Reactively formed copolymer

In contrast to ionic chain polymerizations, free radical polymerizations offer a facile route to copolymers ([9] p. 459). The ability of monomers to undergo copolymerization is described by the reactivity ratios, which have been tabulated for many monomer systems for a tabulation of reactivity ratios, see Section 11/154 in Brandrup and Immergut [14]. These tabulations must be used with care, however, as reactivity ratios are not always calculated in an optimum manner [15]. Systems in which one reactivity ratio is much greater than one (1) and the other is much less than one indicate poor copolymerization. Such systems form a mixture of homopolymers rather than a copolymer. Uncontrolled phase separation may take place, and mechanical properties can suffer. An important ramification of the ease of forming copolymers will be discussed in Section 3.1. 

Blends based on polyolefins have been compatibilized by reactive extrusion where functionalized polyolefins are used to form copolymers that bridge the phases. Maleic anhydride modified polyolefins and acrylic acid modified polyolefins are the commonly used modified polymers used as the compatibilizer in polyolefin-polyamide systems. The chemical reaction involved in the formation of block copolymers by the reaction of the amine end group on nylon and anhydride groups or carboxylic groups on modified polyolefins is shown in Scheme 1. 

Of substantial interest for the synthesis of fibre-forming reactive AN copolymers is the use of methacrolein 4 as second monomer, which affords copolymers of type 5, containing aldehyde groups. 

One can show the drift of copolymer composition with conversion for various comonomer feed compositions by a three-dimensional plot such as that in Fig. 6-4 for the radical copolymerization of styrene (M, )-2-vinylthiophene (M2). This is an ideal copolymerization with r — 0.35 and r-L — 3.10. The greater reactivity of the 2-vinylthiophene results in its being incorporated preferentially into the first-formed copolymer. As the reaction proceeds, the feed and therefore the copolymer become progressivley enriched in styrene. This is shown by Fig. 6-5, which describes the distribution of copolymer compositions at 100% conversion for several different initial feeds. 

A vinyl monomer M, (1 mole) is copolymerized with another monomer M2 (2 moles). The reactivity ratios are r, = 0.6 and r2 = 1.66. Determine the mole fraction of monomer M in the initially formed copolymer. What is the mole fraction of monomer M, in the polymer being formed when two moles of the monomers are converted to a copolymer Draw the graph of the cumulative and the instantaneous copolymer compositions vs. the conversion. 

A copolymerization study of the same two monomers using sodium in liquid ammonia by Landler (36) revealed reactivity ratios of 0.123 for styrene and 6.4 for methyl methacrylate, values which would predict as much as 10% styrene in the initially formed copolymer from a 1 1 molar styrene-methyl methacrylate mixture. However, O Driscoll and Tobolsky have disputed this result (84). 

Both the mini- and macroemulsion copolymerizations of pMS/MMA tend to follow bulk polymerization kinetics, as described by the integrated copolymer equation. MMA is only slightly more soluble in the aqueous phase, and the reactivity ratios would tend to produce an alternating copolymer. The miniemulsion polymerization showed a slight tendency to form copolymer that is richer in the more water-insoluble monomer. The macroemulsion formed a copolymer that is slightly richer in the methyl methacrylate than the co-... 

Kj, rij and r2/, and fei2/ are the fraction, the reactivity ratios and the propagation rate coefficient corresponding to the jth type catalyst species [187]. Monomers of similar structure, such as the substituted styrenes, copolymerize normally but in most instances there are substantial differences in monomer reactivity. Some monomers, (e.g. butene-2) do not homopolymerize but form copolymers with reactive monomers such as ethylene. 

The proportions of the triads vary with the monomer feed. As shown in Fig. 15.2, the proportions of the experimentally found triads agree well with the values calculated from Eq. (15-16a) using r obtained from the content of comonomers in the copolymer (method from Ref. 6)). This agreement means that in this system the penultimate effect, reversibility of propagation, interconversion of active species without propagation [Eq. (15-4)] and redistribution of the initially formed copolymer can be neglected and the process can be described by two reactivity ratios. 

Reactive Compatibilization involves a heterogeneous reaction across a phase boundary. Such a reaction is limited by the interfacial volume available at this phase boundary. Most often, twin screw extruders (having screw diameter from 20 to >120 mm) are employed. The screws are designed using an appropriate sequence of screw elements and auxiliary conditions to promote generation of a large interfacial area for the desired chemical reaction to form copolymer. 

Obviously, because of the difference in the reactivity of styrene and DVB, the networks prepared by free radical copolymerization do not relate to such an ideal system with uniform distribution of DVB units and constant chain lengths between the junction points. Also, it was not possible to eliminate this serious defect by an anionic copolymerization of the comonomers. The anionic copolymerization has often been initiated by n- or sec-hutyl lithium [110-112]. Under such conditions, styrene is consumed faster than p-DVB, the monomer reactivity ratios being ri = 1.5S and r2 = 0.32. Therefore, DVB-enriched domains wUl form toward the end of the anionic process. On the other hand, the styrene—m-isomer reactivity ratio (r = 0.65 and r2 = 1.20) points to the local incorporation of m-DVB crosslinks into the initially formed copolymer [113, 114]. In addition, the anionic process is also accompanied by intramolecular cycUzation, similar to radical styrene DVB copolymerization [115,116]. 

Various copolymers of ethylene with vinyl acetate are prepared by free-radical mechanism in emulsion polymerizations. Both reactivity ratios are close to 1.0 [106]. The degree of branching in these copolymers is strongly temperature-dependent [107]. These materials find wide use in such areas as paper coatings and adhesives. In addition, some are hydrolyzed to form copolymers of ethylene with vinyl alcohol. Such resins are available commercially in various ratios of polyethylene to poly(vinyl alcohol), can range from 30% poly(vinyl alcohol) to as high as 70%. 

Table 22-2. The Sequence Distribution of Monomeric Units mi in the Initially Formed Copolymer for Different Reactivity Ratios, Assuming a Monomer Composition of 1 1...

Generic Processes and Specific Types of Reactions to Form Copolymer in a Reactive Compatibilization Process... 

Some polymers contain reactive functionality but are also themselves subject to mechanochemical radical generation. When such polymers are blended under high-shear mixing with a second functionalized polymer, the architectures of formed copolymer may derive both from the primary, expected reaction and also from an unexpected, radical-radical coupling process. 

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