Polymers with broad MMD

In the literature, various reasons for formation of polymers with broad MMD on heterogeneous ZN catalysts are discussed. Convincing evidence has been obtained using the SF method that the reasrni is heterogeneity of the active centers on a surface of the catalyst [186]. In conditions of quasi-living polymerization there are no transfer reactions of the growing polymer chain and polymer is formed on the surface of catalyst in very small quantities. This polymer cannot cause diffusion restrictions, but nevertheless polymer with broad MMD (Mw/M = 3.2-4.3) is formed. The further increase in time of polymerization does not influence the width of MMD (M ,/M = 3.6). 

It is known that ZN catalysts are multisite catalytic systems containing active centers that differ in their kp values and stereospecificity. This is apparent in the formation of polymers with broad molecular mass distribution (MMD, MJMn > 2) and polyolefins containing separate fractiOTis with varying stereoregularity. Data on the reactivity (/ p values) of separate groups of the active centers and their transformations with the variation in the compositiOTi of catalysts and polymerization conditions have an undoubtedly important role in analysis of this phenomenon. 

If we take into account that the degradation proceeds via the random law then the equilibrium width of MMD should approach value of 2 only after 2-3 breaks. However this is in contrast to the experimentally obtained data illustrated in Fig. 39b. It is seen that MMD depends on the conversion degree and approaches to 1. This effect could be explainded by the variation of the ratio of the values of the elementary constants, which characterize the breaking of C-C bonds in the macromolecule. A decrease of Wj has been registered at the transition to lower molecular polymers, as is the case with fractionated PS and with pol)nners with broad MMD. This might be the explanation of the decrease of the effective upon ozonoly-sis and the narrowing of the MMD [71-73, 108, 111-114]. 

As can be seen from Table 14 the addition order affects reaction rate, molar mass and PDI. The authors suggested that in-situ activation results in the formation of two types of active species the relative concentrations of which were governed by the addition sequence. Addition order (1) EASC + NdV + DIBAH promotes the formation of insoluble species which produced polymer with a broad, bimodal MMD (PDI = 7.5) at low catalyst activity. Addition order (3) DIBAH + NdV + EASC leads to the formation of more soluble catalyst species which exhibit increased catalyst activity and produce BR with a monomodal MMD (PDI = 3.4). The influence of the addition order on cis-1,4-content is negligible. 

The field of application of Osmometry is limited to polymers with masses below 100,000, since the precision depends on the mass, and therefore it becomes inaccurate high masse Furthermore, being a colligative method, osmometry yields Mn but not Mw, and therefore it cannot be used to discriminate a narrow MMD from a broad MMD nor a unimodal MMD from a bimodal MMD. 

Clearly, the characterization of a broad-MMD calibrant requires a considerable amount of effort and expertise, and they have been mainly used where there are problems in producing narrow-MMD calibrants. In practice, the main applications of this approach have been the use of NBS Standard Sample 706 (polystyrene) and, in particular, NBS Standard Sample 1475 (linear polyethylene). SEC has been a very valuable technique for examining the MMDs of polyethylene, but as there has only been limited success in producing narrow-MMD polyethylene calibrants, NBS 1475 has been widely used for calibrating high-temperature SEC systems used for this application. Unfortunately, the molecular mass range of NBS 1475 is rather low and is not really appropriate for many samples. Barlow et al [5] used NBS 1475 and extended the calibration range with additional polymers (see Chapter 4, section 4.5.1.1). 

In the case of polycarbonate and other polymers which possess a broad MMD, MALDI fails to give reliable Mn and Myf values. MALDI underestimates The Mn value falls invariably too close to so that MALDI underestimates the polydispersity index. Generally, MALDI cannot be applied to polymers obtained by free-radical polymerization, since free-radical processes yield polymers which possess a broad MMD. For instance, we recorded [62] the MALDI spectrum of a PMMA sample with a polydispersity index of around 2.5 (the averages were % = 13 kDa and = 33 kDa)... 

Broad MMD HomopolymGrs. Early in the studies on MALDI of synthetic polymers, it was hoped that MALDI could be used successfully for both narrow and broad MMD. Soon it was discovered that for a variety of reasons the correct MMD of broad MMD polymers could not be obtained in one step by MALDI-tof-ms. Good agreement between MALDI and gpc for MMD moments for polymers with polydispersities less than Mw/Mn = 1.1-1.2 was found (50). Polymers with higher polydispersities regularly showed incorrect moments of the MMD (50,66). 

Zimm-Schulz distribution (88,89). Using an analysis method similar to that used previously on the poly(Q -MeSty)-6Zoc -poly(4-vinyl pyridine) system, the MMD of both parts of the copolymer were determined. The data analysis method was claimed to verify the random coupling hypotheses. The hypothesis (90) that the polydispersity of individual blocks is higher than the polydispersity of the whole polymer was confirmed (85). That is, block copolymers with narrow MMD have broad complex chemical composition distribution. 

A serious contradiction with the requirements of a strictly Uving polymerization are broad or even bimodal MMDs which are in the focus of many studies, e.g. [87,178,620]. This observation of broad and at least bimodal MMDs is the result of the presence of at least two active catalyst species which show different activities. This feature is in contradiction with a strictly living polymerization. Wilson attributed the polymer fraction with a high molar mass to insoluble catalyst species which are invisible to the naked eye whereas the low molar mass fraction of the polymer is supposedly produced by soluble sites which operate in a quasi-living manner [89]. In his study Wilson used catalyst systems of the type Nd(carboxylate)3/DIBAH/tBuCl. 

The different averages are differently sensitive to different molar masses. Namely, Mn is sensitive mainly to the fractions with low molar masses while Myy and particularly are sensitive to high molar mass fractions. For monodisperse polymers, all molar mass averages are identical. The order of molar mass averages for polydisperse polymers is Mnpolydispersity index. The ratio MJMyy can be used as an additional parameter or can be alternatively applied instead of MyJM if the Mn value cannot be reliable determined. 

A series of low molecular weight PS and PMMA polymers were produced by free-radical polymerization with a monoacylphosphine oxide,more specifically (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TPO), as a photoinitiator [65]. The MMD of the PS and PMMA samples was broad, with polydispersities of 1.81 and 2.81, respectively. SEC fractionation yielded improved MALDI spectra with respect to the unfractionated polymers. 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

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