Polymer seeding

Popcorn Polymerization CO-Polymerization, frequendy referred to as popcorn polymerization because of the appearance of the product, can be a dangerous side reaction if not carefully controlled. The polymeriza tion appears to proceed without external initia tion (69—71), and is catalyzed by the tightly gelled polymer seeds that are a product of the polymerization. Once seeds are present and immersed either in the Hquid or vapor phase of monomer, their weight increases exponentially with time. 

Three polymer seeds were prepared in a batch reactor. The reactor with styrene and benzene was cooled to 0 C in an ice bath, initiator was injected into the reactor and reaction began with a gradual increase in temperature. Table II presents the initial conditions used in preparing the seed polymer and the molecular weights of the seed polymer. The molecular weight distribution of the pol3nner seeds are shown in Figure 5. 

Figure 5. Weight fraction distributions for polymer Seeds I ( ), 11 (A),...

Experimental Procedure. For the initial start-up of the continuous tirred tank reactor, the mixing speed and bath temperature were adjusted with the reactor full of solvent. The polymer seed and monomer feed rates were then adjusted simultaneously. 

Vacant CA et al (1991) Synthetic polymers seeded with chondrocytes provide a template for new cartilage formation. Plast Reconstr Surg 88(5) 753-759... 

The control and reproducibility of particle size and particle size distribution is important to the quality of acrylic and styrene-acrylic latex products. Particle size has large effects on latex viscosity and the rheology of formulated products and may also exert subtle effects on the end-use peiformaiKe properties. The particle size is controlled primarily by the choice and amount of surfactant, or by the use of seed latexes. A recoit article [32] addresses the use of surfactants to control particle size in semi-continuous acrylic polymmzations. Many surfactants are reconunended by surfactant manuhicturras for the preparation of acrylic and styrene-acrylic latexes [33]. Sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sulfosuccinates and the aUtylphonl ethoxylates ate typical. The patent literature contmns many discussions of the use of single [34] or multiple [3S] polymer seed latexes to control particle size. 

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

Figure 2. IFF Sample. A. Time = 0. Polymer seed and monomer solution are easily distinguishable. B. Time = 2 hours. Monomer solution and currentlyreacting polymer in the reaction zone are indistinguishable.

Vacanti CA, Langer R, Schloo B, Vacanti JP. Synthetic Biodegradable Polymers Seeded with Chondrocytes Provide a Template for New Cartilage Formation In Vivo. Plast Reconstr Surg 1991 87 753-9... 

Vacanti CA, Cima L, Ratkowski D, Upton J, Vacanti JP. Tissue Engineering of New Cartilage in the Shape of a Human Ear Employing Specially Configured Synthetic Polymers Seeded with Chondrocytes. Materials Research Society Symposium Proceedings. 1992 52 367-73. 

A second category of FP exists, that is, IFP. IFP differs from TFP in that the entire process is contained in an isothermal environment and the mechanism requires a polymer seed (a piece of preformed polymer) [7, 8]. Koike discovered IFP and proposed its mechanism in 1988 with additional definitions in 1990 (Figure 5.1b). A reaction vessel contains a polymer seed at the bottom with a... 

Newly formed polymer and remaining polymer seed... 

FP systems must have certain conditions for the front to autocatalytically occur. The necessary and sufficient conditions for TFP include a monomer that will polymerize via free-radical polymerization and a thermal initiator (a photoinitiator may be used to start the reaction, but a thermal initiator is necessary to sustain the reaction) [9,10]. The necessary and sufficient conditions for IFP include a monomer that will dissolve the polymer seed, polymerize via free-radical polymerization, and exhibit the gel effect a thermal initiator and a viscous region in which the gel effect can occur (i.e., the seed dissolving) [6, 11]. Ideally, another necessary IFP condition is a monomer-polymer system that produces an optically clear product because most IFP products are used in optical applications. 

An IFP system containing no dopant consists of the polymer seed, monomer, and thermal initiator. These systems produce a finished product containing no GRIN and ideally possessing a homogeneous RI. These reactions have primarily been studied to provide experimental evidence in favor of the IFP mechanism [6, 11, 41], to verify the predicted results of changing the experimental parameters within the system [6, 11], and to illustrate that the front is truly isothermal [41]. 

To our knowledge, the most comprehensive evidence in support of the IFP mechanism was in Lewis et al. [6] where the differences among three systems were shown (i) an IFP system, (ii) an IFP system with a small-molecule inhibitor added so that only diffusion and no front occurred, and (iii) a system with only monomer solution and no seed so that bulk polymerization occurred. The differences in these systems were shown using Wiener s method [48, 49], a laser sheet deflection technique (LLD) that illuminated changes in RI. The experimental system of this study contained a polymer seed and its monomer solution in a cuvette (Figure 5.8a, side view) [6]. To explain the results of this experiment, the authors first explained how LLD illuminated an IFP sample A low-power laser (about 8mW) passed... 

Evstratova, S.I., Antrim, D., Fillingane, C., and Pojman, J.A. (2006) Isothermal frontal polymerization confirmation of the isothermal nature of the process and the effect of oxygen and polymer seed molecular weight on front propagation. 

Low-molecular-weight polybutadiene oils result when the polymerization is catalyzed by a mixed system of butyllithium, 1,2-bis(dimethylamino)ethane, and potassium z-butanolate [110-112]. With 1,4-dilithium-1,1-4,4-tetraphenylbutane it is possible to get bifunctional living polymers (seeding technique) [113-118]. 

Vacanti C A, Kim W S and Mooney D, Tissue engineered composites of bone and cartilage using synthetic polymers seeded with two cell types , Orthopaed. Trans., 1993, 18, 276. 

Having established that the polymerization of propylene oxide by the zinc hexacyanocobaltate complex catalyst proceeds in the absence of a chain termination reaction, it was of interest to study separately the chain propagation reaction. A non-termi-nated polymer "seed" was prepared and its rate of reaction with propylene oxide was studied. The results of a series of these seeded polymerizations carried out at 30°, 40° and 50° are given in Figvire 6, where log [M]o/[M]t is plotted against time. 

If a polymer seed is swollen with a monomer or a monomer mixture... 

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