Homopolymer formation during

Table III. Homopolymer Formation During UV Grafting of Methyl Methacrylate (30% v/v) in Methanol to Cellulose"...

To avoid homopolymer formation, it is necessary to ensure true molecular contact between the monomer and the polymer. Even if this is initially established, it needs to be maintained during the radiation treatment while the monomer is undergoing conversion. Several methods are used for minimizing the homopolymer formation. These include the addition of metal cations, such as Cu(II) and Fe(II). However, by this metal ion technique, both grafting and homopolymerization are suppressed to a great extent, thus permitting reasonable yield of graft with little homopolymer contamination by the proper selection of the optimum concentration of the inhibitor [83,90,91]. 

Dependence of grafting efficiency and monomeric conversion on reaction time are shown in Figures 2 and 3. It is shown that a rapid polymerization takes place during the first 2 hrs of the reaction the rate then decreases, and after about 8 hrs the reaction almost ceases. It is interest to note that grafting efficiency is directly proportional to time during the first 2 hife of the reaction and after that remains constant at about the 85% level. It seems that the homopolymer formation (see diagrams II-III) is remakably suppressed at pH 5 at which there is not very much ferrous ion in solution to form free OH radical and polymerization takes place mostly on partially xanthated cellulose. 

Shen X-W, Yang Y, Jian J, Wu Q, Chen G-Q (2009) Production and characterization of homopolymer poly(3-hydroxyvalerate) (PHV) accumulated by wild type and recombinant Aeromonas hydrophila strain 4AK4. Bioresour Technol 100 4296-4299 Shenoy SL, Bates WD, Frisch HL, Wnek G (2005a) Role of chain entanglements on fiber formation during electrospinning of polymer solutions good solvent, non-specific polymer-polymer interaction limit. Polymer 46 3372-3384... 

The first arises from the susceptibility of Si—O—Si linkages to undergo scission and bond re-formation by the action of strong bases (equations 26a and 26b). These processes can lead to a randomization ( shuffling or equilibration) of siloxane units between polysiloxane chains and hence to a broadened distribution of MW s and often to the liberation of polysiloxane homopolymer, either during synthesis or on subsequent contact with bases. Care has therefore to be taken in the selection of basic initiators or catalysts for use in polymerization reactions in this series. 

In any application of a copolymer the rate of formation of the product, its molecular weight, and the uniformity of its composition during manufacture are also important considerations. While the composition of a copolymer depends only on the relative rates of the various propagation steps, the rate of formation and the molecular weight depend on the initiation and termination rates as well. We shall not discuss these points in any detail, but merely indicate that the situation parallels the presentation of these items for homopolymers as given in Chap. 6. The following can be shown ... 

Transfer reactions, analogous to those discussed for homopolymer, also occur during the copolymerization reaction and result in formate end groups. 

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

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