Monomer-to-initiator ratio

Multiple covalent bonds are formed in each macromolecule and, in general, statistical, polydispersed structures are obtained. In the case of controlled vinyl polymerizations, the average length of the macromolecule is determined by monomer to initiator ratios. If one views these polymerizations as extraordinarily long sequences of individual reaction steps, the average number of covalent bonds formed/chain may be visualized as shown in Scheme 2 ... 

There are numerous reports available on the optimization of reaction conditions of 2-oxazolines. For instance, the effect of solvent, temperature, pressure, monomer to initiator ratio, and many other critical parameters have been investigated to obtain the optimum conditions [64-68]. Besides these parameters, the initiator structure has also a great effect on the polymerization. The investigation on different initiator structures provided the necessary kinetic parameters for the use of functional initiators [69]. Heterofunctional initiators have been used in polymer science for the combination of different types of monomers that can be polymerized with different polymerization techniques, such as ATRP and CROP [70-72]. 

However, the ROP of lactide and glycolide remains by far the most extensively practiced method in industry and academia for the preparation of polyesters. This method allows a better control of polymerization in terms of high molecular weight, narrow polydispersity, monomer-to-initiator ratio and sequence, well-defined... 

The concept of using group I metal initiators was applied in order to minimize the toxicity generated by heavy metal residues in the end product PLAs when using metals like aluminum, tin, and lanthanides as initiators. In recent years, dinuclear lithium and macro-aggregates with phenolate ligands have attracted substantial interest, mainly due to uncommon strucmral feamres and their ability to catalyze formation of polyester and various other polymeric materials via ROP [28]. A series of lithium complexes supported with 2, 2-ethylidene-bis (4, 6-di-tert-butylphenol) (EDBP-H2) 2-6, (Scheme 6) are excellent initiators for the ROP of L-lactide in CH2CI2 at 0 °C and 25 °C [33-35]. In this case, the PDIs of the obtained PLAs were quite narrow (1.04—1.14) and a Unear relationship between and the monomer-to-initiator ratio ([M]o/[I]o) existed at 0 °C. Dimeric complexes 4 and 6 were the... 

Monomer-to-initiator ratio [M] [I] = 1000 1 Molar feed ratio = LA/GA/TEG Molar composition = LA/GAATEG... 

The discovery of high molecular-weight synthetic polypeptides came as a result of systematic studies by Blout and his co-workers of the conditions which limit the degree of polymerisation of products formed by polymerisation of NCA. Contrary to previous beliefs, it was found that the polymerisation initiated by n-hexyl amine in dry dioxane yields polypeptides of higher molecular weights than expected from the monomer-to-initiator ratio, provided M/I < 100. It should be stressed again that this observation demonstrates that some additional modes of propagation contribute to the reaction, i. e. the "simple poly-addition mechanism cannot account in toto even for the polymerisation initiated by primary amines in dioxane. 

Fig. 2. The influence of l-LA monomer conversion on the number-average molecular weight (O) and the MWD ( ). Polymerization conducted at 60 °C in chloroform with an initial monomer-to-initiator ratio of 100 1...

Fig. 3. The semi-logarithmic plot of -ln([M]/[M]0) versus the reaction time for different monomer-to-initiator ratios. M/I=B 25, 35, 50, 100, 250,0 360, 400...

Table 1 Polymerization of monomers I-n, Il-n and Ill-n with initiator 1 (data given for monomer to initiator ratio= 1 100 for other ratios see references) ...

The monomers XXIV-m-n were polymerized with initiator 4 in a monomer to initiator ratio of about 50. Table 16 summarizes the physico-chemical data for these polymers. Poly-(XXIV-m-n) exhibited smectic C mesophases, but these phases were only observed over a small temperature range and isotropization occurred shortly after the glass transition. It was not possible to identify the mesophase by polarized optical microscopy, but the smectic phase was confirmed by X-ray scattering experiments. Transition temperatures increased with the length of the carbon segments (n) but decreased with increasing siloxane segment (m). 

ROMP of the SCLCP block was carried out with the Grubbs -initiator 6 in a monomer to initiator ratio of about 25. The polymerization was terminated by reaction with 4-(2-bromopropionyloxy)-but-2-enyl 2-bromopropionate, leading to a macroinitiator for ATRP. The radical polymerization was carried out with CuCl, 4,4 -di(n-nonyl)-2,2-bipyridine, and butylacrylate (BA), giving a poly-XXVII-fo-BA copolymer. The homopolymer of poly-XXVII had a narrow PDI of 1.06, while the diblock copolymer showed a PDI of 1.32. 

The ROP of lactide affords high molecular weight PLA polymers with better control of the polymerization process relative to polycondensation. These advantages can be directly attributed to the fact that ROP can be a living polymerization process. Living polymerization is a chain-growth polymerization where chain termination is absent and is characterized by a linear relationship between the monomer to initiator ratio and the experimental molecular weight, and narrow dispersity indicates the... 

Integrating the rate of propagation, given by Eq. (8.30) and assuming [M]sp [M] for high polymers, that is, high monomer to initiator ratio, one obtains... 

As indicated earlier, the initiating system should give products with desired Mns at a practically acceptable rate. The rate of THF polymerization is highest with CFjSOjH as initiator, however, the molecular weights are considerably higher than those given by the monomer to initiator ratio because some of the acid remains unconsumed. 

Thus, during functionalization of PO blends containing styrene polymers, the grafted functionalized groups concentrate within the aliphatic phase. The course of secondary reactions (cross-linking and degradation) is controlled, such as in the case of grafting to neat PO, by variations in the monomer-to-initiator ratios (3). 

Methacrylate functional PIB macromonomers have been synthesized by Hv-ing carbocationic polymerization of IB using the 3,3,5-trimethyl-5-chloro-l-hexyl methacrylate (6)/TiCl4 initiating system in hexane/MeCl 60/40 (v/v) [110, 150]. By varying the monomer to initiator ratio, PIBs in the molecular weight range of 2000-40 000 g moU were obtained with narrow MWDs and close to theoretical ester functionality. 

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