Free-radical polymerizations temperature affect

Free-radical initiation of emulsion copolymers produces a random polymerization m which the trans/ds ratio cannot be controlled. The nature of ESBR free-radical polymerization results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstructurc is not amenable to manipulation, although the temperature of the polymerization affects the ratio of trans to cis somewhat... 

Free-radical polymerizations were conducted under a wide range of conditions that included photochemical, thermochemical, and low-temperature alkylborane-oxygen initiation methods. Both bulk and solution methods were used with somewhat unexpected results. Temperature appeared to have relatively little effect on the tactic order (percent syndiotacticity) of VTFA polymers. Polymerization temperatures ranged from -80 to 150 C and gave syndiotacticities between 50 and 55% by triad analysis. Polar solvents affected tacticity more. For example, polymerization in 1,2-dichloro-ethane resulted in a syndiotacticity of 43%. Presumably, such effects were related to a disruption of the association of VTFA monomer with the growing chain end. 

When choosing an initiator for free-radical polymerization, the important parameters that must be eonsidered are the temperature range to be used for the polymerization and the reactivity of the radicals formed. The presence of certain promoters and accelerators and the nature of the monomer often affect the rate of deeomposition of initiators. For example, the decomposition of benzoyl peroxide may be accelerated at room temperature by employing ternary or quaternary amines. Free-radieal initiation... 

Copolymers of ethylene and vinyl acetate are commercial materials prepared by free-radical polymerization. Explain how copolymeiization with vinyl acetate affects the degree of crystallinity of polyethylene and the polymer chain flexibility at room temperature. How would you expect to vary with increasing vinyl acetate content ... 

Frontal free-radical polymerization is fairly well understood. Studies on the velocity dependence on temperature and initiator concentration have been performed (7,11,23), Frontal polymerization in solution was performed (70), and initiators that do not produce gas were developed (24), The velocity can be affected by the initiator type and concentration but is on the order of a cm/min for monofunctional acrylates and as high as 20 cm/min for multifunctional acrylates (24). 

Free-radical polymerizations are highly exothermic. A typical adiabatic temperature rise for bulk (mass) polymerization of 200-500°C may not be uncommon. The overall activation energy for polymerization is in the order of 80 15 kj mol . The dramatic increase in the heat load during the gel-effect period can result in loss of temperature control, non-isothermal reactor operation and potential rimaways. Non-isothermal operation, aside from safety concerns, can also adversely affect product quality. 

Copolymer composition has a direct effect on the Tg of the polymer, which determines the minimum film forming temperature (MFFT) of the latex and the application. Thus, a 95/5 wt/wt butyl acrylate/methyl methacrylate is an adhesive, whereas a 50/50 copolymer of the same monomers is a binder for paints. Copolymer composition affects properties such as resistance to hydrolysis [4] and weatherability. In situ formed blends of random copolymers of different compositions may be beneficial for application properties [5]. Conventional free-radical polymerization, which is the process used to manufacture almost all commercial emulsion polymers, does not allow the production of block and gradient copolymers (accessible by means of controlled radical polymerization [6], Section 3.3). Nevertheless, graft copolymers are frequently formed, and the extension of grafting largely determines the application properties. Thus, grafting determines the size of the rubber domains in ABS polymers, and the toughness of these polymers increases with rubber size. 

Chemical considerations indicate that the more diffuse the charges the more stable are the ions. Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can, however be greatly influenced by impurities that can act as ion scavengers. These can be water, ammonia, amines, or any other compounds that can be basic in character, affecting rates and molecular weights of the products. Typical cationic polymerizations proceed at high rates even at low temperatures, as low as — 100°C [8]. In the literature one can find many reports of cationic polymerizations of many different monomers with many different initiators. Often, however, such initiators are quite specific for individual monomers and their activities are strongly influenced by the solvents. 

Lao et al. [75] compared the surface functionalization of PHBHV by HEMA grafting via UV treatment and via thermal free radical polymerization. The photochemical grafting was realized with BP and H Oj at room temperature by an indirect method to avoid HEMA homopolymerization. The study of the enzymatic degradability demonstrated that the presence of PHEMA affected the degradability of the grafted PHBHV films (Fig. 6.7). 

The microstructure of free-radically polymerized NBR is affected by the polymerization temperature, as with SBR. For a rubber polymerized at 28 C and containing 28% bound acrylonitrile the ci5-Itrans- ratio of the butadiene units is 12-4/77-6. The lower the temperature the higher the trans- content. It is stated (Hofmann, 1964) that the higher trans- content of low temperature polymerized... 

The direct relationships between activation free energies and tacticity observed for free radical polymerization reactions are seldom encountered in ionic polymerization reactions In these systems, temperature not only affects the propagation rate constant, but it also controls the ion pair equilibrium of the active endgroups. That is, in most ionic polymerization reactions, the active endgroup is in the form of some type of ion pair, and each type of ion pair will most likely have its own ratio of rate constants, kj to k which controls tacticity. As a result, two or more different kinds of active endgroups can exist in dynamic equilibrium with each other and grow concurrently, as indicated below (9)... 

The front velocity is a function of the initial temperature and the AT of the reaction, where AT is determine by the DH X Mo/Cp. The value of AT is also affected by the presence of any inert material. Goldfeder et al. derived an expression for the front velocity in terms of the parameters for a free-radical polymerization. The velodty is a function of k, the thermal diffusivity (0.0014 cm s" ), Tb, fed = the preexponential factor for the initiator decomposition (4xl0 s" ), i = d = the energy activation for the dissociation constant for the initiator = 27 kcal mol" g is the ideal gas constant. 

No phase changes or segregation phenomena occurred up to ca. 20 % conversion of the UV-curable system, in a wide temperature range The LC phase does not affect the free radical polymerization rate Unlike cationic photopolymerization, the free-radical mechanism promotes rate accelerations at conversions related to compositions, for which changes in medium opacity are observed... 

Temperature and pH-sensitive hydrogels composed of polyampholyte CA and poly(NlPAAm] were successfully synthesized via free radical polymerization. The drug release of this hydrogel was determined with salicylic acid employed as the model drug. It found that the release of salicylic acid from the hydrogel particles was affected by temperature, pH, and the cross-linker content in the copolymer hydrogels (Cao et al., 2013). 

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

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