#reversible addition fragmentation chain transfer

[whats-in-a-sentence]

The concept is based on the reversible addition of radicals to carbon-sulfur double bonds in thiocarbonyl thio transfer reagents (R-S(C=S)Z (figure 26.26).

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[tenth-sentence]

CSIRO's RAFT (reversible addition fragmentation chain transfer) technology provides a revolutionary level of control suitable for highly functionalised polymers.

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[materialsscienceandengineering]

Reversible addition-fragmentation chain transfer (RAFT) ( 2 ) polymerization is a form of reversible deactivation polymerization (and therefore chain growth polymerization) wherein the molecular weight of the resulting polymers is carefully controlled through the chain transfer polymerization step. RAFT polymerization is often regarded as one of the most versatile methods for creating polymers with specifically desired structures, such as brush, comb, or star like polymers, as well as dendrimers or cross linked polymers. 

Image sources: ( 1 ) ( 2 )

[mohammedaljuaid]

Controlled radical polymerization (Brief introduction).

مقدمة

تندرج المواد البوليمرية تحت مركبات كيميائية تُسمى “

Macromolecules

" أي الجزيئات الضخمة أوالعملاقة إلا أن البوليمرات تتميز عن بقايا الجزيئات المُدرجة تحت هذا التصنيف بأنها مكونة من جزيئات صغيرة متراصة و مترابطة بروابط تساهمية تُسمى"مونمر". التركيب الكيميائي للمونمرات يسمح لتدرج التحضيرات البوليمرية إلى صنفين:

البلمرة بالتكاثف أو البلمرة الخطية  “Step growth" 

يتميز التركيب الكيميائي للمونمرات المصنفة ضمن هذا النوع بأنها قادرة على التفاعل مع بعضها البعض لينتج البوليمر كناتج رئيسي و نزع جزيء صغيركالماء. ومثال على هذا الصنف هو تفاعل الأحماضالأمينية مع بعضها البعض وبنزع جزيء الماء من طرفي الحمض الأميني و تحضير نايلون ٦,١٠ أو٦,٦. وفي هذا النوع لا يحصل تدخل من قبل أيونات موجبة وسالبة ولا كذلك جذور حرة. 

البلمرة بالإضافة "Chain growth" 

ويلزم في التركيب الكيميائي لمونمرات أن تحتوي على رابطة مضاعفة. وجود الكترونات باي يجعلها أكثرعُرضة للهجوم من قبل الجذر الحر حيث يكون الالكترون الوحيد بالجذر رابطة سيجما قوية مع أحدى الكتروني الرابطة المضاعفة فينتج عن ذلك جذر جديد يهاجم رابطة مضاعفة في مونمر آخر و هكذا تستمرالبلمرة. 

خطوات ميكانيكية البلمرة بالجذر الحر

البدء "Initiation" .1

تُعتبر هذه الخطوة هي الخطوة الأسرع من بين الخطوات الثلاث وفيها تقوم البادئة "Initiator" بإنتاج جذور حرة بطرق مختلفة تختلف بأختلاف التركيب الكيميائي للبادئة أما بتسخين المحلول أو تسليطه بأشعةUV أو بإضافة حفاز. من المعروف بأن الجذور الحرة ذات نشاطية عالية لذلك بمجرد إنتاجها تبدأ الهجوم على المونمرات بعد الهجوم على المونمر يُصبح الناتج جذر جديداً جزيء أكبر يهجم على مونمر آخر وهكذا. 

النمو "Propagation" .2

عندما يهجم الجذر على جزيء أو اثنين من المونمر يبقى الناتج جذراً نشطاً مالم يتم تثبيط بعامل معين. هذا الجذر يستمر بالنمو عن طريق إضافة مونمر إلى سلسلته وتُسمى هذه العملية بالنمو. تختلف سرعة عملية النمو بإختلاف نوع المونمر لذلك كل مونمر يمتلك ثابت يُسمى ثابت النمو ورمزه Kp. ويُعبر هذا الثابت بأنه عدد الوحدات التي تُضاف للسلسلة النشطة في الثانية. 

الإنهاء "Termination" .3

بشكل عام هذه الخطوة تؤدي إلى توقف السلسلة البوليمرية النشطة على التمدد و إضافة وحدات مونمرية أخرى للسلسلة. لسوء الحظ هذه العملية قد تتم بأي لحظة من لحظات البلمرة ولها طُرق مختلفة أبرزها: 

الإندماج "Combination" 

وبهذه الطريقة يلتقي جذريين من سلسلتيين نشطة ويرتبطان مع بعضهما البعض برابطة أحادية. قد تكون السلسلتين لهما نفس الطول أو أحدهما أقصر. ربما يكون أحد الجذريين سلسلة بوليمر و الآخر ليس كذلك. 

 الإنتقال لمونمر أو مذيب  

وفيه يقوم جذر السلسلة النشطة بنقل الجذر إلى جزيء مونمر أو مذيب كنزع هيدروجين بكسر الرابطة الأحادية للهيدروجين بالمونمر أو المذيب والنتيجة تكون سلسلة بوليمرية مثبطة و جذر جديد يقوم بمهاجمة مونمرات أخرى. 

Disproportionation

قد تتواجد سلسلتي بوليمر نشطة إلا أن الجذريين الحريين عوضاً عن الإرتباط مع بعضهما البعض كما في تفاعل الإندماج فأن أحد الجذريي�� يقوم بنزع هيدروجين من ذرة الكربون المجاورة مباشرةً للجذر فينتج عن ذلك سلسلتين مُثبطة أحدهما تحتوي على ميثايل بالنهاية و الأخرى يتفاعل جذريها المجاوريين مكونان رابطة مضاعفة. 

نزع هيدروجين من منتصف بوليمر آخر H-Abstraction 

قد تقوم سلسلة نشطة بنزع هيدروجين من منتصف سلسلة بوليمر نشط آخر فتُصبح مثبطة وغير قابلة للنمو أكثر بينم�� سلسلة البوليمر الأخرى يُصبح لديها جذر جديد غير الجذر الطرفي فإما يستمر كلا الجذريين بالنمو أو في حال كانت المسافة بينهما كافية يُصبح تحلق و يتكون سلسلة حلقية. 

خطوة الإنهاء تمتلك ثابت يُسمى ثابت الإنهاء Kt وهويعتمد على تركيز الجذر الموجود في جميع الطرق لإنهائية التي تمت مناقشتها سابقاً لذلك بوجود هذه الطرق تُصبح إحتمالية تثبيط و قتل السلاسل البوليمرية واردة بشكل أكبر خصوصاً بالثلث الأخير من زمن البلمرة بسبب التنافسية بين Kp و Kt. 

عيوب البلمرة بالجذر الحر 

لذلك فأن البلمرة بالجذر الحر تؤدي إلى سلاسل ذات أطوال مختلفة في المحلول الواحد. كذلك يصعب التحكم بتركيب السلسلة البوليمرية بسبب نشاط الجذرالعالي و خروجه عن السيطرة. كذلك يصعب بالغالب التأكد من تواجد المجاميع الناشئة من تفكك البادئة في بداية أو نهاية السلسلة البوليمرية وإن وُجدت فلن تكون بجميع السلاسل البوليمرية. هذه المجاميع قد تكون مهمة في تطبيقات معينة. على سبيل المثال قد تحتوي على مجاميع نشطة ضوئياً تسمح بتتبعها أو مجاميع منالممكن أن تتفاعل إختيارياً مع مجاميع معينة في مواد حيوية هالجسر الكبريتي أو مجاميع الأمين في الحمض الأميني لايسين أو بالكحولات في السكريات أو مجاميع تسمح بربط بوليمر A ببوليمر B دون الخوض في تفاعلات الجذور. وجود عينة بوليمر تحتوي على سلاسل بوليمر بمدى أطوال كبير يجعلها غير مناسبة للتطبيقات الطبية الحيوية لأن كل مدى داخل هذا المدى الواسع له خصائص مختلفة. 

ويتضح من ذلك بأن البوليمرات المُحضرة بالجذرالحر قد لا تتناسب مع العديد من التطبيقات بينما لابأس بها في تطبيقات صناعية معينة. كذلك تحِد منتحضير بوليمرات بتراكيب مختلفة قد يكون لها تطبيقات جديدة أو تساهم بشكل أفضل. 

 البلمرة الحية "Living polymerization"

قبل عام ١٩٥٦ لم يكون هنا سوى بلمرة بالجذر الحر وكان يُعتقد بأن التحكم بالجذور الحرة أمر ميؤس منه ويجب تقبل هذه الحقيقة حتى قام العالم Szwarc بتحضير بولي ستايرين بواسطة البلمرة الأنيوية.تختلف البلمرة الأنيونية عن البلمرة بالجذر الحر ليس فقط بنوع البادئة و أن الذي يهاجم الرابطة المضاعفة وينمي السلسلة البوليمرية هو الشحنة السالبة بدل الجذرالحر بل كذلك لا تتواجد أي عملية من عمليات الإنهاء السابق ذكرها و بالتالي فأن السلاسل البوليمرية تنمومعاً و بنفس الطول تقريباً ولذلك قيمة PDI تقترب إلى١ بدل من إن تكون عالية ��ثل البلمرة بالجذر الحر. 

( ملاحظة قيمة PDI هي ناتج قسمة Mw على Mn وكلما إقتربت من الواحد يعني بأن جميع السلاسل لها نفس الوزن الجزيئي و أطول. في الجزيئات الصغيرة لأن أوزانها الجزيئية واحدة تكون PDI لهم تساوي ١دائماً).

أُعتبرت هذه الطريقة ثورة في مجال البوليمر وتمتحضير العديد من البولميرات بالبلمرة الكاتيونية والأنيونية لمونمرات كانت تتبلمرة بالجذر الحر ودراسة إختلاف خصائصها عند بلمرتها بالجذر الحر وبالبلمرة الحية. 

لذلك كان التساؤل كيف يتم تطبيق مفهوم living polymerization على الجذور بدل الشحنات ؟ لأن الشحنات المتشابهة تتنافر وليست بنشاطية الجذرالعالية لذلك عملية الإنهاء إحتماليتها قليلة خلال البلمرة والسلاسل متساوية تقريباً. ومن هذا التساؤل ظهر مايُسمى بـ البلمرة الحية "المتحكمة" للجذر (Controlled “living” radical polymerization). 

البلمرة الحية للجذر " Living radical polymerization "

العامل المهم في البلمرة الحية هو التقليل قدر الإمكان من تركيز الجذر الحر. فكما ذُكر سابقاً بأن عملية الإنهاء تعتمد على تركيز الجذر الحر وإلا فأن السلاسل البوليمرية ستتعرض للتثبيط و التوقف عن التمدد.والهدف في البلمرة الحية للجذر هو ألا تتوقف السلاسل البوليمرية بالنمو. لذلك يتم إيقاف بعض السلاسل النشطة مؤقتاً و ترك البعض الآخر ينمو والعملية بينتوقف نشاط الجذر مؤقتاً و تنشيطه عملية عكسية هدفها التقليل من تركيز الجذور الحر. 

 هناك طرق كيميائية عديدة يُستخدم فيها البلمرة عن طريق التحكم بالجذور الحرة للحصول على بوليمرات لها خصائص مشابهة لتلك المُحضرة بالبلمرة الكاتيونية أو الأنيونية. من أوائل و أشهر  تلك الطرق إستخداماً :

  Nitroxide mediated polymerization(NMP) 

هذه الطريقة يُستخدم فيها جذر NO• يقوم هذا الجذربالتفاعل مع جذر بوليمري نشط فيثبطه ثم عند زياد ةدرجة الحرارة تنكسر الرابطة معطية جذر NO• وسلسلة بوليمرية نشطة جذرياً تستمر بمهاجمة المونمرات النمو. هذه الطريقة تسمح بالتحكم بالجذرالحر لكن من عيوبها إستهلاك طاقة حرارية. 

Reversible addition-fragmentation chain transfer (RAFT) 

عامل RAFT هو جزيء يحتوي على مجموعة وظيفية تُسمى thiocarbonylthio  يحتوي  هذا العامل على ذرتي كبريت أحداهما مكونة رابطة مضاعفة مع ذرة كربون مرتبطة بعامل ثابت Z والأخرى مرتبطة بنفس الكربون برابطة أحاديةومرتبطة بمجموعة R. تقوم سلسلة بوليمرية نشطة بالهجوم على الرابطة S=C مكونة جذر على ذرة الكربون التي تقوم بكسر الرابطة من ذرة الكبريت الأخرى مكونة رابطة مضاعفة على الجهة المقابلة وجذر نشط يهاجم المونمرات لتكوين سلسلة بوليمرية وهكذا. عند الهجوم على هذا العامل فأن الجذرالكربوني و الرنين الحاصل تختلف ثباتيته بإختلاف المجموعة Z لذلك تختلف هذه المجموعة بإختلاف نوع المونمر و المذيب المُستخدمين. 

Atom transfer radical polymerization(ATRP) 

تُعتبر طريقة ATRP مُستوحاة من تفاعل atom transfer radical addition (ATRA) وفيها تتم إضافة هاليد الألكيل إلى مجموعة الكين مكونة ناتج هاليد الكيل ذو وزن جزيئي أكبر من الهاليد الالكيل المتفاعل. 

في طريقة ATRP يتم إستخدام هاليد الألكيل كبادئة وبهذه الطريقة يجب إستهلاك البادئة كلياً بالبداية وإلا فأن الوزن الجزيئي يُصبح أكبر من النظري. يتم تنشيط البادئة و إنتاج جذر منها عن طريق حفاز معدن. من مزايا المعدن الحفاز أن يسهل عليه التحول من حالة أكسدة n إلى حالة أكسدة بعد نزع الهاليد أخرى n+1وبذلك يدخل هذا المعدن بحالتي الأكسدة في تفاعل عكسي مع سلسلة البوليمر النشطة بالجذر و نظيرها المثبط بالهاليد القادم من المعدن بحالة الأكسدة العالية.في الغالب يتم إستخدام النحاس Cu كحفاز بسبب وفرته العالية و رخصه مقارنة بمعادن أخرى كالروثينيوم. و يُعتبر مُفضل مقارنة بالنحاس لأنه أعلى نشاطاً منه. في ميكانيكية ATRP يكون النحاس ذوالأكسدة المُنخفضة CuX

هو المنشط ودوره هو نزع الهاليد من الكربون نزعاً متساوياً بحيث ينتج جذر ألكيلي نشط يُهاجم الرابطة المضاعفة بالمونمر و يكون سلسلة نشطة و معدن بحالة الأكسدة العالية CuX2. ثم يتم تثبيط الجذرالنشط بالمعدن ذو الأكسدة العالية CuX2 وهو المثبطب واسطة الهاليد ليتكون CuX و P-X. في هذه العملية العكسية يوجد ثابتين وهما ثابت التنشيط Kact و ثابت التثبيط Kdeact وللحصول على بلمرة حية مُتحكمة لابد يكون  Kdeact > Kact  لتقليل تركيز الجذر النشط و تجنب عملية الإنهاء. وبهذه الطريقة كلما زادت نسبة تحويل المونمر تقل قيمة PDI وهي المطلوبة لتكون البلمرة حية و متحكمة "Controlled". 

يتم معرفة البلمرة إذا كانت حية أو جذر حر حركياً عن طريق رسم منحنى ln(M0/M) مقابل الزمن فتكون العلاقة خطية إذا كانت البلمرة تفاعل من الدرجة الأولى معتمداً على تركيز المونمر أما إذا المعدل يعتمد على تركيز الجذر أيضاً فأن البلمرة تكون غير مُسيطرة و تصبح جذر حر. أحياناً يحصل إنحراف طفيف في العلاقة يكون سببه كفاءة البادئة نفسها لأنها لا تُستهلك بشكل كلي بسرعة في البداية. 

هذه الطرق مكنت المتخصصين بالبوليمر من تحضيربوليمرات بأشكال هندسية مختلفة و تكوين بوليمرات بتراكيب محددة و مختلفة. بالإضافة إلى الحصول على مجاميع بنهاية و بداية السلسلة تكون مطلوبة لتطبيقات معينة أو من الممكن تعديلها للحصول على مجاميع أخرى تُساعد في توظيف السلسلة البوليمرية بطرقأخرى. على سبيل المثال، بطريقة ATRP يتم الحصول على هالوجين بآخر السلسلة يمكن إستبداله بمجموعة أزيد و ربط البوليمر بجسيمات نانوية تمتلك مجموعات الكاين على سطحها بواسطة Click reaction. 

REFERNCE

https://pubs.acs.org/doi/abs/10.1021/acs.chemrev.5b00671

https://www.sciencedirect.com/science/article/pii/S0079670007000044

https://www.cmu.edu/maty/chem/fundamentals-atrp/index.html

[rubberandplasticproducts]

Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies – Polymer Chemistry Blog

Paper of the month: Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies 02 Dec 2019 Chang et al. employ addition-fragmentation chain transfer to generate well-defined block copolymers. The use of a polymethylmethacrylate (PMMA) containing an unsaturated chain end as a macroinitiator during reversible complexation mediated polymerization has been previously reported by Goto and coworkers. Typically, such macroinitiators can also be used as macromonomers to generate branched polymers via propagation. In this work, Goto and co-workers elegantly demonstrate that the occurrence of addition-fragmentation chain transfer and propagation strongly depends on the temperature during the polymerization of styrene. Through carefully monitoring the kinetics of the polymerization of styrene, the authors discovered that propagation is predominant below 60 ̊C, consistent with previous reports. However, upon elevating the temperature (e.g. 120 ̊C), addition-fragmentation chain transfer dominates instead. This discovery then allowed access to the efficient synthesis of block copolymers with PMMA and polystyrene at high temperatures. Importantly, addition-fragmentation chain transfer was also predominant over propagation during the polymerizations of acrylonitrile and acrylates yielding well-defined block copolymers. PMMAs with different molecular weights were also investigated and the polymerization was controlled utilizing iodine transfer polymerization for styrene and reversible complexation mediated polymerization for the other monomers. Such an approach is highly advantageous due to the ease of the operation and it is expected to be a practical alternative for efficient block copolymer synthesis. Tips/comments directly from the authors: The proper purification of polymers and the careful NMR analysis were important for obtaining the accurate kinetic data. The kinetic study provided a useful idea enabling the synthesis of block copolymers of PMMA with polystyrene (PSt). Block copolymers of PMMA with PSt, polyacrylonitrile, and polyacrylates are accessible. Relatively high monomer conversions are achievable. Not only the isolated alkyl iodide but also the alkyl iodide in situ generated from iodine (I2) and azo compound can effectively be used as the initiating dormant species. The in situ method is less expensive and robust and hence can be a practically attractive Read the full article now for FREE until 10th January! Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies, Polym. Chem. , 2019, 10, 5617-5625, DOI: 10.1039/c9py01367a About the web writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[whats-in-a-sentence]

CSIRO's RAFT scientists are continuing the research and commercialisation of multi-functional polymers, particularly in the areas of agriculture, drug delivery, biomedical materials, personal care and cosmetics and flexible electronics (figure 26.22).

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[siva3155]

300+ TOP POLYMERASE CHAIN REACTION Objective Questions and Answers

POLYMERASE CHAIN REACTION Multiple Choice Questions :-

1. The viruses that can achieve neoplastic transformation are called A. DNA tumor viruses B. RNA tumor viruses C. retroviruses HIV D. none of these Answer: A 2. Which of the following is/are? A. Primers B. DNA polymerase C. Nucleotides D. All of these Answer: D 3. For gene transfer to be effective, transforming DNA must be A. incorporated into the bacterial chromosome B. incorporated into a viral genome C. free in the bacterial cytoplasm D. none of the above Answer: A 4. Double stranded DNA denaturation with specified limit of temperature is A. reversible reaction B. irreversible reaction C. either (a) or (b) D. none of these Answer: A 5. From a single molecule of DNA, PCR can make A. one additional copy B. hundreds of copies C. thousands of copies D. millions of copies Answer: D 6. Genomic libraries are made from A. genomic DNA of an organism B. genomic RNA of an organism C. genomic cDNA of an organism D. genomic mRNA of an organism Answer: A 7. Bacteriophages are A. cells in the blood that eat bacteria B. a class of bacteria C. bacterial viruses D. none of the above Answer: C 8. Specialized transduction occurs when A. the bacteriophage incorporates randomly in the bacterial chromosome B. the bacteriophage never incorporates into the bacterial chromosome C. the bacteriophage always incorporates at the same position in the bacterial chromosome D. none of the above Answer: C 9. PCR is used A. to diagnose genetic diseases B. to solve crimes C. to study gene function D. all of these Answer: D 10. To clone into a plasmid vector, both the plasmid and the foreign DNA are cut A. with the same restriction enzyme and mixed together B. with different restriction enzyme and mixed together C. with the combination of enzymes and then seperated D. with the combination of enzymes and mixed together Answer: A

POLYMERASE CHAIN REACTION MCQs POLYMERASE CHAIN REACTION Objective type Questions with Answers 11. PCR can be used to amplify a specific fragment of DNA from which of the following? A. A drop of blood B. A hair follicle C. A fragment of skin D. All of these Answer: D 12. Transformation means A. formation of a pilus B. acquiring DNA from the bacterial cell environment C. plasmid containing a F factor D. F+ and F- strains of bacteria Answer: B 13. Restriction maps A. allows comparison between DNA molecules without the need to determine nucleotide sequence B. allows comparison between DNA molecules but requires to determine nucleotide sequence C. does not allow comparison between DNA molecules D. none of the above Answer: A 14. Which of the following is correct? A. HIV is a retrovirus that kills human helper T cells B. Causes acquired immune deficiency syndrome (AIDS) C. Cripples the immunity systems D. All of the above Answer: D 15. A PCR cycle consists of A. three steps, denaturation, primer annealing and elongation B. three steps, denaturation, initiation and elongation C. three steps, primer annealing, elongation and termination D. three steps, initiation, elongation and termination Answer: A POLYMERASE CHAIN REACTION Questions and Answers pdf Download Read the full article

[materialsscienceandengineering]

Ultraheavy precision polymers

An environmentally friendly and sustainable synthesis of 'heavyweight' polymers with very narrow molecular weight distributions is an important concept in modern polymer chemistry. Thanks to a new photoenzymatic process, Chinese researchers have been able to increase the range of possible monomers. As reported in the journal Angewandte Chemie, the researchers were able to obtain well-defined linear and star-shaped polymers with ultrahigh molecular weights.

Because many polymer properties depend heavily on molecular weight, it is desirable to have as narrow a molecular weight distribution as possible. Precision polymers with ultrahigh molecular weights (> 1 t/mol) would be interesting candidates for high-performance elastomers, low-concentration hydrogels, photonic materials, durable coatings, and flocking agents. However, such heavyweight polymers are not easy to produce with a uniform distribution of molecular weights. The radical polymerizations in widespread use are especially difficult to control in this respect. Modern methods, such as RAFT polymerization (RAFT: reversible addition-fragmentation chain transfer) offer a significantly higher degree of control by keeping the concentration of reactive radicals very small. A special agent reacts reversibly with the growing polymer chains to form a nonradical species. Whenever the intermediate dissociates, new active radicals are formed. This slows the reaction and results in longer, more uniform polymer chains.

Read more.

[bentonpena]

Discovering Bitcoin Part 6: Digital Contracts

Discovering Bitcoin Part 6: Digital Contracts http://bit.ly/30cu1od

This is the sixth installment of bitcoiner Giacomo Zucco’s series “Discovering Bitcoin: A Brief Overview From Cavemen to the Lightning Network.” Read the Introduction to his series, Discovering Bitcoin Part 1: About Time, Discovering Bitcoin Part 2: About People, Discovering Bitcoin Part 3: Introducing Money, Discovering Bitcoin Part 4: A Wrong Turn (New Plan Needed)! and Discovering Bitcoin Part 5: Digital Scarcity. 

In Part 6 of this “Discovering Bitcoin” series, we will build on the idea of using digital puzzles as a way to reproduce scarcity, and on the importance of a supply-control mechanism to grant some hardness to digital money, to explore concepts of proving ownership through signatures and scripts, and the technique known as CoinJoin.

Proving Ownership: Signatures

Our Plan ₿ for money brings us, for the second time, to focus on the topic of people and the question “Who?”

You established the conditions for the issuance of new sats, but what about their transfer? Who is authorized to change the data in the shared balance sheet, transferring ownership?

If there was a central authority in charge of reassigning sats, following instructions by current owners (maybe logged in to the system with the classical username-and-password approach, like in your previous e-gold experiment), there would be a Mallory-vulnerable single point of failure again: Why then even bother moving from physical gold to PoW-based “digital scarcity”? If, on the other hand, each user had an equal right to reassign ownership, then your system could not work at all: Everybody would be encouraged to continuously assign other people’s sats to themselves. You need some kind of consistent authority-defining protocol, which everybody could independently check.

The solution is a cryptographic technique called a “digital signature.” It works like this: First, Alice chooses a random number called a “private key,” which she will keep absolutely secret. Then, she passes this number through a special mathematical function, easy to apply in one direction but practically impossible to reverse. The result is another number called a “public key,” which Alice doesn’t keep secret at all: Instead, she makes sure that Bob gets to know it. Finally, she passes the private key and the message through a second function, again difficult to reverse, which results in a very big number called a “signature.” A third and last mathematical function can be applied by Bob to the message, the signature and Alice’s public key, resulting in a positive or negative verification. If the result is positive, he can be sure that Alice authorized that message (“authentication”), that she will not be able to later deny that authorization (“non-repudiation”) and that the message was not altered in transit (“integrity”).

In a way, it’s similar to handwritten signatures (thus the name), which are easy for everybody to check against some public sample, but difficult to reproduce without being the owner of the “correct hand.” Or to wax seals: easy for everybody to check against a public seal registry, but difficult to reproduce without the correct wax stencil.

So, you change your protocol in order to make fractions of proofs of work independently reusable via digital signatures. The first model you implement is trivial: Each user independently generates a private key and creates a public “account,” labeled with the corresponding public key. When users want to transfer ownership, they create a message including their account, the receiving account and the amount of sats they want to transfer. Then, they digitally sign and broadcast the message, which everybody can verify.

Interestingly enough, a similar scheme can be used by many renowned (yet possibly pseudonymous) developers to sign different versions of your software so that they can freely change, improve, fix, update, audit and review it, and any final user of your system can independently verify said signatures before running their preferred version, leveraging a network of minimized and fragmented trust, without a need for a single authority to centrally distribute the software. This process enables a true decentralization of code.

Script and “Smart Contracts”

You don’t want to limit the conditions that every peer has to check, before accepting any change in the shared balance sheet, to mere digital-signature validity, though. 

You decide that each message can also include a “script”: a list of instructions describing additional conditions that the receiving account (or accounts) will have to satisfy in order to spend again. For example, the sender could require a combination of several secret keys (in conjunction or disjunction) or a specific waiting time before spending. Starting from these very simple (and easy to audit) primitives, complex “smart contracts” can be built, making money effectively “programmable,” even in the absence of central parties.

Darkness (and Scaleness) Problems

Unlike an encrypted messaging system (where if Alice sends Bob some messages, only Bob can read them), your scheme isn’t really optimized for darkness (if Alice sends Bob sats, her message will have to be revealed beyond Bob — at the very least to those who will receive those same sats later on). 

Money circulates. Payees cannot trust any money transfer, even if properly signed, if they cannot verify that the transferred sats have actually been transferred themselves to that specific payer, and so on, upstream, back to the very first PoW-based issuance. With enough circulation of sats, active peers would get to know a huge number of past transactions, and forensic analysis techniques could be employed to statistically correlate amounts, timings, metadata and accounts, thereby deanonymizing many users and stripping them of their deniability. 

This is problematic: As discussed in Part 2, darkness is a fundamental quality for money, both for economical and sociological reasons.

Smart contracts make this problem even worse, since particular spending conditions may be used to identify particular software implementations or specific organization policies.

This lack of darkness is more serious than the one that affected your previous e-gold experiment: It’s true that, back then, you stored most transaction metadata on your central servers, but at least it was only you, as opposed to quite literally anybody (including many of Mallory’s agents), who had access! Furthermore, you could implement some particularly advanced cryptographic strategy to make yourself at least partially “blind” to what was actually going on between your users.

There’s also a minor scaleness problem connected with this design: Digital signatures are quite big, and the chain of transfers that a payee needs to receive in order to validate everything would include many signatures, making validation potentially more expensive. Furthermore, account changes are quite difficult to validate in parallel.

A New Paradigm: “CoinJoin”

To mitigate such problems, you decide to change the fundamental entities of your model from bank-like “accounts” to “Unspent Transaction Outputs” (UTXOs). 

Instead of instructions to move sats from one account to another, each message now includes a list of old UTXOs, coming from past transactions and “consumed” as ingredients, and a list of new UTXOs, “generated” as products and ready for future transactions. Instead of publishing a single, static public key to be used as general account reference (like a bank IBAN or an email address), Bob must provide new, single-use public keys for each payment he wants to receive. When Alice pays him, she signs a message that “unlocks” some sats from some previously created UTXO, and “locks” them again into some new UTXO.

Just like with physical cash, spendable bills don’t always match payment requests — change is often required. If, for example, Alice wants to pay 1,000 sats to Bob, but she only controls several UTXOs locking 700 sats each, she will sign a transaction consuming two of those 700-sats UTXOs (unlocking a total amount of 1,400 sats) and generating two new UTXOs: one associated with Bob’s keys, locking the payment (1,000 sats), and the other associated with Alice’s keys, locking the change (400 sats).

Provided that people don’t reuse keys for different payments, this design increases darkness in and of itself. But even more so when your users start to realize that UTXOs consumed and generated by a single transaction don’t have to come from just two entities! Alice can create a message spending old UTXOs she controls and generating new UTXOs (associated with Bob), then she can pass said message to Carol, who can simply add her old UTXOs she wants to consume and the new UTXOs (associated with Daniel) she wants to create. Finally, Alice and Carol both sign and broadcast the composite message (paying both Bob and Daniel). 

This special use of the UTXO model is called “CoinJoin.” (Trigger warning: Within the actual Bitcoin history, this use wasn’t Satoshi’s design rationale for the UTXO model itself, but was discovered as a potential twist on said design by other developers, many years after the launch.) It breaks the statistical linkability between outputs, while preserving what is called “atomicity”: Transactions are either entirely valid or invalid, thus Alice and Carol don’t have to trust each other. (If one of them tries to alter a partially signed message before adding their own signature, the existent signature becomes invalid.)

There is a possible change to your system that may actually improve the situation even more: a different digital-signature scheme, alternative to the one you’re using now, which is “linear in the signatures.” That means: In taking two private keys (which are nothing but two numbers), signing the same message with each and adding together the resulting signatures (which also are nothing but two very big numbers), the result happens to be the correct signature corresponding to the sum of the two public keys associated with the two initial private keys! 

This sounds convoluted, but the implication is simple: Alice and Carol, when CoinJoining, could add up their individual signatures and broadcast just the sum, which everybody could verify against the sum of their public keys! Since, as we said, signatures are the “heaviest” part of transactions, the possibility of broadcasting just one instead of many would save up a lot of resources. External observers would end up suspecting every transaction of being a CoinJoin, since many users could be after efficiency gains. This assumption would break most of the forensic heuristics.

Image courtesy of CryptoScamHub

Even without this further improvement, the UTXO model already somehow increases scaleness: Unlike state changes in the account model, it allows validation to be efficiently batched and parallelized.

So far, you’ve learned:

that you can decentralize ownership using digital signatures for transfer;

that you can turn transactions into programmable “contracts” with a script system; and

that a more complex paradigm called CoinJoin can further increase darkness and scaleness.

But now that your users can issue sats and transfer them in a completely decentralized way, how can they all be sure that a single chronology is followed, preventing double-spending attacks or attempts to tinker with the inflation schedule? We will answer that in our final installment, “Discovering Bitcoin Part 7: The Missing Pieces.”

The post Discovering Bitcoin Part 6: Digital Contracts appeared first on Bitcoin Magazine.

Trading via Bitcoin Magazine http://bit.ly/2rRO45f September 21, 2019 at 12:40PM

[healthtimetaylor]

Chrysin and docetaxel loaded biodegradable micelle for combination chemotherapy of cancer stem cell.

PMID:  Pharm Res. 2019 Oct 23 ;36(12):165. Epub 2019 Oct 23. PMID: 31646391 Abstract Title:  Chrysin and Docetaxel Loaded Biodegradable Micelle for Combination Chemotherapy of Cancer Stem Cell. Abstract:  PURPOSE: Cancer stem cells (CSCs) have been suggested to represent the main cause of tumour progression, metastasis and drug resistance. Therefore, these cells can be an appropriate target to improve cancer treatment.METHODS: A novel biodegradable brush copolymeric micelle was synthesized by the ring-opening polymerization (ROP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. The obtained micelle was used for co-delivery of the anticancer drug docetaxel (DTX) and Chrysin (CHS) as an adjuvant on the CSCs originated from Human colon adenocarcinoma cell line. Cancer stem cells were enriched by MACS technique and characterized by flow cytometry analysis against CD133 marker.RESULTS: Data demonstrated that the micelles harbouring DTX@CHS had potential to reduce cancer stem cell viability compared to free DTX@CHS, single-drug formulations and the control group (p 

read more

[twitterfaves]

From Twitter: Light-induced self-assembly of cats using visible light mediated photoinduced electron transfer–reversible addition–fragmentation chain transfer polymerization pic.twitter.com/b6aJndIbgI— Sylvain ❄️👨🏻‍🎓 (@DevilleSy) August 30, 2019

http://twitter.com/DevilleSy

[rubberandplasticproducts]

Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors – Polymer Chemistry Blog

Paper of the month: Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors 30 Sep 2019 Knox et al. utilize a benchtop flow-NMR for rapid online monitoring of a range of polymerisation methodologies. To precisely engineer macromolecular materials, close monitoring of the polymerization progress is required. Therefore, real-time online monitoring provides polymer chemists the opportunity to accurately observe and optimize their reactions. To this end, Warren and co-workers utilized benchtop flow-nuclear magnetic resonance (NMR) as a very convenient and powerful tool for real-time monitoring of polymers synthesized either by controlled radical polymerization or free radical polymerization protocols. In particular, reversible addition-fragmentation chain-transfer (RAFT) polymerization was employed to polymerize acrylamides giving very high conversions in less than 10 minutes and the kinetic profile of this reaction was efficiently captured. In a second example where RAFT dispersion polymerization was monitored. In spite of the rapid polymerization rates, high temporal resolution enabled the previse determination of the onset of rate acceleration usually observed for polymerization induced self-assembly (PISA) systems. In addition to the monitoring of the aforementioned complex systems, the free radical polymerization of methyl methacrylate was also studied. In this case, the linear semi-logarithmic plot indicated the expected pseudo-first order kinetics. The results discussed here demonstrate the power of using benchtop NMR spectrometers for online flow applications where both controlled and free radical polymerizations can be employed. It is the author’s opinion that the lower price of these instruments will improve access to NMR spectroscopy while the reduced sample preparation/time taken for analysis will increase research output. Tips/comments directly from the authors: Despite the reduced field strength, detailed polymerization kinetics comparable to traditional ‘high field’ NMR can be obtained since the vinyl protons are easily resolved. Flow-NMR is a powerful tool to improve time-resolution and reduce lab workload but must be used with care – e.g. flow rate and sample cell geometry must be optimized. Hydrogenated solvents can be used with lower-field instruments, but solvent selection is important: minimising any potential solvent overlap is key to reliable data. Spectral corrections such as to the phase and baseline are crucial for reliable data – especially if using an automated system. Read the full article now for FREE until 8th November! Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors,  Polym. Chem. , 2019, 10, 4774-4778, DOI: 10.1039/C9PY00982E About the web writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[whats-in-a-sentence]

It enables the creation of polymer chains that are more uniform in molecular mass (i.e. low polydispersity, see figure 26.21), which means that such polymers exhibit more refined properties.

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[crypto20blog-blog]

How to Get a 3% Discount Genesis Mining Promo Code

The Internet is part of society and is formed by society. And until society is a crime-free zone, the Internet will not be a criminal offense-free zone.

So what's a cryptocurrency? A cryptocurrency is a decentralised payment system, which principally lets people send foreign money to each other over the online with out the need for a trusted third occasion similar to a financial institution or monetary institution. The transactions are low cost, and in lots of instances, they're free. And likewise, the funds are pseudo anonymous as effectively.

In addition to that, the main characteristic is that it's very decentralised, which implies that there isn't any single central point of authority or something like that. The implications of this is performed by everyone having a full copy of all the transactions that have ever occurred with Bitcoin. This creates an extremely resilient community, which means that nobody can change or reverse or police any of the transactions.

The high degree of anonymity in there signifies that it's very hard to hint transactions. It's not totally inconceivable, however it's impractical in most cases. So crime with cryptocurrency-- because you've got fast, borderless transactions, and you have got a excessive stage of anonymity, it in idea creates a system that's ripe for exploitation. So usually when it is a crime online with online fee systems, then they tend to go to the authorities and, say, we will hand over this fee information or we can stop these transactions and reverse them. And none of that may happen with Bitcoin, so it makes it ripe for criminals, in concept.

In light of this, plenty of different businesses are researching into Bitcoin and taking a look at Bitcoin and trying to grasp the way it works and what they can do to police it. It is also been within the media fairly a number of instances, and the media, being the media, like give attention to the dangerous aspect of it. So that they focus very closely on the crime with it. So if there is a theft or a rip-off or something like that, then they have a tendency to blame it on Bitcoin and Bitcoin users.

So probably the most notable is probably Silk Street, which received taken down just lately, and through their $1.2 billion worth of Bitcoins, went to pay for something from drugs to guns to hit males to these sorts of issues. And the media, again, very quickly to blame this on Bitcoins and say that it was the Bitcoin person's fault.

But there's really very little evidence of the scale of the issue of crime with cryptocurrencies. We do not know if there's so much or we do not know if there's a little bit. However despite this, people are very quick to model it as a felony thing, and they overlook the official uses, such as the fast and quick payment.

So a number of analysis questions I am taking a look at in this space is what does crime with Bitcoin appear like? So a lot of people will say that scams and thefts have been occurring for ages. However the means via which they happen changes with the expertise. So a Victorian road swindler would virtually be doing something very completely different to a 419 Nigerian prince scammer.

So the following query that I would prefer to analysis as well is wanting at the scale of the problem of crime with cryptocurrency. So by producing a log of recognized scams and thefts and things like that, we will then cross reference that with the general public transaction log of all transactions and see just how much of the transactions are literally unlawful and legal. So my last query would be, to what extent does the know-how itself actually facilitate crime? By looking again on the crime logs, we are able to see which particular sorts of crime happen, and whether it is really the know-how's fault, or is that this simply the same previous crimes that we've been looking at before. And as soon as we have take into account these items, we will start to consider potential solutions to the problem of crime with Bitcoin.

And we will think about that the one suitable resolution would be one that preserves the underlying values of the know-how itself, which would be privacy and decentralisation. A whole lot of focus from the media is to have a look at the legal points of it. And so they do not give sufficient worth to the reputable uses, because Bitcoin is a expertise that permits quick, fast payments, which is helpful to anybody that is ever paid for anything on the internet.

Crypto-what?

In case you've attempted to dive into this mysterious factor called blockchain, you'd be forgiven for recoiling in horror at the sheer opaqueness of the technical jargon that's often used to frame it. So earlier than we get into what a crytpocurrency is and the way blockchain technology might change the world, let's discuss what blockchain actually is.

In the simplest terms, a blockchain is a digital ledger of transactions, not unlike the ledgers we've got been using for lots of of years to document gross sales and purchases. The perform of this digital ledger is, in ethereum ICO truth, just about identical to a standard ledger in that it data debits and credit between people. That's the core concept behind blockchain; the difference is who holds the ledger and who verifies the transactions.

With traditional transactions, a fee from one particular person to another includes some form of middleman to facilitate the transaction. Let's say Rob wants to transfer ?20 to Melanie. He can both give her cash within the type of a ?20 note, or he can use some kind of banking app to transfer the money directly to her checking account. In both instances, a financial institution is the intermediary verifying the transaction: Rob's funds are verified when he takes the money out of a money machine, or they are verified by the app when he makes the digital switch. The bank decides if the transaction should go forward. The bank also holds the file of all transactions made by Rob, and is solely chargeable for updating it at any time when Rob pays somebody or receives cash into his account. In other words, the bank holds and controls the ledger, and every little thing flows by means of the financial institution.

That's a number of accountability, so it's important that Rob feels he can belief his bank otherwise he wouldn't risk his cash with them. He needs to feel confident that the bank will not defraud him, is not going to lose his cash, is not going to be robbed, and won't disappear in a single day. This want for belief has underpinned pretty much every main behaviour and facet of the monolithic finance industry, to the extent that even when it was discovered that banks had been being irresponsible with our money throughout the financial crisis of 2008, the government (another intermediary) selected to bail them out somewhat than threat destroying the final fragments of belief by letting them collapse.

Blockchains operate in another way in a single key respect: they are totally decentralised. There isn't any central clearing house like a bank, and there's no central ledger held by one entity. As a substitute, the ledger is distributed throughout an enormous network of computers, referred to as nodes, every of which holds a replica of the entire ledger on their respective exhausting drives. These nodes are connected to 1 one other via a chunk of software program referred to as a peer-to-peer (P2P) consumer, which synchronises information across the community of nodes and makes certain that everyone has the same model of the ledger at any given point in time.

When a brand new transaction is entered into a blockchain, it is first encrypted using state-of-the-art cryptographic technology. As soon as encrypted, the transaction is transformed to something known as a block, which is mainly the time period used for an encrypted group of recent transactions. That block is then sent (or broadcast) into the network of pc nodes, the place it's verified by the nodes and, once verified, passed on through the community in order that the block might be added to the top of the ledger on everyone's pc, under the record of all earlier blocks. This is called the chain, therefore the tech is referred to as a blockchain.

As soon as authorised and recorded into the ledger, the transaction may be completed. This is how cryptocurrencies like Bitcoin work.

Accountability and the elimination of belief

What are the benefits of this technique over a banking or central clearing system? Why would Rob use Bitcoin instead of normal forex?

The answer is trust. As talked about earlier than, with the banking system it's crucial that Rob trusts his financial institution to guard his cash and handle it properly. To ensure this happens, monumental regulatory methods exist to verify the actions of the banks and guarantee they are fit for objective. Governments then regulate the regulators, creating a sort of tiered system of checks whose sole goal is to help prevent errors and dangerous behaviour. In other words, organisations just like the Financial Companies Authority exist exactly as a result of banks cannot be trusted on their very own.

And banks frequently make mistakes and misbehave, as we have seen too many occasions. When you might have a single supply of authority, energy tends to get abused or misused. The trust relationship between individuals and banks is awkward and precarious: we do not actually trust them but we do not feel there is much various.

Blockchain methods, then again, do not want you to belief them at all. All transactions (or blocks) in a blockchain are verified by the nodes within the community before being added to the ledger, which suggests there isn't a single level of failure and no single approval channel. If a hacker wished to successfully tamper with the ledger on a blockchain, they must simultaneously hack tens of millions of computers, which is nearly impossible. A hacker would even be just about unable to carry a blockchain network down, as, again, they would wish to have the ability to shut down every single laptop in a network of computer systems distributed around the globe.

The encryption course of itself is also a key issue. Blockchains like the Bitcoin one use intentionally tough processes for his or her verification procedure. In the case of Bitcoin, blocks are verified by nodes performing a deliberately processor- and time-intensive series of calculations, usually in the form of puzzles or advanced mathematical issues, which mean that verification is neither prompt nor accessible. Nodes that do commit the resource to verification of blocks are rewarded with a transaction fee and a bounty of newly-minted Bitcoins.

This has the perform of both incentivising people to grow to be nodes (as a result of processing blocks like this requires fairly highly effective computers and a lot of electrical energy), while additionally handling the process of generating - or minting - models of the forex. This is known as mining, as a result of it includes a considerable quantity of effort (by a pc, in this case) to provide a brand new commodity. It also means that transactions are verified by essentially the most independent means attainable, extra unbiased than a authorities-regulated organisation just like the FSA.

This decentralised, democratic and extremely safe nature of blockchains signifies that they will perform without the necessity for regulation (they are self-regulating), government or other opaque middleman. They work as a result of individuals do not belief one another, reasonably than in spite of.

Let the importance of that sink in for some time and the excitement around blockchain starts to make sense.

Smart contracts

Where issues get really attention-grabbing is the applications of blockchain beyond cryptocurrencies like Bitcoin. Given that one of the underlying rules of the blockchain system is the safe, impartial verification of a transaction, it's easy to imagine different methods by which any such process may be precious. Unsurprisingly, many such applications are already in use or development. Some of the finest ones are:

? Sensible contracts (Ethereum): most likely the most thrilling blockchain growth after Bitcoin, smart contracts are blocks that comprise code that have to be executed in order for the contract to be fulfilled. The code can be something, so long as a computer can execute it, however in simple terms it implies that you should utilize blockchain know-how (with its independent verification, trustless architecture and safety) to create a sort of escrow system for any kind of transaction. For instance, in the event you're a web designer you may create a contract that verifies if a new consumer's web site is launched or not, and then automatically release the funds to you once it's. No extra chasing or invoicing. Sensible contracts are also being used to show possession of an asset comparable to property or art. The potential for lowering fraud with this approach is gigantic.

? Cloud storage (Storj): cloud computing has revolutionised the online and brought in regards to the advent of Large Data which has, in flip, kick began the new AI revolution. But most cloud-based mostly techniques are run on servers saved in single-location server farms, owned by a single entity (Amazon, Rackspace, Google and so on). This presents all the identical problems because the banking system, in that you simply knowledge is managed by a single, opaque organisation which represents a single point of failure. Distributing information on a blockchain removes the belief issue entirely and in addition guarantees to extend reliability as it's so much harder to take a blockchain network down.

? Digital identification (ShoCard): two of the most important problems with our time are identify theft and information protection. With huge centralised services equivalent to Facebook holding so much information about us, and efforts by varied developed-world governments to store digital information about their residents in a central database, the potential for abuse of our private information is terrifying. Blockchain expertise affords a possible answer to this by wrapping your key knowledge up into an encrypted block that may be verified by the blockchain network whenever it's worthwhile to show your identification. The functions of this range from the obvious alternative of passports and I.D. cards to different areas such as replacing passwords. It might be enormous.

? Digital voting: highly topical in the wake of the investigation into Russia's affect on the latest U.S. election, digital voting has long been suspected of being both unreliable and highly susceptible to tampering. Blockchain expertise affords a method of verifying that a voter's vote was efficiently despatched while retaining their anonymity. It guarantees not solely to reduce fraud in elections but additionally to increase common voter turnout as people will have the ability to vote on their cell phones.

[itsmedicinesfakianakis]

Thermodynamic Parameters of Temperature-Induced Phase Transition for Brushes onto Nanoparticles: Hydrophilic versus Hydrophobic End-Groups Functionalization

Quantification of the stimuli-responsive phase transition in polymers is topical and important for the understanding and development of novel stimuli-responsive materials. The temperature-induced phase transition of poly(N-isopropylacrylamide) (PNIPAm) with one thiol end group depends on the confinement—free polymer or polymer brush—on the molecular weight and on the nature of the second end. This paper describes the synthesis of heterotelechelic PNIPAm of different molecular weights with a thiol end group—that specifically binds to gold nanorods and a hydrophilic NIPAm end group by reversible addition-fragmentation chain-transfer polymerization. Proton high-resolution magic angle sample spinning NMR spectra are used as an indicator of the polymer chain conformations. The characteristics of phase transition given by the transition temperature, entropy, and width of transition are obtained by a two-state model. The dependence of thermodynamic parameters on molecular weight is compared for hydrophilic and hydrophobic end functional-free polymers and brushes.

Thermodynamic parameters of the temperature-induced phase transition of heterotelechelic poly(N-isopropylacrylamide) brushes grafted onto gold nanorods are studied by 1H high-resolution magic-angle sample spinning NMR spectroscopy and compared to free polymer in aqueous solution. Transition temperature, entropy, and width of transition demonstrate remarkable differences depending on the molecular weight and polarity of the free, terminal end groups.

from # All Medicine by Alexandros G. Sfakianakis via alkiviadis.1961 on Inoreader http://ift.tt/2ik4Zhw from OtoRhinoLaryngology - Alexandros G. Sfakianakis via Alexandros G.Sfakianakis on Inoreader http://ift.tt/2ihWZNY

[rubberandplasticproducts]

Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity – Polymer Chemistry Blog

Stace et al. employ small cationic RAFT agents to produce low dispersity polymers in ab initio emulsion polymerization. Reversible addition fragmentation chain transfer (RAFT) polymerization has revolutionized the field of polymer chemistry providing access to a wide range of materials with controlled molecular weight, functionality, end-group fidelity and dispersity. In their current contribution, the groups of Moad, Keddie and Fellows joined forces to report a range of low molar mass cationic RAFT agents that allow for predictable molecular weight and dispersity in ab initio emulsion polymerization. In particular, upon utilizing the protonated RAFT agent ((((cyanomethyl)thio)carbonothioyl)(methyl)amino)pyridin1-ium toluenesulfonate and the analogous methyl-quaternized RAFT agents, 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, styrene could be efficiently polymerized yielding polystyrene with narrow molecular weight distributions (Đ m 1.2–1.4). The authors attribute the success of ab initio emulsion polymerization with the former RAFT agent to the hydrophilicity of the pyridinium group which allows for the predominant partition of the water-soluble RAFT agent into the aqueous phase.  The RAFT agent also gives minimal retardation. In addition, by employing 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, a “surfactant-free” RAFT emulsion can be achieved producing a low Đ m  polystyrene although the RAFT end-group was lost upon isolating the polymer. Additional preliminary experiments were also performed demonstrating that this class of RAFT agents can be broadly applicable in ab initio emulsion polymerization of a range of other more-activated monomers including acrylates and methacrylates producing low dispersity polymers while the polymerization of less activated monomers such as vinyl acetate showed good control over the molecular weight, albeit broader molecular weight distributions. The authors are currently investigating such systems to establish their full utility in emulsion polymerization and develop robust and scalable conditions for the formation of block copolymers. Tips/comments directly from the authors: There are two significant challenges in implementing successful ab initio emulsion polymerization in a high throughput platform such as the Chemspeed® Devising a protocol for vortexing/agitating so as to form, and then maintain, a stable latex. The protocol reported was the end-result of many experiments. Degassing the reaction medium. RAFT polymerization can be successfully carried out in non-degassed media.  However, for good reproducibility, optimal dispersity, high end group fidelity and acceptable polymerization rates, degassing remains important.  In conducting experiments on the Chemspeed®, it is important to make sure the media to be dispensed by the robot are degassed, and that all of the solvent lines, and the solvent used to prime and wash the syringe needles are degassed. Read the full article for FREE until 6th December! Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity, Polym. Chem., 2019, 10, 5044-5051, DOI: 10.1039/C9PY00893D About the Web Writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.