Polymerization groups produced, effect

The concept of PO macroinitiators centers on the introduction of an initiation moiety into an olefinic polymer chain for polymerization. The most effective route for preparing PO macroinitiators is by employing functional polyolefins containing hydroxyl groups or other reactive groups. These functional POs are prepared by copolymerization of olefins with functional monomers and post-polymerization reaction, as mentioned above. In the case where an initiation moiety was at the chain-end of the polyolefins, a block type copolymer is produced. It has been reported that thiol-terminated PP was used as polymeric chain transfer agent in styrene and styrene/acrylonitrile polymerization to form polypropylene-b/odc-polystyrene (PP-b-PS) and polypropylene-btock-poly(styrene-co-acrylonitrile) (PP-b-SAN) block copolymer [19]. On the other hand, polymer hybrids with block and graft structures can be produced if initiation moieties are in the polymer chain. 

Polymerizations of this type in solution produce predominantly shorter than 5-mer oligonucleotides with a mixed and variable composition of 2, 5 and 3, 5 linkages (86). The distribution of products is highly sensitive to reaction conditions, including the presence of metal ions, modification of the leaving group, and sequence context. The addition of the clay montmo-rillonite to otherwise similar reactions has a favorable effect on polymerization (87). The effect is likely due to the concentration of monomers within the clay s cationic layers. These... 

The effect of drying temperature and pressure of Mg on the catalytic activity can be seen clearly by looking at Fig. 3 for polymerization runs at 5 atm. monomer pressure. As published before (22), silica dried at 600"C and having Isolated hydroxyl groups produce catalysts with higher activities, compared with those prepared on silica dried at 150 C. In both cases, acceleration-type kinetic curves were obtained with maximum activities (A ) of 2000 and 200 gPE x gTi x h x atm." respectively. 

They also prepared silica-immobilized imidazolium ILs of different alkyl groups and used them as the catalysts for the same copolymerization [125]. Although the immobilized IL catalysts were easily separated by filtration after the polymerization, they produced copolymers whose Mn and f(C02) were smaller than those of the copolymers prepared with the corresponding homogeneous IL catalysts. Effects of the alkyl chain length of IL and the reaction conditions on the nature of the copolymer synthesized showed similar trends to those observed with the homogeneous ILs. The immobilized ILs could be reused up to four consecutive runs without considerable loss of activity and f(C02). 

Another innovative approach to initiating the polymerization of acrylic structural adhesives is illustrated by a family of adhesives that uses the adherend surface itself as part of the chemical reaction that generates the free radical initiator. The compositions contain an acidic material, a sulfonyl halide group, and a transition metal compound. Upon contact with a metal surface such as iron, zinc, copper, cadmium, and their alloys, the components react with the metal surface through a free radical pathway, producing effective initiators for the polymerization of the acrylic monomers. Later extensions of this concept provide two-part adhesives employing... 

Chain Transfer. A number of materials act as tme transfer agents in THF polymerization notable examples are dialkyl ethers and orthoformates. In low concentrations, water behaves as a transfer agent, and hydroxyl end groups are produced. The oxygen of dialkyl ethers are rather poor nucleophiles compared to THF and are therefore not very effective as transfer agents. On the other hand, orthoformates are effective transfer agents and can be used to produce alkoxy-ended PTHFs of any desired molecular weight (169). 

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