Incremental monomer addition

To prove that under these conditions, the IB polymerization is living, a monofunctional analogue of 1,2-p-methoxyphenyl-2-methoxypropane, was used to study the kinetics by incremental monomer addition technique. Results of this study indicated Hving polymerization with slow initiation [61,62]. 

Having demonstrated that the polymerization could be continued by incremental monomer addition, studies of the reaction products of such experiments were made. The results of one such study are shown in Table VI. It shows a comparison of the observed and expected number-average molecular weights for an experiment involving two incre-... 

The initial charge ratio and incremental monomer additions required to produce a constant composition 90-10 methyl methacrylate-< -methyl-styrene are shown in Figure 3. 

Most controlled/living cationic systems provide well-defined polymers of relatively low molecular weight, typically in the range M = 5,000 to 20,000. The degrees of polymerization are equal to the ratio of concentrations of the reacted monomer to that of the introduced initiator (DP = d[M]/[I]0), and the polydispersities remain relatively low, MJMn < 1.2. The addition of new portions of monomer (sometimes called incremental monomer addition, IMA) [5] leads to the expected increase of molecular weights, and the addition of another comonomer results in block copolymers, whereas the addition of terminating reagents provides end-functionalized macromolecules (cf. Chapter 5). 

As mentioned in Sect. 2.2.1.1 slow initiation is controlled either by the rate of ion-generation or cationation (cf. Eqs. (1)—(4). Based on earlier results [58,59] a convenient diagnostic method has been developed [1,29] to elucidate the nature of the rate determining step for rapid carbocationic polymerizations and in IMA (incremental monomer addition) experiments. For only one monomer addition, i.e., for AMI (all monomer in or for the first monomer addition in IMA) experiments, the relevant expressions are as follows ... 

Already before reporting this combined inifer and living polymerization approach, Kennedy and coworkers developed a controlled isobutene polymerization method based on cumyl ester initiators (Scheme 8.6) with boron trichloride as activator and incremental monomer addition [28], The livingness of the polymerization was demonstrated by the linear increase of number-average molar mass and the constant number of polymer chains (A) with the amount of PIB obtained (wp, as measure for conversion) as well as the narrowing of the molar mass distribution with conversion (Fig. 8.1) [28]. 

A series of high-cis B-I-B triblock copolymers were prepared with lanthanide catalysts by incremental monomer addition. TTie hydrogenated products lead to E-(E/P)-E triblock copolymers with hard (semi-crystalline) ethylene block at both chain ends and soft (amorphous rubbery) alternating E/P block in the center segment. 

Incremental monomer addition studies, bearing on the mechanism of initiation and termination, were done under a nitrogen atmosphere in bottles. The bottles were fitted with a perforated cap having a butyl gasket which was extracted using a mixiure of ethanol with 32 by weight of toluene. These polymerizations were carried out at an initial catalyst concentration of O.OW weight percent and at 50°C. 

Figure 5. Gel permeation chromatograms of poly-(propylene ether) dials prepared by incremental monomer addition...

Semibatch processes with incremental monomer additions are particularly useful in copolymer production through free-radical polymerization when the reactivities of comonomers vary widely. For a binary copolymerization system, the mass balances for the two monomer types are... 

The solvent and initiator are charged to the reactor and heated to reflux (ca 80°C). Forty percent of the monomer charge is then added. The remainder of the monomer is added in four equal increments at 24, 50, 79, and 110 min after addition of the initial monomer charge. The reaction mixture is kept at reflux overnight, then cooled and packaged (96). 

The existence of an azeotropic composition has some practical significance. By conducting a polymerization with the monomer feed ratio equal to the azeotropic composition, a high conversion batch copolymer can be prepared that has no compositional heterogeneity caused by drift in copolymer composition with conversion. Thus, the complex incremental addition protocols that arc otherwise required to achieve this end, are unnecessary. Composition equations and conditions for azeotropic compositions in ternary and quaternary eopolymerizations have also been defined.211,21... 

Clear impact-resistant polystyrene is a commercial plastic with the desirable combination of toughness and exceptional clarity. It is a styrene-1,3-butadiene multiblock copolymer containing more than 60% styrene. Most of these products are mixtures of block copolymers formed by incremental additions of initiator and monomers followed by coupling (Sec. 5-4c). The products generally have a tapered and multiblock composition with branching (due to the coupling agent). 

Use of these polar randomizers also increases the vinyl unsaturation in the copolymer. Butadiene-styrene random copolymers can also be prepared by a very slow and continuous addition of monomers22. or by an incremental addition of butadiene to a styrene-rich monomer mixture during polymerization. These two... 

Figure 8 shows the characterization of these poly (propylene ether) diols by gel-permeation chromatography. There is a shift in the peak position to lower elution volumes, in accord with an increase in molecular weight with each monomer increment polymerized. The molecular-weight distributions of the three diols are similar and remain narrow after the addition of monomer increments. Since all of the molecules apparently continued to grow, this polymerization must proceed with very little chain termination under these conditions. 

Thus, the use of alkyllithium initiation offers the synthetic chemist a tool of enormous flexibility for "tailor-making" polymers of precise structure. Control of molecular weight, molecular-weight distribution, diene structure, branching, monomer-sequence distribution, and functionality can conveniently be achieved by such techniques as incremental or sequential addition of monomer, initiators, or modifier, programming of temperature, continuous polymerization, or the use of multifunctional reagents. 

However, in some cases, additional effects on (dn/dc)copo must be considered. Due to cooperative interactions between the monomer units in the polymer chain, copolymer refractive index increments may deviate from the summation scheme. As a result of different sequence length distributions, different values of (dn/dc)copo can be obtained for the same gross composition. Copolymer (dn/dc)copo values can be obtained by multiple detection SEC providing the chemical composition at each slice of the elution curve. 

Growing the 2000-A latex by multiple additions involving surfactant, monomer, catalyst, and a chain transfer agent to control the molecular weight was successful. Each increment was added every 26 hr for three increments from the start of the polymerization. The first two additions consisted of fresh monomer (not exceeding a 2 1 monomer to polymer ratio) with 0.25 phm each of catalyst and chain transfer agent. The first addition also included half of the total surfactant used the second and third had catalyst alone. 

Consider computing the excess chemical potential for a chain molecule composed of M monomers, denoting such a molecule type by a - The longer the chain, the more difficult the computation. It is easier to compute the incremental chemical potential for addition of a single segment. Following the notation discussed in Section 1.3, p. 16,... 

To achieve optimal properties in an AB block copolymer, it is important to control the molecular weight of the blocks, and minimize the amount of A homopolymer produced on addition of the second monomer. Termination reactions do occur in these systems [20], but the rate is fairly slow, particularly at temperatures below about 100 °C. In a practical sense, protic impurities present a much greater challenge. In a two-reactor system it is common practice to prepare the first block in one reactor, titrate out impurities in the B monomer charge in a second reactor by adding small increments of butyllithium to a solution of the B monomer until the first sign of color or exotherm, and then the transfer poly(A)Li solution to the second reactor. 

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