes place in particles, as shown by results concerning copolymer composition, these results seem surprising. The possibility of continuous nucleation is quite normal and explained through the Fitch theory (12) but questionable points are ... 

Poly(M,M-diethylacrylamide-co-M,W-dimethylacrylamide) P(DEA-co-DMA) copolymers with different amounts of DMA can be synthesized by free radical polymerization in THF with AIBN as the initiator (1 mol%). In a typical reaction, the solution mixture is bubbled with dry nitrogen for 30 min prior to polymerization. The temperature is then gradually raised to 68 °C in a period of 2 h and maintained for 18 h. Each reaction mixture was precipitated in ether or hexane after the polymerization. The copolymer composition determined by JH NMR spectra is normally close to the feed ratio of monomers prior to polymerization. The nomenclature used hereafter for these copolymers is P(DEA-co-DMA/x), where x denotes the mol % content of DMA. The chemical structure of P(DEA-co-DMA) is as shown in Scheme 6. 

Figure 7-3 records the changes of monomer feed and copolymer compositions with conversion in the case of glycidyl methacrylate and styrene. This copoly-merizalion would produce an essentially styrenic polymer which is cross-linkable through the pendant epoxy groups of the methacrylate residues. The last 10% of copolymer formed is practically pure polystyrene. In the styrene-butadiene copolymerization depicted in Fig. 7-4, the product composition is almost constant for the first 70% of the reaction where this polymerization would normally be halted anyway (Section 7.2.3). 

Vq PB diluent concentration at the solubility limit under a standard state Vp Poisson s ratio of block copolymer composite Vp Atomic frequency factor in molecular chain scission Q Active craze front length per unit volume a Negative pressure (mean normal stress)... 

Fig. 19.7 Plots of normal probability vs. studentized residuals for models describing solvent resistance of BPA-HQ-RS copolymer compositions in (a) THF, (b) chloroform, (c) MEK. Reprinted with permission from Potyrailo et al.23 Copyright 2006 American Chemical Society...

It should, however, be pointed out that in the above case, if one simply ascribes a single solubility parameter to each monomer, it is Impossible to predict an overall negative enthalpy of mixing. It has also been noted that a window of miscibility can be explained by a favorable specific interaction without recourse to a cross term. If one separates the normal dispersive forces from the specific interaction, then as a first approximation, when the solubility parameters of the two polymers are similar the unfavorable dispersive interactions are small and specific interactions yield miscibility. For a copolymer/polymer mixture the solubility parameters might be expected to match at some specific copolymer composition (32). A method of combining the features of both the specific interaction and the cross term is to use something similar to the UNIFAC group contribution system and model all the interactions, both favorable and unfavorable within the system. 

The difficulties in forming copolymers of certain olefins with SO2 have been discussed in a number of papers [61d]. The highest copolymerization rates were found in systems containing an electron-donor monomer with low resonance stabilization and gave alternating copolymer compositions. Monomers with high resonance stabilization with elechon-acceptor groups result mainly in homopolymerization under normal conditions. For example, styrene forms only a... 

All latex samples prepared by emulsion polymerization are characterized by a broad distribution of molar masses, and in the case of copolymer latexes, a distribution of copolymer composition. Since the diffusion coefficient for a polymer depends upon both the chain length and the chemical structure, the polymers in any one film sample will be characterized by a rather broad distribution of Dcm values. Experiments to detormine in such systems actually yield a value averaged over the distribution, Dts. As will be seen below, since different components of the system contribute to the measured signal at different times, and the fastest diffusing species dominate the diffusion at early times, experi-mental values of Detr decrease with the ext t of interdiffiision. For such sanqiles, one is normally less int sted in the absoluie values of than in how extonal... 

The cloud-point titration method (Section 6.6.5) can also be applied to copolymers whose monomeric units are chemically not very different. In this method, solutions of various concentrations are titrated with nonsolvent to the first cloud point. By extrapolation to 100% polymer, a critical volume fraction 3 of the nonsolvent is obtained, which normally depends linearly on the copolymer composition. 

Aside from block copolymer composition and the quahty of the block-selective solvent, polymer concentration can also affect micellar morphology. For example, the length of cylindrical micelles normally increases with the concentration of diblock copolymers in a block-selective solvent [38-40]. If the aggregates are kinetic products, the morphologies will be affected by sample preparation conditions and history as well. 

In the case of styrene-acrylonitrile copolymers, the method of choice for batch suspension polymerization is normally that involving the azeotropic monomer/comonomer composition to minimize copolymer compositional drift. Nevertheless, complications often arise because considerably more acrylonitrile than styrene dissolves in the continuous aqueous phase. As conversion proceeds, acrylonitrile diffuses into the polymer particles and the monomer ratio in the bead changes, causing the composition of the copolymer to change as well [4]. 

The previous discussion leads to the definition of the fourth classical control problem, which is the control of the copolymer composition along the reaction batch (or at the end of the batch). This objective is normally attained through manipulation of monomer feed flow rates [44, 45]. The feed stream usually contains the most reactive monomer species, so that composition control is obtained by keeping the concentration of the most reactive monomer concentration at the desired low levels throughout the batch time. It is important to emphasize that implementation of monomer feed strategies may lead to runaway conditions in the presence of heat transfer limitation [ 46 ], which partially explains why control of copolymer composition in emulsion reactors is normally attained by working under starved conditions. 

Estimation of Copolymer Composition Distribution. A copolymer is normally polydisperse not only in molar mass but also in chain composition. A combination of static and dynamic LLS can be used to estimate its composition distribution. Consider a copolymer sample consisting of monomers A and B and suppose that the copolymer species i is characterized by the molar mass M and the weight fraction wpSMi)). Assume that for a given M, there is no further composition heterogeneity. For a given copolymer in solvents 1 and 2, eq (12) applies (43,44)... 

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