Monomer constitution polymerizations

To broaden our overall knowledge of process kinetics the first chapter of this volume deals with elementary reactions in radical and anionic polymerization it was written by G. V. Schulz, the first recipient of the H. Staudinger Award. It is followed by a discussion on monomer constitution and stereocontrol in radical polymerization by H. G. Elias et al. 

Monomer Constitution and Stereocontrol in Free Radical Polymerizations... 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

These findings imply that the use of probabilities for i-ad formation at a given temperature in a given solvent is insufficient to describe the monomer constitutions influence on the stereocontrol in free radical polymerizations. The lack of correlation is either the result of the combined action of more than one parameter (size of substituent, resonance stabilization and/or structure of propagating radicals, etc.) or the result of noncomparable experimental conditions. 

The high number of monomers constituting M and M is referred to as the degree of polymerization (DP). Being predetermined by the manufacturer s methodologies, the average DP is constant but widely distributed from the mean, and is thus unsuitable without fractionation for most experiments involving purchased samples. 

Two factors contribute to the stability of the gels prepared by the two-step concentrated emulsion. The repulsive forces between the charged surfactant molecules adsorbed on the surface of neighboring cells of the dispersed phase is one of them. The increased viscosity of the dispersed phase which contains the monomer constitutes the second factor, since the increased viscosity opposes the separation of the phases. The partial polymerization increases the viscosity of the dispersed phase, thus increasing the stability of the concentrated emulsion. Monomers that could not lead to gels in the one-step concentrated emulsion method were able to generate them when the two-step pathway was employed. Using this pathway, almost all monomers could be employed to prepare polymer materials. 

The impact strength of brittle thermoplastic materials is generally improved by adding small amounts of rubber, either pure or modified by grafting with the monomer or monomers constituting the matrix to be reinforced (1, 2, 3, 4, 5). As a rule, modification is achieved by monomer polymerization in the presence of the reinforcing elastomer, which is usually a butadiene polymer or copolymer (6, 7). 

Only the translational and external rotational entropy components are lost in the polymerization of olefins. The loss in translational entropy, of course, is independent of constitution. The loss in external rotational entropy is also independent of the monomer constitution since the bond moments and moments of inertia of most monomers are of about the same magnitude. The internal rotational and vibrational entropy components are indeed different from monomer to monomer, but their absolute values are quite small (Table 16-7). Thus, the standard entropy ASl is practically independent of constitution in the case of compounds with olefinic double bonds (Table 16-8). Differences in the ceiling temperature are, in practice, caused by the polymerization enthalpy alone. 

The stereocontrol of most free radical polymerizations appears to be governed by an end-controlled mechanism. It generally follows first-order Markov statistics with respect to diads (see also Section 16.5.2.3). The tactic-ity of the formed polymer is also influenced by the solvent used. The cause of this solvent control effect is unclear, and possibly is due at least partly to different degrees of solvation. A compensation effect (see Section 16.5.4.) exists in the relationship between the activation entropies and enthalpies for diad formation in various solvents. The compensation temperature TJj varies with monomer constitution (Table 20-11). The compensation enthalpies AAHI vary strongly according to both monomer and placement type. 

Polyheterocycles. Heterocychc monomers constitute a third class of monomers that can he polymerized to form fully conjugated polymers the most common of these monomers, shown in equation 7, are p5urole (X = NH), thiophene (X = S), and furan (X = O). They can he doped to give electrical conductivity when pol5unerization occurs in the 2,5-positions. These monomers can be polymerized both by electrochemical and chemical methods. The polyheterocycles have received considerable attention for their electron-rich nature, which leads to materials that are easily oxidized and therefore more stable in the oxidized state. Additionally, the increased structural complexity of polyheterocycles relative to polyacetylenes makes structural modifications possible for improved processability. 

Cyclic ethers constitute an important class of heterocyclic monomers that polymerize by ionic mechanisms. Studies of the mechanism, kinetics, and thermodynamics of cyclic ether polymerization were essential in establishing basic principles of ionic ring-opening polymerization (ROP). There are several book chapters and reviews summarizing this field and although some of them date back to early 1980s, the main conclusions are still t id and provide a basis for more recent developments. " ... 

Because the conditions for final protecting-group removal can be harsh, eliminating this step allows for the synthesis of materials containing sensitive bioactive epitopes. Consequently, the ability to synthesize substituted polymers utilizing both defined initiators and unprotected monomers constituted a critical advance in the synthesis of bioactive substituted polymeric displays via ROMP. Methods that have been used successfully for the incorporation of polar bioactive ligands are highlighted. 

Recent reviews on the coordination polymerization of polar vinyl monomers constitute excellent and comprehensive accounts regarding the latest developments in Al-mediated alkyl methacrylate polymerization [67-69]. Noteworthy results in this area are highlighted in this section. 

Polymerization processes yielding polymers, whose mers are constitutionally identical to the reacting monomers are now classified as addition polymerizations. Thus styrene can be converted, by addition polymerization, to polystyrene ... 

It is not possible to apply (C2.1.1) down to the level of monomers and replace by the degree of polymerization N and f by the sum of the squares of the bond lengths in the monomer because the chemical constitution imposes some stiffness to the chain on the length scale of a few monomer units. This effect is accounted for by introducing the characteristic ratio defined as C- — The characteristic ratio can be detennined... 

Figure 6.3 shows some data which constitute a test of Eq. (6.26). In Fig. 6.3a, Rp and [M] are plotted on a log-log scale for a constant level of redox initiator. The slope of this line, which indicates the order of the polymerization with respect to monomer, is unity, showing that the polymerization of methyl methacrylate is first order in monomer. Figure 6.3b is a similar plot of the initial rate of polymerization—which essentially maintains the monomer at constant con-centration—versus initiator concentration for several different monomer-initiator combinations. Each of the lines has a slope of indicating a half-order dependence on [I] as predicted by Eq. (6.26).

As with the rate of polymerization, we see from Eq. (6.37) that the kinetic chain length depends on the monomer and initiator concentrations and on the constants for the three different kinds of kinetic processes that constitute the mechanism. When the initial monomer and initiator concentrations are used, Eq. (6.37) describes the initial polymer formed. The initial degree of polymerization is a measurable quantity, so Eq. (6.37) provides a second functional relationship, different from Eq. (6.26), between experimentally available quantities-n, [M], and [1]-and theoretically important parameters—kp, k, and k. Note that the mode of termination which establishes the connection between u and hj, and the value of f are both accessible through end group characterization. Thus we have a second equation with three unknowns one more and the evaluation of the individual kinetic constants from experimental results will be feasible. 

Unsaturated polyester resins prepared by condensation polymerization constitute the largest industrial use for maleic anhydride. Typically, maleic anhydride is esterified with ethylene glycol [107-21-1] and a vinyl monomer or styrene is added along with an initiator such as a peroxide to produce a three-dimensional macromolecule with rigidity, insolubiUty, and mechanical strength. 

Acrylic Polymers. Although considerable information on the plasticization of acryUc resins is scattered throughout journal and patent hterature, the subject is compHcated by the fact that acryUc resins constitute a large family of polymers rather than a single polymeric species. An infinite variation in physical properties may be obtained through copolymerization of two or more acryUc monomers selected from the available esters of acryUc and methacryhc acid (30) (see Acrylic esterpolya rs Methacrylic acid and derivatives). 

Two kinds of monomers are present in acryUc elastomers backbone monomers and cure-site monomers. Backbone monomers are acryUc esters that constitute the majority of the polymer chain (up to 99%), and determine the physical and chemical properties of the polymer and the performance of the vulcanizates. Cure-site monomers simultaneously present a double bond available for polymerization with acrylates and a moiety reactive with specific compounds in order to faciUtate the vulcanization process. 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

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