Copolymer addition, preformed

Sterically stabilised vesicles prepared with the addition of triblock copolymers of the poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO) type, namely Poloxamers or Pluronics (PEO-PPO-PEO), have shown enhanced stabilisation [43]. Steric stabilisation of phospholipid vesicles by the copolymer molecules has been attempted by two different techniques (1) Addition of the block copolymer to preformed vesicles (method A) and (2) addition of the block copolymer to the lipid before formation of the vesicles (method I). In the latter, both the lipid and copolymer participate in the construction of the vesicle. A schematic picture of the resulting vesicle structure for the two methods is given in Figure 13.28. 

Among the preformed polymers cured by minor additions of aHyl ester monomers and catalysts followed by heat or irradiation are PVC cured by diallyl fumarate (82), PVC cured by diallyl sebacate (83), fluoropolymers cured by triaHyl trimeUitate (84), and ABS copolymers cured by triaUyl trimeUitate (85). 

Addition reactions were frequently used to create MAIs capable of forming block copolymers. Thus, one possible pathway is to react preformed polymer-contain-... 

An outstanding property of EPS is its extremely low density (when compared to other processes), that by alteration of the preforming treatment can be varied according to the end use. Other types of plastics are employed to produce expandable plastic foam (EPF), including PE, PP, PMMA, and ethylene-styrene copolymers. They can use the same equipment, with only slight modifications. These plastics have different properties from those of EPS and open up different markets. They provide improved sound insulation, resistances to additional heat deformation, better recovery of shapes in moldings, and so on. 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

The possible chain transfer reactions between the initiator radical, the activated monomer, and a growing chain of the monomer applied on the one hand and the preformed polymer on the other are given under (I). The graft copolymer originates either from addition of the monomer applied to the polymer radical formed by the chain transfer (reaction II) or from recombination of two polymer radicals (reaction III), the latter one being less probable. 

In addition to block copolymer synthesis by subsequent polymer growth along one polymer chain, Eq. (21), and the reaction sequence of Eqs. (19, 22—24), preformed polymer blocks can be linked via reactive end groups. Polynorbomene with one titanacyclobutane end group was reacted in a Wittig-type reaction with... 

Interest in block copolymers is based on their capacity to combine in an additive manner the properties of the constitutive blocks. They can be synthesized by coupling of the preformed blocks or by sequential polymerization of the corresponding monomers. 

Polymer-supported triphenylphosphine ditriflate (37) has been prepared by treatment of polymer bound (polystyrene-2% divinylbenzene copolymer resin) triphenylphosphine oxide (36) with triflic anhydride in dichloromethane, the structure being confirmed by gel-phase 31P NMR [54, 55] (Scheme 7.12). This reagent is effective in various dehydration reactions such as ester (from primary and secondary alcohols) and amide formation in the presence of diisopropylethylamine as base, the polymer-supported triphenylphosphine oxide being recovered after the coupling reaction and reused. Interestingly, with amide formation, the reactive acyloxyphosphonium salt was preformed by addition of the carboxylic acid to 37 prior to addition of the corresponding amine. This order of addition ensured that the amine did not react competitively with 37 to form the unreactive polymer-sup-ported aminophosphonium triflate. 

When a copolymer seed is preformed and then a preemulsified mixture of vinyl acetate and butyl acrylate in water (using sodium dodecylbenzenesulfo-nate, Siponate DS-10, as surfactant) is added slowly, the composition of the product approaches that calculated from the reactivity ratios. The particle size and the size distribution is related to the ionic strength of the medium. With low levels of initiator and buffer, the particle size tends to be small and the size distribution narrow. As ionic strength increases, the particle size increases. At low monomer addition rates, the particle size increases. Typically, particle sizes range fi"om 0.206 to 1.05/im with coefficients of variation ranging from 10.5 (for the smaller particles) to 5.9 (for the larger ones) [173, 174]. 

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