Molecular weight distribution addition polymers

Figure 4B. Molecular weight distribution of polymer from slow initiation or programmed continuous initiator addition.

Anionic polymerization is widely used to prepare polymers with narrow molecular weight distribution. Addition of styrene to the living poly(l,l-dialkylsilabutane)s provided a poly(l,l-dialkylsilyl-/ -styrene) block copolymer (Scheme 11) in 99% yield with MJMn = 1.19 C1997PSA3207, 1995MM7029>. 

Big. 34. Molecular weight distribution of polymers formed by continuous addition of monomer to living oligomers solution containing some terminating impurities... 

J. Pojman, J. Willis, D. Fortenberry, V. Ilyashenko, and A. Khan, Factors affecting propagating fronts of addition polymerization velocity, front curvature, temperature profile, conversion and molecular weight distribution, J. Polym. Sci. Part A Polym Chem., 33 (1995), pp. 643-652. 

Polymers are found in such a large variety of products that they have shaped modern life. The extraordinary versatility of polymers in terms of end-use properties is due to the variety and complexity of the microstructure of the polymeric material. The polymeric material includes both the polymer and the additives with which it is compounded. The microstructure of the polymeric material is determined by the molecular and morphological characteristics of the polymer itself, the way in which the polymer is processed and the additives used for compounding (Figure 1.1). The molecular characteristics of the polymer include chemical composition, monomer sequence distribution (MSD), molecular weight distribution (MWD), polymer architecture, chain configuration and morphology. 

Simultaneous control of stereosequence and molecular weight distribution has long been one of the holy grails in the field of radical polymerization. Nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and RAFT all offer control over molecular weight distribution. However, polymers produced by these methods show similar tacticity to those obtained by the conventional process. Recently there have been reports of tacticity control of homopolymers " (which enables the synthesis of stereoblock copolymers ) and control of the alternating tendency for copolymerizations in ATRP or RAFT polymerization through the use of Lewis acids as additives. 

In Table 6.2 a number of rheological and thermal properties have been tabulated for several important generic polymers. These data have been gathered from numerous sources, including the author s own measurements. The data should be used as estimates only, because measurement techniques may differ and because considerable differences in properties can occur in one particular polymer as a result of variations in molecular weight distribution, additives, thermomechanical history, etc. Actual measurement of polymer properties should always be preferred above published data. However, actual measurement is not always possible, in which case the table may provide useful information. 

Pojman, J. A. Willis, J. Fortenberry, D. Ilyashenko, V. Khan, A. 1995b. Factors Affecting Propagating Fronts of Addition Polymerization Velocity, Front Curvature, Temperature Profile, Conversion and Molecular Weight Distribution, J. Polym. Sci. Part A Polym. Chem. 33, 643-652. 

The Poisson distribution of the molecular weight of addition polymers (which do not have termination) is given by... 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Using both condensation-cured and addition-cured model systems, it has been shown that the modulus depends on the molecular weight of the polymer and that the modulus at mpture increases with increased junction functionahty (259). However, if a bimodal distribution of chain lengths is employed, an anomalously high modulus at high extensions is observed. Finite extensibihty of the short chains has been proposed as the origin of this upturn in the stress—strain curve. 

In addition to this drive to look beyond manufacturing to specifications, new analytical methods such as molecular weight distribution, Mooney relaxation, and other measures of polymer processibiHty are being explored. 

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

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