Homopolymerization, mixed

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

The compatibilization of clay with LDPE and HDFE is accomplished by the in situ polymerization of MAH or its precursor maleic acid, in the presence of a radical catalyst. The latter must be capable of initiating the homopolymerization of MAH, i.e. it must be present in high concentration and/or have a half-life of less than 30 min at the reaction temperature, e.g. t-butyl per-benzoate (tBFB) at 150°C. In a one-step process, the clay and PE are mixed with MAH-tBPB in the desired PE/clay ratio. In the preferred two-step process, a 70/30-90/10 clay/PE concentrate is prepared initially in the presence of MAH-tBPB and then blended with additional PE to the desired clay loading. The compatibil-ized or coupled PE-MAH-clay composites have better physical properties, including higher impact strengths, than unfilled PE or PE-clay mixtures prepared in the absence of MAH-tBPB. 

Monomer 12 is a crystalline solid which, when homopolymerized, affords a high Tg, thermally stable polymer that has potential application in both the microelectronic and aerospace areas. Monomer 13 (mixed isomers) is a liquid at room temperature and, when homopolymerized, also provides a high Tg, thermally stable polymer that has a low moisture uptake and a low dielectric constant. This polymer has been targeted into the microelectronics area because of this interesting set of properties, combined with the prepolymer s unique ability to planarize over underlying topography. 

Aluminum-porphyrin complexes, in epoxide homopolymerization, 11, 599 Aluminum(III)-sulfur bonds, mixed covalent and non-covalent systems, 9, 258... 

Early studies on the homopolymerization of E-l,3-pentadiene yielded polymers with a high cis-1,4-content and an isotactic structure, whereas E-2-methyl-l,3-pentadiene resulted in a polymer with a mixed czs-1,4/transit-structure [487-492]. Investigations on the polymerization of E-1,3-pentadiene with the system NdN/TIBA/DEAC partially support these findings as a poly(l,3-pentadiene) with a cis- 1,4-threo-disyndiotactic structure was obtained [492]. A somewhat lower cis- 1,4-content of 70% was obtained when the polymerization of E-l,3-pentadiene was catalyzed by (CF3COO)2NdCl/TEA [493,494]. When 2,3-Dimethyl-1,3-butadiene is polymerized with the catalyst NdN/TIBA/EtAlC the resulting poly(2,3-dimethyl-butadiene) predominantly contains cis-1,4-units [495,496]. 

When one is using primary amines, for example diethylene triamine (DETA), the mix ratio should be closer to stoichiometric amounts. However, there is always a percentage of homopolymerization in practice, especially at the temperature of reaction. Smaller amounts of DETA than stoichiometric, therefore, will cause a complete cure. But this will generally occur at the expense of increasing brittleness. 

Spontaneous 1 1 copolymerization has been noted when sulfur dioxide was bubbled through bicycloheptene at —40°C. (88), when isobutylene was bubbled through methyl a-cyanoacrylate (54), when 1,3-dioxole was mixed with maleic anhydride (17), and when vinylidene cyanide was mixed with styrene (20), the latter reactions at room temperature. None of these monomers undergoes homopolymerization under the same experi-... 

The high tendency to homopolymerization of the 4-acetoxybenzoic acid relative to transesterification is responsible for the nonuniform distribution. In order to obtain copolymers with HBA units randomly distributed along the chains, the monomer 4-acetoxybenzoic acid may be added in portions rather than in a singla batch. Unitika (Suenaga and Okada, 1989) advanced the production process so that the HBA and PET moieties are uniformly distributed in the copolymers. With the advanced process the resins ( Rodrun LC-5000 ) showed almost 100% solubility in a hot (150 °C-160 °C) 50/50 mixed solvent of tetrachloroethane and phenol, while that produced by the old (Jackson and Kuhfuss, 1973) method has an insoluble residue of about 30 wt-%. The 13C-NMR spectra also showed that... 

Copolymerization of THF with DXL proceeds in the unusual way. When these two monomers are mixed in solution and the initiator is added, some polymerization of THF is observed at the very beginning 35). Then DXL polymerizes and throughout its polymerization the rate of THF polymerization is low. When DXL reaches its equilibrium concentration, THF polymerizes until its concentration becomes slightly lower than its homopolymerization [THF]e under similar conditions (cf. Fig. 15.3)35,... 

We measured the heat of copolymerization in emulsion for copolymer containing 13.5 mole % styrene at 60°C. Measurements were made with a styrene feed at 20°C therefore, some reaction heat was used to warm the styrene to 60°C. Experiments indicated that this effect was partially compensated for by the heat of mixing because the mixing of styrene with the reactants was very slightly exothermic. If the styrene were preheated to 60°C, the effect would be to raise the measured heat of copolymerization by about 0.09 kcal./ mole. A value of 20.2 kcal/mole (i.e., 337 cal/g) was obtained for the heat of copolymerization. In order to elucidate this unexpectedly high finding, we also measured the heat of homopolymerization of acrylonitrile under similar emulsion conditions in the same apparatus a value of 22.1 kcal/mole was obtained. 

Different monomers are mixed, then homopolymerized and cross linked simultaneously... 

Processive DNA synthesis is measured on either homopolymeric or heteropoly-meric (natural) DNA templates. We have used primed M13 DNA below, although it tends to produce a less uniform distribution of products that reflects sequence-dependent pause sites unique to each DNA polymerase. In the protocol described below, we have also used a DNA trap (unlabeled activated DNA) to trap dissociated polymerase molecules, thereby ensuring that DNA substrates virtually never react with more than one enzyme during the reaction. To use a trap, the polymerase and DNA substrate are mixed in the absence of a necessary reaction component, such as dNTPs. To start the reaction, the dNTPs are added, along with a large excess of the trap. The polymerase molecules already bound to the DNA substrate carry out one round of processive synthesis and then dissociate and bind the trap. 

In addition to the homopolymeric systems discussed above, a wide variety of mixed copolymeric systems incorporating isothianaphthene, thieno[3,4-i ]pyrazines, and related materials have also been reported. The most common approach is the endcapping of a tailored fused-ring system (e.g., isothianaphthene) with external thiophene units to produce symmetrical mixed terthienyl precursors, which can in turn be polymerized by oxidative polymerization to produce the desired copolymeric material. 

A key consideration in conducting polymerization is maintenance of the proper recipe or ingredient mix. In most homopolymerizations and all copolymerizations, more than a single ingredient is required in the reacting mass. For example, in some emulsion polymerizations as many as 20 different components must be fed into the reaction at controlled concentrations. 

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